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Angle of prop shaft - theoretical question.
JAXAshby wrote:
the original (and incorrect) term was "P-Torque". that was replaced (to some extent) by the term "P-Thrust" as in asymetrical thrust due to the angling of the disc of the prop relative to forward motion. "P-Factor" is now used by idiots and for idiot who don't have a clew what is going on. On Jaxworld maybe ... You truly are an idiot. So how about a cite for the use of those terms on this planet. Oh, silly me, asking Jax to backup one of his febrile claims. btw ricky, you must the only person on the planet who claims to have an Airline Transport rating who doesn't know the difference between slots and slats. Another bad drug reaction, Jax? Please show where I wrote anything that would indicate such an absurd position on my part. Oh, silly me, asking Jax to backup one of his febrile claims. Take your pills, Jax and get some rest. Rick |
Angle of prop shaft - theoretical question.
gene, *you* are the guy who said prop wash spirals and thus twists an airplane
one way or the other, and no one else( and thus a smaller rudder would reduce the effect, AND a larger rudder would reduce it). AND, prop wash on a boat must push against something or the boat won't go. So, who is the clewless one? no, gene. the prop pushes against the water, NOT the prop wash. and yes you did state that prop walk is due to the prop wash spiralling, and thus pushing against god knows what. dumb, gene, as in stupid on your part. No... stupid on my part would be continuing this thread with the terminally clueless.... -- 23' Grady White, out of Southport, NC. http://myworkshop.idleplay.net/cavern/ Homepage http://www.southharbourvillageinn.com/directions.asp Where Southport,NC is located. http://southharbourvillageinn.linksysnet.com Real Time Pictures at My Marina http://www.thebayguide.com/rec.boats Rec.boats at Lee Yeaton's Bayguide |
Angle of prop shaft - theoretical question.
rickie, recess is over. go back to class.
the original (and incorrect) term was "P-Torque". that was replaced (to some extent) by the term "P-Thrust" as in asymetrical thrust due to the angling of the disc of the prop relative to forward motion. "P-Factor" is now used by idiots and for idiot who don't have a clew what is going on. On Jaxworld maybe ... You truly are an idiot. So how about a cite for the use of those terms on this planet. Oh, silly me, asking Jax to backup one of his febrile claims. btw ricky, you must the only person on the planet who claims to have an Airline Transport rating who doesn't know the difference between slots and slats. Another bad drug reaction, Jax? Please show where I wrote anything that would indicate such an absurd position on my part. Oh, silly me, asking Jax to backup one of his febrile claims. Take your pills, Jax and get some rest. Rick |
Angle of prop shaft - theoretical question.
Cornered and, as seen here a thousand times before, unable to support a
single word of his paranoid fantasies, Jax could only respond with this brilliant retort: rickie, recess is over. go back to class. That is SO Jax ... come on out and play Jax, tell us all about your taildragger time and show those cites, show us those posts ... Bwahahahahahahahahahahahah what a poseur you are. But, sadly, and typically Jax, not even a good poseur. Rick |
Angle of prop shaft - theoretical question.
rickie, recess is over. go back to class.
Cornered and, as seen here a thousand times before, unable to support a single word of his paranoid fantasies, Jax could only respond with this brilliant retort: rickie, recess is over. go back to class. That is SO Jax ... come on out and play Jax, tell us all about your taildragger time and show those cites, show us those posts ... Bwahahahahahahahahahahahah what a poseur you are. But, sadly, and typically Jax, not even a good poseur. Rick |
Angle of prop shaft - theoretical question.
Subject: Angle of prop shaft - theoretical question.
From: Wayne.B Date: 06/05/2004 21:28 Pacific Standard Time Message-id: On 05 Jun 2004 22:44:53 GMT, (Shen44) wrote: the basic issue (propwalk) is caused by prop rotation and all other factors may assist or decrease it, but the basic "phenom" occurs when you rotate the prop. ============================================ Of course, but what is the cause of the asymetric thrust? THAT is the question. Everyone agrees that a prop with a horizontal shaft still exhibits prop walk, implying that the bottom the prop is more efficient at providing thrust than the top. Lots of theories have been provided but none that seem totally convincing since prop walk still exists to one degree or another on deep props, that have plenty of hull clearance. First off, ignore any parallel discussions of aircraft propellors. An aircraft propellor work in one medium ... air. A boat propellor impacts two mediums ... water and air (if you need confirmation of this, look astern of a boat underway, under power ... you will see a wash/water, being lifted above the surface in a rooster tail or lessor version thereof. Now, go to a boatyard and stand in front of a propellor on some boat out of the water. Look at the angle/pitch of the blade, then start slowly rotating the blade (assuming RH and you are standing in front of it) and visualize you are pushing against water. As you rotate, (starting in the vertical up position) you will note that the blade angle/pitch is pushing back and down against the water. Water is not compressible. By pushing back and down, you are pushing against a "solid" mass of water and are most efficient. Now, as the propellor rotates it also pushes to the side against that "solid mass of water, but as it starts rotating up, the solid mass becomes a relative small column of water with air above it (air - highly compressible). The propellor blade is now pushing back, but also up, and lifting that water up and away - i.e., no longer a solid wall of water, into the air, and it becomes less efficient. The same applies as the blade re aproaches the vertical. The forces of greatest efficiency are on the downward and sideward rotation of the blade (pushing the stern to stbd when going ahead -left turn). The same applies to backing, but because the prop is now pulling the boat, the effect is far more noticeable. Someone is bound to discuss the overhang of the hull, over the prop, should negate this. Not so. That overhang is relatively short and variable, so that although it may affect the degree, it will not stop the results. BG And this is my theory and I'magonna stick to it......for now. Shen |
Angle of prop shaft - theoretical question.
Overall, Shen and I are in agreement on this issue, as to the cause when
it relates to boats (for what that's worth), but again, my main point is that you should normally expect to see this reaction in a specific direction for a specific prop rotation no matter the angle of the shaft or hull configuration or relative depth of the prop. In fact, as someone else has said, this is much the basis of the "surface piercing" Arnesson prop setup. As a kind of further note, I was reading some comments from guys running "go fast" Donzi types, discussing switching from outboard turning to inboard turning props. This seemed to give them an increase in speed (unrelated to this discussion) but also made the boats handle poorly around the dock (totally understandable to me as it completely changes the handling characteristics), but the important point was that these were I/O's and the reason for this handling change was simple - propwalk - and the propwalk was/is basically, caused by the propellor rotation, as this shows - at least to me. otn |
Angle of prop shaft - theoretical question.
On Sun, 6 Jun 2004 08:49:02 -0400, "Charles T. Low"
wrote: The main effect is from the spiral prop wash. In reverse, where asymmetric thrust is virtually always more pronounced, the top half of the prop wash vortex strikes the hull, and pushes it sideways. The effect will vary depending on the underwater hull shape, the angle of the prop shaft, design of prop ... So, a right hand propeller in reverse turns counter-clockwise. The top half of the spiralling prop wash is moving to port, and pushes on the hull, yawing the stern to port. The bottom half othe spiral is mostly in clear water, pushing on nothing. ================================================== ==== Charles, I think you've got it, thanks. |
Angle of prop shaft - theoretical question.
