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Thrust vectoring
"JAXAshby" wrote in message
... an object placed on the "suction side" of a propeller will there is no force in natural called "suction". Everyone with even one semester in physics knows that. how many semesters did you say you had, jeffies? There may not be a force called "suction" but I did not use the word in that context. "Suction Side" is a commonly used engineering term, used in the context of both propellers and compressors. If you had the even most basic knowledge of refrigeration (you claimed family in the business) you would know that. You've just demonstrated that you're a complete fraud, jaxie. But we all knew that. |
Thrust vectoring
Subject: Thrust vectoring
From: (JAXAshby) Jaxass, your comment has nothing to do with what he's been discussing. It is, however, very basic knowledge, which I'm surprised you know...... it can also be applied to moving ahead, but that's probably too much "input" for you for one day. Shen schlackoff, you were told to never post in English. Adults are posting and they use big words like "docking" and such. Huh? What does that have to do with what he is saying .... "back up" is not being discussed. Shen careful. you *must* be moving through the water for a rudder to be effective when trying to back up. Thanks. One of the things I tend to talk about is the potential advantage of using rudders when twisting (swivel) as it tends to help when you work into how you need to set your rudders to "walk". o |
Thrust vectoring
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Thrust vectoring
t can
also be applied to moving ahead, no, it can not. water *pushed* over a rudder can cause a rudder to turn a boat, while water "pulled" over a rudder can not. |
Thrust vectoring
wayne, you are out of your league.
*push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. |
Thrust vectoring
Thanks ....comments interspersed:
Rod McInnis wrote: With this in mind, for those with twin screw boats, if I told you that rudders were important tools of boat handling, but not to be considered for steering, rather for "thrust vectoring", when maneuvering around a dock, etc., when kicking an engine ahead, both positive and negative ...... would you understand what I was saying? No, I would think that most people would be more confused. Why? To begin with, I would start by pointing out that there is a big difference between how an inboard reacts and how I/Os or outboards react. There is also a big difference between what I call "large rudder" and "small rudder" boats. Sorry, I should have made it more specific that I was just talking about "inboards" not IO's or outboards. On inboards, there can be "large rudder" as well as "small rudder" boats (generally they tend toward "small" but it's "not written in stone"). It's also interesting that few talk about type of rudder when talking about it's "size" (can also be read as "power"). A typical sail boat has a "large rudder" which becomes effective, in either forward or reverse, as soon as the boat is making way. A typical power boat has a "small rudder" which either needs a significant amount of boat speed or to have the prop pushing water past it. These rudders are generally ineffective in reverse. see above The basic concept of twin engine inboards is that you consider the rudder to only be a factor when the engine is in forward. Ignore it for the engine in reverse. Interesting comment that I think a few have made. To check on myself, I went back to my original post to see if I'd indicated anywhere that I was applying this "thrust vectoring" to astern "kicks" ... I wasn't, and in fact specifically stated "when kicking an engine ahead", so I don't know where your last two sentences are coming from. Hmmmm I see something ... "both positive and negative", refers to rudder angle when doing, say, a twist or walk. otn |
Thrust vectoring
On Thu, 25 Mar 2004 02:07:31 GMT, otnmbrd
wrote: For years, on occasion, I've have been involved with teaching someone boat handling, using single and/or twin screw inboards.....if I told you that rudders were important tools of boat handling, but not to be considered for steering, rather for "thrust vectoring", when maneuvering around a dock, etc., when kicking an engine ahead, both positive and negative ...... would you understand what I was saying? otn Probably not, judging by one or two of the responses here. Perhaps it would be easier for you to demonstrate the effect of sucking a fluid past a board placed in the forward stream line. Take a hand vacuum (I used a Bissett) and close to the suction nozzle, place a sheet of paper parallel to the air flow into the nozzle. Place the edge quite close to the nozzle's side. You will see the paper move sidewards towards the airflow into the vacuum if the flow speed is unequal on each side of the paper. Bernouilli of course. The effect is quite small, but readily visible. Faster flow leads to lower pressure, of course. Extending the demo to the rudder placed behind the propellor spinning in reverse to show the small side force on the rudder should then be easier to communicate, I'd think. Brian Whatcott |
Thrust vectoring
You're absolutely wrong about this jaxie. Feynman would think you're a complete