JAXAshby could only repeat his infantile retort:
rickie, recess is over. go back to class. What's the matter, Jax? Has Oz got you so whipped so badly over on the siphon breaker thread that you can't even keep track of where you are getting your ass kicked worse? C'mon, Jax, don't be a Speedo Queen all your life, take a day off and tell us all about your taildragger flying experiences and cite those posts that you claim I made ... Thom kicked your ass pretty good too, so maybe that's why you can only come back with cut and pastes of your childish little hissy fit retorts. This just hasn't been your week has it, Jax? Historically this is where you fade away again ... good timing too, you have done an excellent job showing all the new readers here just how amazing you are. Rick |
Angle of prop shaft - theoretical question.
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Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: That theory sounds compelling at explaining a difference in the efficiency of the blade during it's downward vs. upward travel (if there's no overhang.:) This part of your explanation would create an upward force. It would also create a listing torque force since the upward force of your theory above is applied off the center of the prop. But since the column of water is the same in both the sideways directions, where does the sideways force come from to create prop walk? Steve The part of his explanation which is not being stressed enough, is that you must follow the blade through it's complete revolution. The blade starts pushing down/back (pitch of the blade) but as it rotates it begins to push to the side/back (RH prop going ahead), pulling/pushing the stern to stbd. Again, the blade is more efficient during the lower portion of this arc (solid water) than it is in the upper portion (water being lifted into air) which causes the propwalk. The next time you get to look at a ship in ballast or riding light, with it's prop just beneath the surface, watch the "wash" from the prop at the surface. You'll see it being "thrown" up/back and to the side/back. As for the "overhang" of the hull,issue, look again at the pitch of the blade. When the blade is pushing up/back, it's not straight up, it's G BACK/up, then look at the wash astern of your boat .... you'll see it breaking the surface astern of you. otn |
Angle of prop shaft - theoretical question.
Wayne.B wrote: On Sun, 6 Jun 2004 08:49:02 -0400, "Charles T. Low" wrote: The main effect is from the spiral prop wash. In reverse, where asymmetric thrust is virtually always more pronounced, the top half of the prop wash vortex strikes the hull, and pushes it sideways. The effect will vary depending on the underwater hull shape, the angle of the prop shaft, design of prop ... So, a right hand propeller in reverse turns counter-clockwise. The top half of the spiralling prop wash is moving to port, and pushes on the hull, yawing the stern to port. The bottom half othe spiral is mostly in clear water, pushing on nothing. ================================================== ==== Charles, I think you've got it, thanks. This is one of those explanations which I feel may have an effect on propwalk amount, but is not the "root" cause of propwalk. Reason ... you get propwalk ahead and astern. When going ahead, the wash does not push against the hull. otn |
Angle of prop shaft - theoretical question.
Steve wrote:
That theory sounds compelling at explaining a difference in the efficiency of the blade during it's downward vs. upward travel (if there's no overhang.:) This part of your explanation would create an upward force. It would also create a listing torque force since the upward force of your theory above is applied off the center of the prop. But since the column of water is the same in both the sideways directions, where does the sideways force come from to create prop walk? Steve Not true, in my opinion. During the lower rotation of the blade it is pushing sideways in "solid" water . Looking at the blade from astern (RH) and using a 360 deg circle and watching the pich angle for angle of push. As the blade approaches 45* it is pushing back/down, as it approaches 90* it starts pushing back/sideways against "solid" water..... pulling the stern to stbd. As it rotates to 180* it is still pushing against the water, back/sideways against "solid" water, but as it starts coming up to 270* it begins to push back/up and loses efficiency, as it starts to lift the water up/back. This continues through 360* where it's pushing back/sideways (and to a degree, up), but at that reduced efficiency because it can lift that water column up and back into the air. I realize this is not the easiest visualization and my writing skills may not be explaining the point at it's best. Shen |
Angle of prop shaft - theoretical question.
On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:
As for the "overhang" of the hull,issue, look again at the pitch of the blade. When the blade is pushing up/back, it's not straight up, it's G BACK/up, then look at the wash astern of your boat .... you'll see it breaking the surface astern of you. Not on my boat. There's still maybe 5 feet of boat hull in the water above the prop behind where the prop exits. There's no prop wash at all breaking the surface astern of me. And since we're talking about backing up from standing still, there really is nothing of the sort you've described above. And yet I still get prop walk. Steve |
Angle of prop shaft - theoretical question.
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Angle of prop shaft - theoretical question.
On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote:
This is one of those explanations which I feel may have an effect on propwalk amount, but is not the "root" cause of propwalk. Reason ... you get propwalk ahead and astern. When going ahead, the wash does not push against the hull. I don't think there is a "root" cause of prop walk. It's just a sum of many factors, some of which even counteract eachother which is why you can't be sure of the direction of prop walk just by which way the prop turns. |
Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote: As for the "overhang" of the hull,issue, look again at the pitch of the blade. When the blade is pushing up/back, it's not straight up, it's G BACK/up, then look at the wash astern of your boat .... you'll see it breaking the surface astern of you. Not on my boat. There's still maybe 5 feet of boat hull in the water above the prop behind where the prop exits. There's no prop wash at all breaking the surface astern of me. And since we're talking about backing up from standing still, there really is nothing of the sort you've described above. And yet I still get prop walk. Steve G There's always the boat which doesn't appear to show the obvious reaction. Considering the angle of pitch of the prop, when ahead, 5 feet is relatively nothing..... also, what is the shape of your hull aft of the prop? I've seen the same results, but my feeling is it takes very little "lift" of the wash from the prop blade to create that unequal thrust back and to the side, we know as "propwalk". You are saying there's nothing of the sort that I've described. I'm saying it's not always readily apparent, but it IS there. Back a boat from a dead start .... you won't immediately see the wash. Back a boat that has headway .... it will be even longer before you see the wash .... but .... the unequal thrust WILL be occurring. If you have a low power to weight ratio, the visual results will be greatly lessened. However, this does not mean they aren't occurring. Again, BG I'm no scientist, engineer, Naval architect, or prop designer .... my opinions have developed over a good many years of talking to individuals, handling all kinds of boats, watching wakes and reactions .... the "root" cause is prop rotation .... everything else is a variable, adding or detracting from the mix. otn |
Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote: This is one of those explanations which I feel may have an effect on propwalk amount, but is not the "root" cause of propwalk. Reason ... you get propwalk ahead and astern. When going ahead, the wash does not push against the hull. I don't think there is a "root" cause of prop walk. It's just a sum of many factors, some of which even counteract eachother which is why you can't be sure of the direction of prop walk just by which way the prop turns. G I disagree. My feeling is rotation in a water medium is the root cause. The amount of propwalk however is dependent on many factors, some of which can and do counteract it's effect. On 9,999.99 out of 10,000 boats fitted with right hand fixed pitch props, starting at DIW with no wind and current ..... the boat will back to port and vice versa for left hand props. However, add wind, current, headway, sternway, turning, a BARN door rudder, etc., etc., and you are absolutely correct .... that boat may go in the opposite direction at the least opportune time, if you are not aware of the possibilities. otn |
Angle of prop shaft - theoretical question.