fool for invoking his "sprinkler paradox" in this case. The boat is not turned directly by the propeller, it is turned because a water flow is pressing against the rudder. "Push" and "pull" are irrelevant, and the water flow could even come from a current, or the wash from another boat. For a variety of reasons, the affect is far more powerful in foreword, but it is still there in reverse. USSailing, and Boat/US both describe this on their websites. http://www.videos.sailingcourse.com/...pring_line.htm http://www.boatus.com/seaworthy/swlines.asp And the Coast Guard http://www.uscg.mil/hq/g-o/cgaux/Pub...crew/ch10d.pdf "JAXAshby" wrote in message ... wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. |
Thrust vectoring
Thanks. G Wasn't really interested in how you reacted to others
responses, only in how YOU reacted. As for the rest, you seem to have also grabbed onto the "reverse" aspect, which is NOT what I was discussing at all. I repeat, "When kicking an engine ahead" Thrust vectoring does not apply when going astern, unless G you have Flanking Rudders.... which is another story entirely. otn Brian Whatcott wrote: Probably not, judging by one or two of the responses here. Perhaps it would be easier for you to demonstrate the effect of sucking a fluid past a board placed in the forward stream line. Take a hand vacuum (I used a Bissett) and close to the suction nozzle, place a sheet of paper parallel to the air flow into the nozzle. Place the edge quite close to the nozzle's side. You will see the paper move sidewards towards the airflow into the vacuum if the flow speed is unequal on each side of the paper. Bernouilli of course. The effect is quite small, but readily visible. Faster flow leads to lower pressure, of course. Extending the demo to the rudder placed behind the propellor spinning in reverse to show the small side force on the rudder should then be easier to communicate, I'd think. Brian Whatcott |
Thrust vectoring
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Thrust vectoring
BG Jeff, you still wasting time on that imbecile? The odds on him
knowing anything about real world boat handling fall into the "minuscule to none" category. otn Jeff Morris wrote: You're absolutely wrong about this jaxie. Feynman would think you're a complete fool for invoking his "sprinkler paradox" in this case. The boat is not turned directly by the propeller, it is turned because a water flow is pressing against the rudder. "Push" and "pull" are irrelevant, and the water flow could even come from a current, or the wash from another boat. For a variety of reasons, the affect is far more powerful in foreword, but it is still there in reverse. USSailing, and Boat/US both describe this on their websites. http://www.videos.sailingcourse.com/...pring_line.htm http://www.boatus.com/seaworthy/swlines.asp And the Coast Guard http://www.uscg.mil/hq/g-o/cgaux/Pub...crew/ch10d.pdf "JAXAshby" wrote in message ... wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. |
Thrust vectoring
Jax,
I may not be the brightest bulb in the box, and the last physics course I took was well over twenty years ago (I still break out in a cold sweat when I hear the words "Virial Theorem"), so if you can explain it to me I'd appreciate it. How does the rudder (or the rudder stock & pintles through which the force is applied to the hull) know whether the water flowing past it is being pushed or pulled? Now if you want to argue that the water flow across the rudder is so small that the effect of the rudder is overpowered by the prop walk, that I can buy. But to say that the water flowing past the rudder (and being deflected by it has no effect because it's motion was started by some mysterious sucking force makes no sense. Because the water column being pushed aft when in forward is of constant diameter (at least in gross terms across the distances we are toaling about), the velocity in the column is for practical purposes constant. resulting in a high velocity stream being deflected by the rudder and a large resultant lateral force. In reverse, however, there is no such water column aft of the prop. The water is being sucked in from all directions and thus it's velocity falls off as the square of the distance from the prop (again, we are taling in gross terms here). This results in a comparatively slow, but non-zero velocity as it passes the rudder. Movement of the water (regardless of it's cause) past the rudder, and its' being deflected by it causes a lateral force. JAXAshby wrote: wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. -- Dan Best - (707) 431-1662, Healdsburg, CA 95448 B-2/75 1977-1979 Tayana 37 #192, "Tricia Jean" http://rangerbest.home.comcast.net/TriciaJean.JPG |
Thrust vectoring
otnmbrd wrote...
Thanks. In answer to your question, yes, but "torque steer" would not work for me, as I would be apt to apply that to "prop walk". Yep, much more appropriate; but that leaves me with no opinion. Hope you derive something useful here. Have you asked your students if they have any analogies or concepts to offer when you see the lightbulb of understanding go off in their heads that first time? |
Thrust trolling
Yea, but he always gets an "A" in the trolling category.