That was a good explanation and I got what you're trying to say. The only problem with it is the overhang. My prop is about 3 ft down. The clearance to the hull above it is around 3 to 4 inches. There's still about 5 or 6 feet of waterline behind the prop. So on the upward stroke, the blade is pushing the water column against the hull. First, what is the shape of your hull? Secondly, look at the pitch of your prop - the main push is back, the "UP" angle is only slight and not directly "up" against you hull, even WITH (I'll bet) 5-6' of overhang. Even in reverse, with any degree of deadrise, the push to the side and up will not be mainly impacting on your hull for the full revolution, if at all, and even if you have no deadrise, the water will shortly leave the confines of your hull and be allowed to push up into the air. Not only is there no noticable bulge of water behind the boat from the prop but there isn't even a sign on the surface that there's a prop turning at all. I just don't get that stream of bubbles behind me or a bulge in the water that powerboats get or anything other than the same sort of wake I get when sailing. And yet there's pronounced prop walk when reversing. Steve My feeling on this is that the visual effects of prop thrust are not always readily apparent, but this does not in and of itself, alter what is occurring, i.e., you don't need a big white water wash, astern or ahead of you to be, in reality, experiencing a lessor degree of efficiency from the prop during half (maybe a little less, maybe a little more) of it's rotation, which is causing the unequal pull to stbd or port, depending on it's rotation. Shen |
Angle of prop shaft - theoretical question.
On Tue, 08 Jun 2004 02:33:44 GMT, otnmbrd wrote:
Steven Shelikoff wrote: On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote: As for the "overhang" of the hull,issue, look again at the pitch of the blade. When the blade is pushing up/back, it's not straight up, it's G BACK/up, then look at the wash astern of your boat .... you'll see it breaking the surface astern of you. Not on my boat. There's still maybe 5 feet of boat hull in the water above the prop behind where the prop exits. There's no prop wash at all breaking the surface astern of me. And since we're talking about backing up from standing still, there really is nothing of the sort you've described above. And yet I still get prop walk. Steve G There's always the boat which doesn't appear to show the obvious reaction. Considering the angle of pitch of the prop, when ahead, 5 feet is relatively nothing..... also, what is the shape of your hull aft of the prop? I've seen the same results, but my feeling is it takes very little "lift" of the wash from the prop blade to create that unequal thrust back and to the side, we know as "propwalk". You are saying there's nothing of the sort that I've described. I'm saying it's not always readily apparent, but it IS there. Back a boat from a dead start .... you won't immediately see the wash. Back a boat that has headway .... it will be even longer before you see the wash .... but .... the unequal thrust WILL be occurring. It's absolutely obvious that there's unequal thrust. If not, there wouldn't be prop walk. The question is, where does it come from? And I still think the answer is multiple sources all contribute, some more than others on any given boat. I just don't think the effect you're describing here plays as much a part on my boat as it might on others because there's more above the prop than just a column of water and then air. Steve |
Angle of prop shaft - theoretical question.
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Angle of prop shaft - theoretical question.
On Tue, 08 Jun 2004 02:45:24 GMT, otnmbrd wrote:
Steven Shelikoff wrote: On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote: This is one of those explanations which I feel may have an effect on propwalk amount, but is not the "root" cause of propwalk. Reason ... you get propwalk ahead and astern. When going ahead, the wash does not push against the hull. I don't think there is a "root" cause of prop walk. It's just a sum of many factors, some of which even counteract eachother which is why you can't be sure of the direction of prop walk just by which way the prop turns. G I disagree. My feeling is rotation in a water medium is the root cause. Um, yeah. That's kind of obvious. Not really an explanation of why rotation in a water medium causes prop walk though. It's sort of the same thing as saying the root cause of prop walk is a different amount of force pushing the stern left vs. right. You'd be correct, but it doesn't really tell you anything you don't already know. Steve |
Angle of prop shaft - theoretical question.
I have often wondered if the phenomenon is partly caused by the same
forces that act on a helicopter rotor (or any other rotating mass) ... a control input force to the rotor is applied 90 degrees ahead of the desired output force. Applying that to the marine propeller means that the right hand prop applying a force (better "bite") at 6 o'clock would generate a resultant force at 9 o'clock in ahead rotation and 3 o'clock astern. The force at 3 o'clock would result in the stern moving to port when backing. Rick |
Angle of prop shaft - theoretical question.
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Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: It's absolutely obvious that there's unequal thrust. If not, there wouldn't be prop walk. The question is, where does it come from? And I still think the answer is multiple sources all contribute, some more than others on any given boat. I just don't think the effect you're describing here plays as much a part on my boat as it might on others because there's more above the prop than just a column of water and then air. Steve Actually, for the most part we are in agreement. When you go back through my post on this issue, you will always note that I am mentioning a number of factors which will enhance and detract from propwalk (wind, current, hull form, speed, pitch, kort nozzles, etc.) and the fact that we are not always sure what that reaction will be, until we experience it on a particular boat under particular conditions. However, there has to be a "root" cause .... an initial action/reaction which starts the process. Can we say that the directional rotation of the prop (right or left) is by itself the cause? I can't say with certainty that it is or isn't, but, in my experience handling ships with CP props, when the prop is set to zero pitch (judged to be zero due to lack of creep ahead or astern) I generally (note, I'm saying "generally") see no side movement, yet put in even the most minimal pitch and I will experience "walk" (loaded condition, prop down deep with hull overhang above, and light condition, prop close to the surface with hull overhang out of the water .... makes no difference .... both ahead and astern ... which is why I discount hull overhang). What does this lead me to believe. If the rotating prop at zero pitch, generally eg showed me no "walk" and as soon as I added pitch, I got "walk" then I look closely at the effects of a rotating prop and it's pitch for the root cause. Let's now look at prop efficiency (these are MY views based on what I see, read and feel). If we follow the rotation of a RH fixed pitch prop, looking at it from astern starting at top dead center (ooo*) the particular blade is pushing water to the right (and back ...always back, but we will ignore that component for this discussion) at minimal efficiency. this efficiency, however, is increasing as the propellor turns towards 45* and the direction is changing to a increasingly down direction. As the blade reaches 45*, efficiency is close to maximum and from here the direction is more down than to the side. Someplace just prior to 90* the efficiency becomes maximum and as the blade rotates toward 135* the angle of push changes to the left (pulling/pushing the stern to stbd) the blade continues at maximum efficiency through 180* (pushing left) but as it begins it's rotation upwards, that efficiency, slowly begins to drop off and the direction of push begins to angle upward/left until you reach 270*. From this point, efficiency drops off at a marked rate and the blade is pushing up and beginning to push slightly right. As you reach @315* you are close to being back to minimal efficiency and pushing right/up, which continues back to 000*. You can see from this (my visualization) that the force pushing left (pulling/pushing the stern to the right) occurs during that time when the blade is pushing most efficiently, whereas the force pushing to the right (countering that left push) occurs when the blade is pushing with less efficiency...... propwalk. I would love someone who designs props and is far more technically versed in the goings on of a prop beneath the water, to critique this. Also, so there's no misunderstanding, remember, my feeling about props efficiency decrease on it's upward rotation is about the prop pushing water up and into air, losing efficiency. .... and most importantly, these are my views/visualizations .... G yours may vary. otn |
Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: On Tue, 08 Jun 2004 02:45:24 GMT, otnmbrd wrote: Steven Shelikoff wrote: On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote: This is one of those explanations which I feel may have an effect on propwalk amount, but is not the "root" cause of propwalk. Reason ... you get propwalk ahead and astern. When going ahead, the wash does not push against the hull. I don't think there is a "root" cause of prop walk. It's just a sum of many factors, some of which even counteract eachother which is why you can't be sure of the direction of prop walk just by which way the prop turns. G I disagree. My feeling is rotation in a water medium is the root cause. Um, yeah. That's kind of obvious. Not really an explanation of why rotation in a water medium causes prop walk though. It's sort of the same thing as saying the root cause of prop walk is a different amount of force pushing the stern left vs. right. You'd be correct, but it doesn't really tell you anything you don't already know. Steve BG see my other response .... no way I could write that again. otn |
Angle of prop shaft - theoretical question.
On Tue, 08 Jun 2004 12:08:33 -0400, Wayne.B
wrote: On Tue, 08 Jun 2004 01:28:20 GMT, (Steven Shelikoff) wrote: My prop is about 3 ft down. The clearance to the hull above it is around 3 to 4 inches. There's still about 5 or 6 feet of waterline behind the prop. So on the upward stroke, the blade is pushing the water column against the hull. ====================================== Charles Low conjectured, and I think he may be right, that it is this force of the prop wash hitting the hull that causes the "walk". It's the only explanation that seems to hold up to scrutiny with different hull and prop configurations. It could be a contributing factor but not the only one, and maybe not even the most important for some boats. You can have propwalk even with no prop wash hitting the hull, like when you first start out in forward from a standstill with a boat that has the prop beyond the stern of the boat (an outboard or sterndrive.) It's just not as noticable because you can control it better in forward. Steve |
Angle of prop shaft - theoretical question.
On Tue, 08 Jun 2004 17:29:47 GMT, otnmbrd wrote:
Steven Shelikoff wrote: It's absolutely obvious that there's unequal thrust. If not, there wouldn't be prop walk. The question is, where does it come from? And I still think the answer is multiple sources all contribute, some more than others on any given boat. I just don't think the effect you're describing here plays as much a part on my boat as it might on others because there's more above the prop than just a column of water and then air. Steve Actually, for the most part we are in agreement. When you go back through my post on this issue, you will always note that I am mentioning a number of factors which will enhance and detract from propwalk (wind, current, hull form, speed, pitch, kort nozzles, etc.) and the fact that we are not always sure what that reaction will be, until we experience it on a particular boat under particular conditions. However, there has to be a "root" cause .... an initial action/reaction which starts the process. *The* root cause is a net force sideways. [...] Let's now look at prop efficiency (these are MY views based on what I see, read and feel). If we follow the rotation of a RH fixed pitch prop, looking at it from astern starting at top dead center (ooo*) the particular blade is pushing water to the right (and back ...always back, but we will ignore that component for this discussion) at minimal efficiency. this efficiency, however, is increasing as the propellor turns towards 45* and the direction is changing to a increasingly down direction. As the blade reaches 45*, efficiency is close to maximum and from here the direction is more down than to the side. Someplace just prior to 90* the efficiency becomes maximum and as the blade rotates toward 135* the angle of push changes to the left You haven't explained why the prop is less efficient at 0 and gains efficiency on it's way from 0 to 90. I can think of some reasons why that may be correct. But the reason given having to do with a column of water only backed by air and a bulge at the surface isn't it. That's because the whole way from 0 to 180 degrees there is either an infinite column of water (right a 0 and 180) or the column of water is supported by the sea floor (everywhere else between 0 and 180) [...] Also, so there's no misunderstanding, remember, my feeling about props efficiency decrease on it's upward rotation is about the prop pushing water up and into air, losing efficiency. .... and most importantly, these are my views/visualizations .... G yours may vary. I realize that. It just doesn't support your discussion about what happens to the prop efficiency from 0 to 180 degrees. All it does is explain why the prop is less efficient on the upward part of it's trip vs. the downward part of it's trip which creates a net upward force off the centerline, which lifts the stern and lists the boat. It does nothing to explain why there's a net sideways force (if there's no overhang:), which I think we've agree is the root cause of prop walk that must be explained. Steve |
Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: *The* root cause is a net force sideways. G read the next part carefully [...] Let's now look at prop efficiency (these are MY views based on what I see, read and feel). If we follow the rotation of a RH fixed pitch prop, looking at it from astern starting at top dead center (ooo*) the particular blade is pushing water to the right (and back ...always back, but we will ignore that component for this discussion) at minimal efficiency. this efficiency, however, is increasing as the propellor turns towards 45* and the direction is changing to a increasingly down direction. As the blade reaches 45*, efficiency is close to maximum and from here the direction is more down than to the side. Someplace just prior to 90* the efficiency becomes maximum and as the blade rotates toward 135* the angle of push changes to the left You haven't explained why the prop is less efficient at 0 and gains efficiency on it's way from 0 to 90. Imagine the blade is just beneath the surface Again, ignoring the after component, up until the blade reached o, coming from 315, the pitch of the blade was not pushing towards a solid wall of water, it was pushing the water up and to the right into air ... it was less efficient, compared to it's opposite blade which was rotating from 135 to 180 which was pushing down and to the left against solid incompressible water. As the blade starts rotating to 090 it is pushing to the right and as it rotates, also begins to push down(into more solid water) , so that it's efficiency begins to increase as the angle it's pushing down, increases and it stops pushing to the right. I can think of some reasons why that may be correct. But the reason given having to do with a column of water only backed by air and a bulge at the surface isn't it. How do you know that? That's because the whole way from 0 to 180 degrees there is either an infinite column of water (right a 0 and 180) or the column of water is supported by the sea floor (everywhere else between 0 and 180) Not initially, but shortly after 000* it begins to be and increases. Why aren't you considering 180 - 000? Looking at the prop just beneath the surface, are you saying that the pitched blade is pushing against a solid column of water as it goes from 180* and approaches the surface at 000*? [...] Also, so there's no misunderstanding, remember, my feeling about props efficiency decrease on it's upward rotation is about the prop pushing water up and into air, losing efficiency. .... and most importantly, these are my views/visualizations .... G yours may vary. I realize that. It just doesn't support your discussion about what happens to the prop efficiency from 0 to 180 degrees. All it does is explain why the prop is less efficient on the upward part of it's trip vs. the downward part of it's trip which creates a net upward force off the centerline, which lifts the stern and lists the boat. It does nothing to explain why there's a net sideways force (if there's no overhang:), which I think we've agree is the root cause of prop walk that must be explained. If you can see a net upwards force, I'm halfway there. You agree that the blade is more efficient from 000-180 than from 180-000? If so, forget those numbers, consider the prop just beneath the surface (to help the visualization) and look at the blade rotation from 090-270 and 270-090, considering a 12 inch dia prop on a boat in 3,000 feet of water. At 090 the blade is pushing directly down. as the blade rotates past 090, it continues to push down, but also begins to push to the left. As the blade rotates toward 180 the downward push decreases as the sideways push (to the left) increases,until you reach 180 where the blade is pushing directly left. During this time, the blade has been pushing against a solid column of water, 3,000 feet deep (maximum efficiency). As the blade passes 180, it continues to push left, but also begins to push up (against a 12" column of