-- Keith __ What did you forget? "otnmbrd" wrote in message .net... BG Jeff, you still wasting time on that imbecile? The odds on him knowing anything about real world boat handling fall into the "minuscule to none" category. otn |
Thrust vectoring
I'm always learning.
Actually, most people I get into these discussions with (I'm not an instructor in any sense of a formal course, though I've taught more than one "newbie") are people with experience that have a problem or need to figure out a reason for what they are seeing/experiencing. Frequently the discussion revolves around "inboard" turning and "outboard" turning props (no one here picked up on that difference, which I find interesting, yet many were concerned with using rudders when going astern, which I was not discussing). I'm still hoping to see some other responses to my responses, since how others see things, can be as educational as how I see things BG. otn Frank Maier wrote: otnmbrd wrote... Thanks. In answer to your question, yes, but "torque steer" would not work for me, as I would be apt to apply that to "prop walk". Yep, much more appropriate; but that leaves me with no opinion. Hope you derive something useful here. Have you asked your students if they have any analogies or concepts to offer when you see the lightbulb of understanding go off in their heads that first time? |
Thrust vectoring
Boat static in slip. Put in forward gear. move rudder port and
starboard, stern moves accordingly. Put in reverse gear, again same result. 'splain dis to me,Luci? On 25 Mar 2004 21:57:40 GMT, (JAXAshby) wrote: t can also be applied to moving ahead, no, it can not. water *pushed* over a rudder can cause a rudder to turn a boat, while water "pulled" over a rudder can not. |
Thrust vectoring
geesh, jeffies, you CLAIM to have a degree in physics, yet it is plainly
obvious you don't even begin to understand what is going on. You're absolutely wrong about this jaxie. Feynman would think you're a complete fool for invoking his "sprinkler paradox" in this case. The boat is not turned directly by the propeller, it is turned because a water flow is pressing against the rudder. "Push" and "pull" are irrelevant, and the water flow could even come from a current, or the wash from another boat. For a variety of reasons, the affect is far more powerful in foreword, but it is still there in reverse. USSailing, and Boat/US both describe this on their websites. http://www.videos.sailingcourse.com/...pring_line.htm http://www.boatus.com/seaworthy/swlines.asp And the Coast Guard http://www.uscg.mil/hq/g-o/cgaux/Pub...crew/ch10d.pdf "JAXAshby" wrote in message ... wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. |
Thrust vectoring
over the nee, you be a stew pid as jeffies.
go ahead. TRY to back that thing up. BG Jeff, you still wasting time on that imbecile? The odds on him knowing anything about real world boat handling fall into the "minuscule to none" category. otn Jeff Morris wrote: You're absolutely wrong about this jaxie. Feynman would think you're a complete fool for invoking his "sprinkler paradox" in this case. The boat is not turned directly by the propeller, it is turned because a water flow is pressing against the rudder. "Push" and "pull" are irrelevant, and the water flow could even come from a current, or the wash from another boat. For a variety of reasons, the affect is far more powerful in foreword, but it is still there in reverse. USSailing, and Boat/US both describe this on their websites. http://www.videos.sailingcourse.com/...pring_line.htm http://www.boatus.com/seaworthy/swlines.asp And the Coast Guard http://www.uscg.mil/hq/g-o/cgaux/Pub...crew/ch10d.pdf "JAXAshby" wrote in message ... wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. It is a fact of physics that you can NOT control using rudder by *pulling* water over it. you MUST push. =================== Absolutely not true. If there is water moving past the rudder, regardless of direction or cause, it can be used to create a directed thrust simply by angling the rudder away from the flow direction. The confusion arises because the prop in forward pushes a large flow across the rudder, whereas the prop in reverse pulls only a relatively small amount of water across the rudder. Small, but not zero. You don't need a degree in physics to understand this, just a little common sense. Richard Feynman would no doubt find the discussion amusing however. |
Thrust vectoring
if you can explain it to me
I'd appreciate it. Dan, will do. it is getting late tonight. will do tomorrow. |
Thrust vectoring
shlackoff, you are lost to this universe. metaphor does not count for squat.
btw schlackoff, did you know that the climb rate/service ceiling on a Cessna 336/337 was greater single engine on the aft engine than the forward engine? of course you knew that. wayne, you are out of your league. *push* is required under the laws of physics. If you can't see that, just take Feynman's word for it. I have to agree with Jax on this one. Why else do you think all airplanes have the propellor in the back. Steve |
Thrust vectoring
brian!!! knock it off!!
wrote: For years, on occasion, I've have been involved with teaching someone boat handling, using single and/or twin screw inboards.....if I told you that rudders were important tools of boat handling, but not to be considered for steering, rather for "thrust vectoring", when maneuvering around a dock, etc., when kicking an engine ahead, both positive and negative ...... would you understand what I was saying? otn Probably not, judging by one or two of the responses here. Perhaps it would be easier for you to demonstrate the effect of sucking a fluid past a board placed in the forward stream line. Take a hand vacuum (I used a Bissett) and close to the suction nozzle, place a sheet of paper parallel to the air flow into the nozzle. Place the edge quite close to the nozzle's side. You will see the paper move sidewards towards the airflow into the vacuum if the flow speed is unequal on each side of the paper. Bernouilli of course. The effect is quite small, but readily visible. Faster flow leads to lower pressure, of course. Extending the demo to the rudder placed behind the propellor spinning in reverse to show the small side force on the rudder should then be easier to communicate, I'd think. Brian Whatcott |
Thrust vectoring
over the nee, brian was sucking smoke.
Thanks. G Wasn't really interested in how you reacted to others responses, only in how YOU reacted. As for the rest, you seem to have also grabbed onto the "reverse" aspect, which is NOT what I was discussing at all. I repeat, "When kicking an engine ahead" Thrust vectoring does not apply when going astern, unless G you have Flanking Rudders.... which is another story entirely. otn Brian Whatcott wrote: Probably not, judging by one or two of the responses here. Perhaps it would be easier for you to demonstrate the effect of sucking a fluid past a board placed in the forward stream line. Take a hand vacuum (I used a Bissett) and close to the suction nozzle, place a sheet of paper parallel to the air flow into the nozzle. Place the edge quite close to the nozzle's side. You will see the paper move sidewards towards the airflow into the vacuum if the flow speed is unequal on each side of the paper. Bernouilli of course. The effect is quite small, but readily visible. Faster flow leads to lower pressure, of course. Extending the demo to the rudder placed behind the propellor spinning in reverse to show the small side force on the rudder should then be easier to communicate, I'd think. Brian Whatcott |
Thrust vectoring
manana
Boat static in slip. Put in forward gear. move rudder port and starboard, stern moves accordingly. Put in reverse gear, again same result. 'splain dis to me,Luci? On 25 Mar 2004 21:57:40 GMT, (JAXAshby) wrote: t can also be applied to moving ahead, no, it can not. water *pushed* over a rudder can cause a rudder to turn a boat, while water "pulled" over a rudder can not. |
Thrust vectoring
Gould 0738 wrote in message ... If the wind has you pinned against the dock, you won't realize enough kick to get the stern free. Take a spring from bow to shore. Put lots of fenders up front. Apply full helm towards the shore. Slowly add power. The stern will walk away from the shore until you can back off from the quay in comfort. As Jere said earlier, 'Kick Ass'. JimB |
Thrust vectoring
Wayne.B wrote in message ... It seems to be a consensus among experienced twin screw captains that leaving the rudders amidship is good practice for most maneuvers. There are exceptions of course but having the rudders amidship leads to more predictable response in my experience. Comments? True. You only need rudder if you're trying to achieve a lateral shift in position. This can be nicely illustrated in zero wind and current if you position close to a buoy, apply full rudder, then balance one engine astern and the other forward to give zero yaw rate and boat speed, and the boat will slowly (deep keel) or quickly (shallow keel) move laterally away from the direction you've applied rudder. The 'Kick Ass' effect, with rotation cancelled by differential power! JimB |
Thrust vectoring
JAXAshby wrote in message ... water *pushed* over a rudder can cause a rudder to turn a boat, while water "pulled" over a rudder can not. See my other post. If conditions are such that water is ingested from one direction, and exits in another, there is a change in lateral momentum. That's a lateral force, usually behind the C of G. That's a yaw. That's turning a boat. I think you're making the assumption (without stating it) that, engine in reverse, water will always be ingested from dead astern; ie, the rudder is small and of such a distance from the prop that it has no effect on the direction of ingestion. JimB |
Thrust vectoring