water), efficiency decreases and as the blade approaches 270, the upward component increases as the left component decreases until you reach 270* where the blade is now pushing directly up against 6" of water. From 090 to 180 the blade was pushing at maximum efficiency in an ever increasing left component. From 180 to 270 the efficiency was decreasing at the same time as the left component was decreasing. Now, at 270, the blade is pushing directly up against 6" of water and as soon as it passes 270 begins to push to the right in a decreasing column of water. As before, as the right component increases the up component decreases, until it disappears at 000* in zero inches of water....the prop is relatively inefficient during this whole period or arc of rotation, compared to it's opposite 090-180 As the blade passes 000* it is pushing directly right and as it rotates past 000* begins to push downward (and decrease pushing right) and consequently begins to increase in efficiency as it gets further down and into more solid water, until we once again reach 090*. If we consider that we've got a 2 bladed prop, blade A passing between 090 and 270 started out at maximum efficiency and continued at that to 180* where it's efficiency began to decrease. Blade B passing between 270 and 090, on the other hand started out at poor efficiency, which it maintained until 000, where it started to pick up efficiency, going to maximum at 090. The net greater push is to the left (hull goes right) .... propwalk G I've exaggerated numbers, for clarity (at least, for my attempt at it) and don't really know how I could explain this in another way to make the point I'm trying to get across, clearer. otn |
Angle of prop shaft - theoretical question.
On Wed, 09 Jun 2004 04:56:51 GMT, otnmbrd wrote:
Steven Shelikoff wrote: *The* root cause is a net force sideways. G read the next part carefully [...] Let's now look at prop efficiency (these are MY views based on what I see, read and feel). If we follow the rotation of a RH fixed pitch prop, looking at it from astern starting at top dead center (ooo*) the particular blade is pushing water to the right (and back ...always back, but we will ignore that component for this discussion) at minimal efficiency. this efficiency, however, is increasing as the propellor turns towards 45* and the direction is changing to a increasingly down direction. As the blade reaches 45*, efficiency is close to maximum and from here the direction is more down than to the side. Someplace just prior to 90* the efficiency becomes maximum and as the blade rotates toward 135* the angle of push changes to the left You haven't explained why the prop is less efficient at 0 and gains efficiency on it's way from 0 to 90. Imagine the blade is just beneath the surface Again, ignoring the after component, up until the blade reached o, coming from 315, the pitch of the blade was not pushing towards a solid wall of water, it was pushing the water up and to the right into air ... it was less efficient, compared to it's opposite blade which was rotating from 135 to 180 which was pushing down and to the left against solid incompressible water. As the blade starts rotating to 090 it is pushing to the right and as it rotates, also begins to push down(into more solid water) , so that it's efficiency begins to increase as the angle it's pushing down, increases and it stops pushing to the right. There's no such thing as "more solid water" unless you're talking about ice. Once the blade goes past 0 degrees, all the way from there until 180 degrees it's pushing against the same thing. The water may be minutely more dense and under a tiny bit of less pressure for the 0 to 90 part of the trip than from the 90 to 180 part of the trip. But that has absolutely nothing to do with your theory of the water column being backed by air vs. something else. Your theory gives no reason why there's less efficiency for the 0 to 90 part of the rototation than for the 90 to 180 part of the rotation. It also gives no reason why there's a difference in efficiency between the 180 to 270 part of the rotation than from the 270 to 360 part since it's backed by air for that entire trip (unless there's hull overhang). To explain that, you have to start looking at the amount of water before you reach air. There's also one more thing you haven't thought about and that's the fact that we're dealing with 2 interfaces on the way down as well as on the way up. On the way up, the water column being pushed by the prop can bulge the air at the surface. On the way down, the water column being pushed by the prop can bulge the bottom, especially if the bottom is made of something like soft mud. It may provide more support than air at the surface, but the difference isn't as large as if there truly was a single interface on the way down. I can think of some reasons why that may be correct. But the reason given having to do with a column of water only backed by air and a bulge at the surface isn't it. How do you know that? Because there's no difference in the "backing" of the column of water for the entire travel of the prop from 0 to 180 degrees. It's not backed by air for any portion of that rotation. It's backed by practically non-compressable water against the bottom of the sea for that entire period of the rotation. So that theory provides no explanation for why there would be a difference in efficiency from 0 to 90 degrees vs. 90 to 180 degrees. That's because the whole way from 0 to 180 degrees there is either an infinite column of water (right a 0 and 180) or the column of water is supported by the sea floor (everywhere else between 0 and 180) Not initially, but shortly after 000* it begins to be and increases. Yes, initially and for the entire trip it doesn't change. It doesn't increase at all beyond 0. It's the same the whole way to 180. Why aren't you considering 180 - 000? Looking at the prop just beneath the surface, are you saying that the pitched blade is pushing against a solid column of water as it goes from 180* and approaches the surface at 000*? Your theory that the difference in efficiency is due to the column of water being backed by air vs. not backed by air doesn't explain any difference during the way up either since it' backed by air the whole way from 180 up to 0. You have to start looking at the amount of water before you get to air. [...] Also, so there's no misunderstanding, remember, my feeling about props efficiency decrease on it's upward rotation is about the prop pushing water up and into air, losing efficiency. .... and most importantly, these are my views/visualizations .... G yours may vary. I realize that. It just doesn't support your discussion about what happens to the prop efficiency from 0 to 180 degrees. All it does is explain why the prop is less efficient on the upward part of it's trip vs. the downward part of it's trip which creates a net upward force off the centerline, which lifts the stern and lists the boat. It does nothing to explain why there's a net sideways force (if there's no overhang:), which I think we've agree is the root cause of prop walk that must be explained. If you can see a net upwards force, I'm halfway there. I'm agreeing that if your theory is correct, i.e., that there is a difference in efficiency whether the column of water being pushed by the prop is backed up by air vs. something other than air, then there will be a net upwards force. You agree that the blade is more efficient from 000-180 than from 180-000? If so, forget those numbers, consider the prop just beneath the surface (to help the visualization) and look at the blade rotation from 090-270 and 270-090, considering a 12 inch dia prop on a boat in 3,000 feet of water. At 090 the blade is pushing directly down. as the blade rotates past 090, it continues to push down, but also begins to push to the left. As the blade rotates toward 180 the downward push decreases as the sideways push (to the left) increases,until you reach 180 where the blade is pushing directly left. During this time, the blade has been pushing against a solid column of water, 3,000 feet deep (maximum efficiency). As the blade passes 180, it continues to push left, but also begins to push up (against a 12" column of water), efficiency decreases and as the blade approaches 270, the upward component increases as the left component decreases until you reach 270* where the blade is now pushing directly up against 6" of water. From 090 to 180 the blade was pushing at maximum efficiency in an ever increasing left component. From 180 to 270 the efficiency was decreasing at the same time as the left component was decreasing. Now, at 270, the blade is pushing directly up against 6" of water and as soon as it passes 270 begins to push to the right in a decreasing column of water. As before, as the right component increases the up component decreases, until it disappears at 000* in zero inches of water....the Move the blade down in the water a few feet and this is no longer true. I.e., if the blade is 12" and it's 3 feet below the surface, it's pushing against the smallest column of water just past 270 degrees. As it rotates from just beyond 270 until it reaches 0 degrees, the size of the water column before it reaches air is ever increasing, thus increasing it's efficiency during the trip from 270 back to 0. prop is relatively inefficient during this whole period or arc of rotation, compared to it's opposite 090-180 As the blade passes 000* it is pushing directly right and as it rotates past 000* begins to push downward (and decrease pushing right) and consequently begins to increase in efficiency as it gets further down and into more solid water, until we once again reach 090*. Again, not true. Once it passes 0 degrees, there's no difference in what it's pushing against all the way until 180 degrees. So you can forget that portion of the trip giving any net force in the sideways direction. If we consider that we've got a 2 bladed prop, blade A passing between 090 and 270 started out at maximum efficiency and continued at that to 180* where it's efficiency began to decrease. Blade B passing between 270 and 090, on the other hand started out at poor efficiency, which it maintained until 000, where it started to pick up efficiency, going to maximum at 090. No. Actually, if the blade is at the surface, the efficiency decreased from 270 to 0 (if we're now talking about the amount of water before air) and then got to maxumum right at 0 and stayed there until 180. If the blade is a few feet down, it was at minimum efficiency just past 270 and then gains efficiency on it's way to 0 degrees. The net greater push is to the left (hull goes right) .... propwalk G I've exaggerated numbers, for clarity (at least, for my attempt at it) and don't really know how I could explain this in another way to make the point I'm trying to get across, clearer. The problem is that geometry doesn't support what you're trying to show. To support your theory, you're stating assumptions about what the prop is doing during it's rotation and for several parts of the rotation, those assumptions are just wrong. Also, you're assuming more than just a difference in efficiency in water being backed by air vs. non-air. You're also assuming that the efficiency changes due to how much water there is before you get to air. Except for a very tiny amount of water (i.e., right at the surface) that's not much of a factor. That's because right at the surface, the size of the prop is huge relative to the depth of the prop. So there's a large relative difference between the amount of water above the blade when it's at 180 degrees vs. 0 degrees. But move a small prop down a few feet and that becomes less and less of a factor. Steve |
Angle of prop shaft - theoretical question.
Read Chapman's piloting book.
G "otnmbrd" wrote in message k.net... Steven Shelikoff wrote: It's absolutely obvious that there's unequal thrust. If not, there wouldn't be prop walk. The question is, where does it come from? And I still think the answer is multiple sources all contribute, some more than others on any given boat. I just don't think the effect you're describing here plays as much a part on my boat as it might on others because there's more above the prop than just a column of water and then air. Steve Actually, for the most part we are in agreement. When you go back through my post on this issue, you will always note that I am mentioning a number of factors which will enhance and detract from propwalk (wind, current, hull form, speed, pitch, kort nozzles, etc.) and the fact that we are not always sure what that reaction will be, until we experience it on a particular boat under particular conditions. However, there has to be a "root" cause .... an initial action/reaction which starts the process. Can we say that the directional rotation of the prop (right or left) is by itself the cause? I can't say with certainty that it is or isn't, but, in my experience handling ships with CP props, when the prop is set to zero pitch (judged to be zero due to lack of creep ahead or astern) I generally (note, I'm saying "generally") see no side movement, yet put in even the most minimal pitch and I will experience "walk" (loaded condition, prop down deep with hull overhang above, and light condition, prop close to the surface with hull overhang out of the water .... makes no difference .... both ahead and astern ... which is why I discount hull overhang). What does this lead me to believe. If the rotating prop at zero pitch, generally eg showed me no "walk" and as soon as I added pitch, I got "walk" then I look closely at the effects of a rotating prop and it's pitch for the root cause. Let's now look at prop efficiency (these are MY views based on what I see, read and feel). If we follow the rotation of a RH fixed pitch prop, looking at it from astern starting at top dead center (ooo*) the particular blade is pushing water to the right (and back ...always back, but we will ignore that component for this discussion) at minimal efficiency. this efficiency, however, is increasing as the propellor turns towards 45* and the direction is changing to a increasingly down direction. As the blade reaches 45*, efficiency is close to maximum and from here the direction is more down than to the side. Someplace just prior to 90* the efficiency becomes maximum and as the blade rotates toward 135* the angle of push changes to the left (pulling/pushing the stern to stbd) the blade continues at maximum efficiency through 180* (pushing left) but as it begins it's rotation upwards, that efficiency, slowly begins to drop off and the direction of push begins to angle upward/left until you reach 270*. From this point, efficiency drops off at a marked rate and the blade is pushing up and beginning to push slightly right. As you reach @315* you are close to being back to minimal efficiency and pushing right/up, which continues back to 000*. You can see from this (my visualization) that the force pushing left (pulling/pushing the stern to the right) occurs during that time when the blade is pushing most efficiently, whereas the force pushing to the right (countering that left push) occurs when the blade is pushing with less efficiency...... propwalk. I would love someone who designs props and is far more technically versed in the goings on of a prop beneath the water, to critique this. Also, so there's no misunderstanding, remember, my feeling about props efficiency decrease on it's upward rotation is about the prop pushing water up and into air, losing efficiency. .... and most importantly, these are my views/visualizations .... G yours may vary. otn |
Angle of prop shaft - theoretical question.
Gordon wrote: Read Chapman's piloting book. G Sorry Gordon, don't have one and at this stage of my life, too cheap to buy one. Maybe you could quote the relevant sections from that book? otn |
Angle of prop shaft - theoretical question.
Sheesh, Steve, I just printed your last, out, so I could read it in peace.
G You and I are a couple of "long winded" writers, ain't we? Back in a bit otn |
Angle of prop shaft - theoretical question.