JAXAshby wrote in message ... it is caused by asymetrical thrust of an angled prop shaft. When backing up the blade coming up to the hull has a much greater "angle of attack" than the blade going down from the hull. Thus more thrust on one side than the other. Interesting theory. Can't think where you got that from. Better check the trig though. 1 ft/sec astern, typical prop tip speed about 50ft/sec, lets say one 1 degree. Differential effect of 20deg shaft angle, 1-cos20 = 0.06deg. Lets say 1/20 degree. Compared to a typical prop pitch of 20deg or so that means that 1/400 of your thrust (800 lb? reduced to 2lb) is being exercised over a moment arm of 16 inches to turn your vessel. OK, that's coarse maths from the back of an envelope, with a margin of error of maybe an order. But I still don't think that even 30 ft/lb is going to turn your vessel. That's what I use to tighten my nuts. And it reduces to zero when you have zero stern way. So your theory can only true when the boat is actually travelling in reverse (your definition of backing up?). It is utterly trivial compared to the paddle wheel effect. You can test this statement by selecting reverse while moving slowly forward. The vessel won't kick first one way, then the other. It'll go the paddle wheel way. It is also caused to some extent by the contrainment of the prop wash against the hull on the up side blade, compared to no constrainment on the down side blade. Don't understand that. All forces are the result of changes in momentum. The wash spirals away from the prop. Read on. On the upper side, the lateral speed of the spiral is slowed by friction against the ship's hull. The lower side much less so. So the lower lateral momentum added is greater than the upper. The result is a force as if paddled by the lower blades. You could also think of it as the frictional force exerted on the hull by slowing the lateral speed of the upper part of the spiral. Whichever, it's the opposite direction to your theory, which, in turn, doesn't tie in with my experience. JimB |
Thrust vectoring
JAXAshby wrote in message ... careful. you *must* be moving through the water for a rudder to be effective when trying to back up. Not quite true. When 'trying to back up' (attempting rearward motion) you will often use bursts of ahead over the rudder to alter the pointing of a boat, whether you're static or actually moving astern. Very large diameter props, well sheltered from prop walk, with a large rudder close to the prop (as on single prop tugs and some fishing vessels), will, in astern, cause rudder to alter the direction from which water is ingested into the prop. If the water is ingested from one direction, and exits in another, that deflection causes a change in lateral momentum - creating a force which in turn causes a yaw. Of course, single screw tugs are not so common now, and this thread was about twin screw vessels. But this used to be a useful manoeuvring trick. Note that the effect is in the same direction as the burst of ahead; ie, full left rudder yaws the vessel to the left in either fwd or reverse (no 'suction' effect). And note that I was talking about props sheltered from prop walk - either through their depth, the use of shields or a lateral offset. JimB |
Thrust vectoring
Take a spring from bow to shore. Put lots of fenders up
front. Apply full helm towards the shore. Slowly add power. The stern will walk away from the shore until you can back off from the quay in comfort. Of course, but what does that have to do with an observation that with the helm amidships you won't achieve enough side thrust from the prop to kick the stern away from the dock? In a *moderate* wind, the spring line is usually unneccessary. Removing from the question originally posed to the board any variable that says the rudder cannot be used allows the rudder to be turned toward the dock, and a brief application of forward will indeed "kick out" the stern. (Just have to make sure that you don't whack the stem in the process) |
Thrust vectoring
See my other post. If conditions are such that water is
ingested from one direction, and exits in another, there is a change in lateral momentum. That's a lateral force, usually behind the C of G. That's a yaw. That's turning a boat. I think you're making the assumption (without stating it) that, engine in reverse, water will always be ingested from dead astern; ie, the rudder is small and of such a distance from the prop that it has no effect on the direction of ingestion. JimB JimB: Are you attempting to make the case that a rudder controls the direction of a power driven vessel primarily by changing the side of the prop on which water is "ingested"? You've got my confusulated if you are...... Please elaborate. Thanks |
Thrust vectoring
Boat static in slip. Put in forward gear. move rudder port and
starboard, stern moves accordingly. Put in reverse gear, again same result. 'splain dis to me,Luci? If you have a large enough rudder and you can swing it fast enough, you will see some movement of the stern when static in the slip. In fact, the movement will be exactly the same whether forward or reverse is selected....................as long as the engine isn't running. :-) |
Thrust vectoring
It seems to be a consensus among experienced twin screw
captains that leaving the rudders amidship is good practice for most maneuvers. There are exceptions of course but having the rudders amidship leads to more predictable response in my experience. Much depends on the size of the rudders. Twin screw boats tend to have much smaller rudders than single screw for a variety of reasons. Some of these reasons have to do with rudder support when the rudder is not directly aft of the keel, others involve the reduction of drag to achieve greater speed, and still others calculate the combined area of both rudders. In the final analysis, on most twin screw vessels the rudder has a very marginal ability to change the direction of the boat through the water compared to the application of unequal thrust from the engines. I'll be out on a speedy twin screw boat later this morning to collect some data and get some photos. I fully expect that at crusing speed or better and with equal thrust from the engines the turning circle of this 42 footer will be close to 1/8 mile in diameter. And that won't be particularly unusual. Obviously not much rudder in play. When close quarter maneuevering we consider the wind and current and compensate for any significant forces. Seems to make sense that one should respond with the most efficient and significant force available....whether that's unequal thrust, a big rudder, or an oar. |
Thrust vectoring
Gould 0738 wrote in message ... Take a spring from bow to shore. Put lots of fenders up front. Apply full helm towards the shore. Slowly add power. The stern will walk away from the shore until you can back off from the quay in comfort. Of course, but what does that have to do with an observation that with the helm amidships you won't achieve enough side thrust from the prop to kick the stern away from the dock? Sorry! Missed the point that the helm was amidships, and therefore agree your point. I have walked a stern out in reverse with the bow tethered to the shore though. In a *moderate* wind, the spring line is usually unneccessary. Removing from the question originally posed to the board any variable that says the rudder cannot be used allows the rudder to be turned toward the dock, and a brief application of forward will indeed "kick out" the stern. (Just have to make sure that you don't whack the stem in the process) JimB |
Thrust vectoring
Single screw .... With full left rudder, kick ahead, bow will swing to
port .... due mainly to rudder and some prop walk (right hand prop). With rudder hard left, kick astern, stern will swing to port .... all prop walk, has nothing to due with rudder placement(rudder could be hard left or hard right, initial affect will be the same). The only way to shield a prop from prop walk, that I know of is a Kort Nozzle or similar shield. otn JimB wrote: Note that the effect is in the same direction as the burst of ahead; ie, full left rudder yaws the vessel to the left in either fwd or reverse (no 'suction' effect). And note that I was talking about props sheltered from prop walk - either through their depth, the use of shields or a lateral offset. JimB |
Thrust vectoring
Gould 0738 wrote in message ... See my other post. If conditions are such that water is ingested from one direction, and exits in another, there is a change in lateral momentum. That's a lateral force, usually behind the C of G. That's a yaw. That's turning a boat. I think you're making the assumption (without stating it) that, engine in reverse, water will always be ingested from dead astern; ie, the rudder is small and of such a distance from the prop that it has no effect on the direction of ingestion. JimB JimB: Are you attempting to make the case that a rudder controls the direction of a power driven vessel primarily by changing the side of the prop on which water is "ingested"? No. I'm starting from the point that the rudder changes the lateral momentum of water traveling past the stern, and this creates a lateral force. It's obvious that, engine ahead, the water comes from straight ahead and can then be deflected laterally by the rudder to create a lateral force. With the engine in astern, I was saying that *if* the water entering the prop is constrained to come from one side (big rudder etc) then it's momentum is changed from having a lateral component to having no lateral component, therefore a similar force is developed. However, you've made me reflect on that. Yes, there will be a force on the rudder, since it is deflecting the water (and you can feel this on the tiller). And I've witnessed (and had explained to me) the effect on trawlers and tugs. But I can't square this with the idea that, in astern with no motion, the water ultimately has started with zero momentum (if we're standing still) and is accelerated to have solely a fore and aft element. That implies no net lateral force. So something is missing here! Perhaps there's a subtle change in prop walk in these types of boats caused by the rudder angle in astern Thanks for making me think . . . JimB |
Thrust vectoring
jim, asym thrust has been a known factor since before WW2.