I've always wondered if a submarine, 1,000 feet underwater, experiences
"propwalk" .... I'm betting it doesn't G. At any rate, to save some unnecessary reading, by the numbers on some points at issue: 1. We are not talking about some highly visible, easily measured, variations in efficiency. These are very subtle, but sufficient to cause the condition. 2. Due to it's relative proximity to the surface, I do not feel that a blade is at maximum efficiency at this angle of rotation. My sense/guess/feel, is that isn't doesn't reach maximum until about 045*. Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for awhile. The wash is pushed to the side, breaking the surface during the initial part of it's rotation. However .... 3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop efficiency is in transition during these portions of rotation. In one the efficiency is increasing and the other it is decreasing ... 0% net difference.)the blades are still pushing back, but there is no net effect (arguably) which we can readily apply to "propwalk". Instead ..... 4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the blade is pushing back against a relatively solid column of water, down against a relatively solid column of water and increasingly LEFT against a relatively solid column of water. During this entire quadrant of rotation, the blade is at maximum efficiency...... BUT, from 270-000 the blade is pushing back relatively nearer the surface, up toward the surface, and RIGHT towards and relatively close to the surface, where it can and does break the surface or at least bulge the surface. So..... 5. My sense from this. The blade, in these two all important quadrants, is more efficient between 090-180, than it is between 270-000. The differences if you add in depth of the hub of the prop, may diminish, but for a boat floating on the water surface, the efficiency will never be equal...... VBG ..... propwalk. I disagree that there is no such thing as a "more solid" column of water. I disagree that a propellor is at maximum efficiency at 000* (on a boat floating on the surface of the water). I disagree that by moving the blade a few feet down in the water, you will totally negate the effects or differences in blade efficiency. I think we've probably hashed, thrashed, and rehashed this enough in the NG, Steve. However, feel free to continue via E-mail if you wish. otn |
Angle of prop shaft - theoretical question.
On Wed, 9 Jun 2004 08:30:12 -0700, "Gordon" wrote:
Read Chapman's piloting book. G I have it but it's on the boat, not available right now. I read it but don't remember a discussion about what exactly causes prop walk. What does it say? Steve |
Angle of prop shaft - theoretical question.
On Wed, 09 Jun 2004 17:57:52 GMT, otnmbrd wrote:
I've always wondered if a submarine, 1,000 feet underwater, experiences "propwalk" .... I'm betting it doesn't G. At any rate, to save some unnecessary reading, by the numbers on some points at issue: 1. We are not talking about some highly visible, easily measured, variations in efficiency. These are very subtle, but sufficient to cause the condition. But the force generated is actually pretty high. For instance, due to the full keel of my boat it's much easier to move it fore and aft vs. spin it sideways. Yet when I throw the thing in reverse, all that happens at first is prop walk. By keeping the rudder fully to port I can, by alternating forward and reverse power every few seconds, spin my boat completely around in very little more than it's LOA. I use that "feature" to get into very tight spaces. 2. Due to it's relative proximity to the surface, I do not feel that a blade is at maximum efficiency at this angle of rotation. My sense/guess/feel, is that isn't doesn't reach maximum until about 045*. Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for awhile. The wash is pushed to the side, breaking the surface during the initial part of it's rotation. However .... No, that wash is coming from the upward and to the right (for a right handed prop). For example, say the prop is 2 feet deep. When the blade is at 315 degrees, that's when it's pushing water out 45 degrees to the right, which works out to be 2 feet to the right. By the time the blade goes past 0 degrees, it's only pushing water sideways and then down. The upward wash to the left of the boat is from the 180 to 270 part of the rotation and the upward wash to the right is from the 270 to 0 part of the rotation. 3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop efficiency is in transition during these portions of rotation. In one the efficiency is increasing and the other it is decreasing ... 0% net Separating out all the other effects and only discussing the efficiency of the effect we're talking about here, it's not increasing from 0 to 90. It's at maximum at 0 and stays there until 180. It's decreasing as you go from 180 to 270 and then increasing again as you go from 270 to 360. For a prop that's not right at the surface the decreasing force to the right as it goes from 180 to 270 balances out the increasing force to the left as it goes from 270 to 360 because the balance point (i.e., where the force is at a minimum due to minimum efficiency) is right at or very near 270 degrees. I.e., the size of the water column when it's at 280 degrees is the same as when it's at 350, just in the other direction. 290=340, etc. But for a prop that's very near or at the surface, those forces don't balance out. That's because as the prop gets closer to the surface the balance point (where the force is at a minimum due to minimum efficiency because of the smallest water column before you get to air) move further around the rotation. In this case, the force at 280 /= the force at 350 and you have a net sideways force. difference.)the blades are still pushing back, but there is no net effect (arguably) which we can readily apply to "propwalk". Instead ..... 4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the blade is pushing back against a relatively solid column of water, down against a relatively solid column of water and increasingly LEFT against a relatively solid column of water. During this entire quadrant of rotation, the blade is at maximum efficiency...... BUT, from 270-000 the blade is pushing back relatively nearer the surface, up toward the surface, and RIGHT towards and relatively close to the surface, where it can and does break the surface or at least bulge the surface. So..... And the quadrant from 0 to 90 exactly balances out the quadrant from 90 to 180. And the quadrant from 180 to 270 "almost" exactly balances out the quadrant from 270 to 0. The smaller the ratio of prop size/prop depth, the closer those quadrants (180-270 and 270-0) will balance out. Of course, all this assumes no hull overhang.:) 5. My sense from this. The blade, in these two all important quadrants, is more efficient between 090-180, than it is between 270-000. The Most definitely. But you're ignoring the other two all important quadrants. differences if you add in depth of the hub of the prop, may diminish, but for a boat floating on the water surface, the efficiency will never be equal...... VBG ..... propwalk. I disagree that there is no such thing as a "more solid" column of water. That doesn't really matter since it's not important as long as you realize that the "solidity" of the column of water (if there is such a thing) is the same for the same angle to the right vs. to the left of the prop. I disagree that a propellor is at maximum efficiency at 000* (on a boat floating on the surface of the water). If it's not then there's something else at work then the effect we're talking about here. Because at 0 degrees the blade is pushing against an infinite column of water directly to the right (for a RH prop). Well, maybe not infinite because of the curvature of the earth and because there may be a shoreline before the curvature of the earth comes into play. I disagree that by moving the blade a few feet down in the water, you will totally negate the effects or differences in blade efficiency. Not totally. But it doesn't take going very far down before everything *nearly* balances out left and right due to the effect we're talking about. I think we've probably hashed, thrashed, and rehashed this enough in the NG, Steve. However, feel free to continue via E-mail if you wish. Nah. I'd rather continue it here. Finally a civil boating related thread in the midst of a sea of bickering and political crap. Steve |
Angle of prop shaft - theoretical question.