it is caused by asymetrical thrust of an angled prop shaft. When backing up the blade coming up to the hull has a much greater "angle of attack" than the blade going down from the hull. Thus more thrust on one side than the other. Interesting theory. Can't think where you got that from. Better check the trig though. 1 ft/sec astern, typical prop tip speed about 50ft/sec, lets say one 1 degree. Differential effect of 20deg shaft angle, 1-cos20 = 0.06deg. Lets say 1/20 degree. Compared to a typical prop pitch of 20deg or so that means that 1/400 of your thrust (800 lb? reduced to 2lb) is being exercised over a moment arm of 16 inches to turn your vessel. OK, that's coarse maths from the back of an envelope, with a margin of error of maybe an order. But I still don't think that even 30 ft/lb is going to turn your vessel. That's what I use to tighten my nuts. And it reduces to zero when you have zero stern way. So your theory can only true when the boat is actually travelling in reverse (your definition of backing up?). It is utterly trivial compared to the paddle wheel effect. You can test this statement by selecting reverse while moving slowly forward. The vessel won't kick first one way, then the other. It'll go the paddle wheel way. It is also caused to some extent by the contrainment of the prop wash against the hull on the up side blade, compared to no constrainment on the down side blade. Don't understand that. All forces are the result of changes in momentum. The wash spirals away from the prop. Read on. On the upper side, the lateral speed of the spiral is slowed by friction against the ship's hull. The lower side much less so. So the lower lateral momentum added is greater than the upper. The result is a force as if paddled by the lower blades. You could also think of it as the frictional force exerted on the hull by slowing the lateral speed of the upper part of the spiral. Whichever, it's the opposite direction to your theory, which, in turn, doesn't tie in with my experience. JimB |
Thrust vectoring
nope. *pull* does NOT affect the rudder, only *push* does.
careful. you *must* be moving through the water for a rudder to be effective when trying to back up. Not quite true. When 'trying to back up' (attempting rearward motion) you will often use bursts of ahead over the rudder to alter the pointing of a boat, whether you're static or actually moving astern. Very large diameter props, well sheltered from prop walk, with a large rudder close to the prop (as on single prop tugs and some fishing vessels), will, in astern, cause rudder to alter the direction from which water is ingested into the prop. If the water is ingested from one direction, and exits in another, that deflection causes a change in lateral momentum - creating a force which in turn causes a yaw. Of course, single screw tugs are not so common now, and this thread was about twin screw vessels. But this used to be a useful manoeuvring trick. Note that the effect is in the same direction as the burst of ahead; ie, full left rudder yaws the vessel to the left in either fwd or reverse (no 'suction' effect). And note that I was talking about props sheltered from prop walk - either through their depth, the use of shields or a lateral offset. JimB |
Thrust vectoring
Single screw .... With full left rudder, kick ahead, bow will swing to
port .... due mainly to rudder and some prop walk (right hand prop). With rudder hard left, kick astern, stern will swing to port .... all prop walk, has nothing to due with rudder placement(rudder could be hard left or hard right, initial affect will be the same). true. otn JimB wrote: Note that the effect is in the same direction as the burst of ahead; ie, full left rudder yaws the vessel to the left in either fwd or reverse (no 'suction' effect). And note that I was talking about props sheltered from prop walk - either through their depth, the use of shields or a lateral offset. JimB |
Thrust vectoring
ject: Thrust vectoring
From: "Rod McInnis" Date: 03/25/2004 11:53 Pacific Standard Time Message-id: With this in mind, for those with twin screw boats, if I told you that rudders were important tools of boat handling, but not to be considered for steering, rather for "thrust vectoring", when maneuvering around a dock, etc., when kicking an engine ahead, both positive and negative ...... would you understand what I was saying? No, I would think that most people would be more confused. I'd be interested to know why you think this, as I don't consider the rest of what you wrote to be applicable to this statement. Shen |
Thrust vectoring
Subject: Thrust vectoring
From: (JAXAshby) Date: 03/25/2004 13:57 Pacific Standard Time Message-id: t can also be applied to moving ahead, no, it can not. water *pushed* over a rudder can cause a rudder to turn a boat, while water "pulled" over a rudder can not. Jax, quit worrying about push and pull. Take a sailboat and start backing it with the engine.... get up the right amount of speed and shut down the engine. You can now steer that boat because of the forces of the water PASSING over that rudder, exert a steering force. Hoist sails and trim them to start getting headway (you are not pushing water over the rudder you are passing water over the rudder) and once you have sufficient speed, this water passing over the rudder will exert a force to steer the vessel.... hence it applies astern or ahead. Too simple for you to understand? ...... oh well...... Shen |
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