Steven Shelikoff wrote: But the force generated is actually pretty high. Not really. If it was, it would overcome the ahead or astern component. For instance, due to the full keel of my boat it's much easier to move it fore and aft vs. spin it sideways. Yet when I throw the thing in reverse, all that happens at first is prop walk. Because of propwalk and the fact that your prop is in the stern and everything forward of that follows your prop, much like the "tail of a dog". By keeping the rudder fully to port I can, by alternating forward and reverse power every few seconds, spin my boat completely around in very little more than it's LOA. I use that "feature" to get into very tight spaces. If I remember correctly you have a LH prop which means this is totally understandable and expected. No, that wash is coming from the upward and to the right (for a right handed prop). No, only some of it. G it's amazing what you can see and watch over a period of years. Because of the proximity to the surface, the blade, starting at 000*, pushes water right, then down, BUT, again, because of the proximity to the surface (no real column of water above that directly in line with the pitch of the blade) it pushes water up into the air, thus not having full efficiency. Once again, my feeling is that it doesn't reach full efficiency until @ 045*, and moving the prop further underwater doesn't totally negate this fact/effect, just reduce it. For example, say the prop is 2 feet deep. When the blade is at 315 degrees, that's when it's pushing water out 45 degrees to the right, Maybe, depends on the pitch, but for our discussion, ok. (as long as you realize the direction is also UP toward the surface ..... which works out to be 2 feet to the right. By the time the blade goes past 0 degrees, it's only pushing water sideways and then down. Understood, but I feel the column of water above this is not "solid" ... when you push on something, if it can't go straight, it goes to the side, in this case up and to the surface .... path of least resistance..... less efficiency. The upward wash to the left of the boat is from the 180 to 270 part of the rotation and the upward wash to the right is from the 270 to 0 part of the rotation. The main left component is from 090* to 180* during maximum prop efficiency. From 180* - 270* the "left" component starts at maximum and constantly decreases, as the upward component increases (nothing happens individually, everything happens concurrently). 3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop efficiency is in transition during these portions of rotation. In one the efficiency is increasing and the other it is decreasing ... 0% net Separating out all the other effects and only discussing the efficiency of the effect we're talking about here, it's not increasing from 0 to 90. It's at maximum at 0 and stays there until 180. NO It's decreasing as you go from 180 to 270 and then increasing again as you go from 270 to 360. No. The "left" component is decreasing as you go from 180-270, but the right component (from 270-000) is staying relatively low (compared to the left component from 090 t0 180) because the direction is up toward the surface at the same time as it is to the right, rather than (090-180) down toward "solid" water and to the left. For a prop that's not right at the surface the decreasing force to the right as it goes from 180 to 270 balances out the increasing force to the left as it goes from 270 to 360 because the balance point (i.e., where the force is at a minimum due to minimum efficiency) is right at or very near 270 degrees. I.e., the size of the water column when it's at 280 degrees is the same as when it's at 350, just in the other direction. 290=340, etc. Rather than going into detail .... NO. You cannot compare a force from 180-270, to a force from 270-360. you MUST compare a force from 180-270, to 000-090, and a force from 270-360, to 090-180. The "length" of the water column from 090-180, is far greater than the "length" of the water column from 270-360. The body (propellor pitch) is constantly changing direction of "push". But for a prop that's very near or at the surface, those forces don't balance out. That's because as the prop gets closer to the surface the balance point (where the force is at a minimum due to minimum efficiency because of the smallest water column before you get to air) move further around the rotation. In this case, the force at 280 /= the force at 350 and you have a net sideways force. True, but that sideways force does NOT equal the force at 100-170, because the column of water above it is less than the column of water below 100-170. difference.)the blades are still pushing back, but there is no net effect (arguably) which we can readily apply to "propwalk". Instead ..... 4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the blade is pushing back against a relatively solid column of water, down against a relatively solid column of water and increasingly LEFT against a relatively solid column of water. During this entire quadrant of rotation, the blade is at maximum efficiency...... BUT, from 270-000 the blade is pushing back relatively nearer the surface, up toward the surface, and RIGHT towards and relatively close to the surface, where it can and does break the surface or at least bulge the surface. So..... And the quadrant from 0 to 90 exactly balances out the quadrant from 90 to 180. And the quadrant from 180 to 270 "almost" exactly balances out the quadrant from 270 to 0. The smaller the ratio of prop size/prop depth, the closer those quadrants (180-270 and 270-0) will balance out. NO,NO,NO You cannot compare 000-090, to 090-180 or 180-270, to 270-360. You Must compare opposites !! 000-090 and 180-270 or 090-180 and 270-360. Of course, all this assumes no hull overhang.:) It may be a factor, but not a cause EG as explained from my point of view earlier. 5. My sense from this. The blade, in these two all important quadrants, is more efficient between 090-180, than it is between 270-000. The Most definitely. But you're ignoring the other two all important quadrants. Nope, I'm narrowing down the important quadrants of push, germane to the discussion. differences if you add in depth of the hub of the prop, may diminish, but for a boat floating on the water surface, the efficiency will never be equal...... VBG ..... propwalk. I disagree that there is no such thing as a "more solid" column of water. That doesn't really matter since it's not important as long as you realize that the "solidity" of the column of water (if there is such a thing) is the same for the same angle to the right vs. to the left of the prop. No. The column of water is important to the "net" right and left forces. I disagree that a propellor is at maximum efficiency at 000* (on a boat floating on the surface of the water). If it's not then there's something else at work then the effect we're talking about here. Because at 0 degrees the blade is pushing against an infinite column of water directly to the right (for a RH prop). No. This would only be true if the column of water was contained within a pipe. It's not, so the water above the column can escape above the surface, reducing efficiency. Well, maybe not infinite because of the curvature of the earth and because there may be a shoreline before the curvature of the earth comes into play. I disagree that by moving the blade a few feet down in the water, you will totally negate the effects or differences in blade efficiency. Not totally. But it doesn't take going very far down before everything *nearly* balances out left and right due to the effect we're talking about. No, if that were the case, then your boat wouldn't experience propwalk. I think we've probably hashed, thrashed, and rehashed this enough in the NG, Steve. However, feel free to continue via E-mail if you wish. Nah. I'd rather continue it here. Finally a civil boating related thread in the midst of a sea of bickering and political crap. Steve LOL Well, it seems no one is really complaining, so have at it. otn |
Angle of prop shaft - theoretical question.
I have it but it's on the boat, not available right now. I read it but
don't remember a discussion about what exactly causes prop walk. What does it say? Steve That unequal blade thrust is the result of the difference in effective pitch between the descending and ascending blade when a propeller is rotated on an angle, (90 degrees from the angle at which the shaft exits the shaft log) though a horizontally advancing medium (water under the boat). (Page 143 in my 1985 edition, may be different in later versions) |
Angle of prop shaft - theoretical question.
Gould 0738 wrote: I have it but it's on the boat, not available right now. I read it but don't remember a discussion about what exactly causes prop walk. What does it say? Steve That unequal blade thrust is the result of the difference in effective pitch between the descending and ascending blade when a propeller is rotated on an angle, (90 degrees from the angle at which the shaft exits the shaft log) though a horizontally advancing medium (water under the boat). (Page 143 in my 1985 edition, may be different in later versions) Thanks Chuck. (Damn ... G more people are reading this thread than I thought). hmmmmm ... If I'm reading this correctly, Chapman is saying the shaft must be angled "down" from the horizontal flow of water, and this is the "root" cause? UHOH. I disagree that an angle down or up for that matter, makes a difference to the root cause. I do agree that it MAY increase propwalk. I also agree that the main cause appears between the upward and downward ascent of the blade, but feel that the main difference occurs between 090-180 and 270-360 ( eg I believe 180-270 is marginally more efficient than 000-090). Thanks again otn |
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