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Angle of prop shaft - theoretical question.
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Angle of prop shaft - theoretical question.
On 4 Jun 2004 11:32:26 -0700, (basskisser) wrote:
(Steven Shelikoff) wrote in message ... On 4 Jun 2004 04:17:35 -0700, (basskisser) wrote: (Steven Shelikoff) wrote in message ... On Thu, 3 Jun 2004 12:06:33 -0700, "Gordon" wrote: "Gary Warner" wrote in message ... We have an inboard and, of course, the prop shaft exits the hull at an angle. I just drove by a similar (but different brand) of boat and it appears it's prop shaft comes out at less of an angle. My first though it that a angle that gives a more push forward and less push "up" would be more efficient. Is that about right?? Yes but the real gain in effiency comes from the fact that the pitch on the blade on one side now more closely equals the pitch on the opposite side. This also means it takes less rudder to maintain a straight line and therefore less drag. You'll also get less propwalk when reversing the boat with the lesser shaft angle from horizontal. Why? My theory is that , all else being equal, a boat with a greater shaft angle will have less thrust in the horizontal direction to counteract the sideways force from the prop paddle-wheeling and at the same time will increase the sideways "spinning" component of the rotational force from the prop. Think of it this way: If you have the shaft completely vertical and spin the prop, the main motive force will be straight up and not push the boat anywhere. You've minimized the "paddle-wheel" sideways effect but that total force is very small. However, you've also maximized the torque rotational force that will try and spin the boat in the same way as putting a single mixer blade in a bowl of batter will try and spin the bowl and that force is huge compared to the paddle-wheel force. As you lessen the angle of the shaft, you start to increase the sideways paddle-wheel force but again, that's a very small force. The direction of the "mixing bowl" rotational force changes from trying to spin the boat to trying to list the boat. Also, you get more motive force in the proper direction to allow you to counteract both the "paddle-wheel" force and the "mixing bowl" force. When the shaft is completely horizontal, the rotational force is all in the direction that creates list as you apply power and none is trying to rotate the boat. I believe this has the greatest effect on reducing prop walk since the rotational force can be very large. While a small change in shaft angle has a small effect on the ratio of the vector components of that force, the total force is so great that a small change in the ratio of the vectors can have a large effect on the handling of the boat. Also, you have the maximum amount of thrust in the proper direction to help handle the remaining sideways paddle-wheel force, which is the least of all of them. That's my theory and I'm sticking with it. lol Steve I don't agree. If the direction of the prop and shaft, relative to the centerline of the boat never changes, and the cause of the "prop walk" But it is changing. That's the variable we're changing, the angle of the shaft. I.e., as the shaft angle gets closer and closer to coming straight out of the boat with no downward angle, the prop walk gets less and less. Or stated another way, the greater the angle of the shaft the greater the prop walk. And that's due to the torque of the shaft, which is applied more and more as a force to rotate the boat as the downward angle of the shaft is increased. When the shaft is straight back with no downward angle, the torque force from the shaft causes the boat to list but doesn't rotate it in the water. is from the rotational force, it would make no difference what the angle of the prop is relative to the horizon. It could be anywhere from horizontal, through 90 degrees to horizontal, and the rotational force would remain the same. The rotational force from the prop remains the same. But what it does to the boat depends on the angle of the shaft relative to the boat. If the shaft is vertical, it will try and rotate the boat in the horizontal plane, i.e., prop walk. If the shaft is horizontal, it will try and rotate the boat in the vertical plane, i.e., list. Anywhere in between vertical and horizontal and some of the force will try and rotate the boat while some will cause list with the list increasing and the prop walk decreasing the closer to horizontal you get. Steve |
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#4
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Angle of prop shaft - theoretical question.
Steven Shelikoff wrote:
But it is changing. That's the variable we're changing, the angle of the shaft. I.e., as the shaft angle gets closer and closer to coming straight out of the boat with no downward angle, the prop walk gets less and less. Or stated another way, the greater the angle of the shaft the greater the prop walk. And that's due to the torque of the shaft, which is applied more and more as a force to rotate the boat as the downward angle of the shaft is increased. When the shaft is straight back with no downward angle, the torque force from the shaft causes the boat to list but doesn't rotate it in the water. The rotational force from the prop remains the same. But what it does to the boat depends on the angle of the shaft relative to the boat. If the shaft is vertical, it will try and rotate the boat in the horizontal plane, i.e., prop walk. If the shaft is horizontal, it will try and rotate the boat in the vertical plane, i.e., list. Anywhere in between vertical and horizontal and some of the force will try and rotate the boat while some will cause list with the list increasing and the prop walk decreasing the closer to horizontal you get. Steve Using your theory, Steve, the stern would walk the same way whether the prop was a left hand or right hand wheel... and this is not the case. I have always been under the impression, on straight inboards, that the lower half of the propellor, the arc furthest from the boat/hull surface, is the one that does the most work/thrust (eg. surface piercing drives, etc.). The prop has better "bite" in the less turbulent water away from the hull. This has been my experience too... Correct Craft & Century inboards: RH prop, walks to port in reverse, favors left turns in forward... (the opposite of your explanation if taken to the extreme of a 90 degree propshaft angle). Mastercraft inboard: LH prop, walks to starboard in reverse, favors right turns in forward. (again, the opposite of your explanation if taken to the extreme of a 90 degree propshaft angle). The rotational force of the shaft/prop is transferred primarily to the attitude of the boat on a single screw craft. Rob |
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#5
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Angle of prop shaft - theoretical question.
On Sat, 05 Jun 2004 11:06:08 -0400, trainfan1
wrote: Steven Shelikoff wrote: But it is changing. That's the variable we're changing, the angle of the shaft. I.e., as the shaft angle gets closer and closer to coming straight out of the boat with no downward angle, the prop walk gets less and less. Or stated another way, the greater the angle of the shaft the greater the prop walk. And that's due to the torque of the shaft, which is applied more and more as a force to rotate the boat as the downward angle of the shaft is increased. When the shaft is straight back with no downward angle, the torque force from the shaft causes the boat to list but doesn't rotate it in the water. The rotational force from the prop remains the same. But what it does to the boat depends on the angle of the shaft relative to the boat. If the shaft is vertical, it will try and rotate the boat in the horizontal plane, i.e., prop walk. If the shaft is horizontal, it will try and rotate the boat in the vertical plane, i.e., list. Anywhere in between vertical and horizontal and some of the force will try and rotate the boat while some will cause list with the list increasing and the prop walk decreasing the closer to horizontal you get. Steve Using your theory, Steve, the stern would walk the same way whether the prop was a left hand or right hand wheel... and this is not the case. Actually, no it wouldn't. The torque is in a different direction depending on whether the prop is right or left handed so the boat would turn in a different direction. I have always been under the impression, on straight inboards, that the lower half of the propellor, the arc furthest from the boat/hull surface, is the one that does the most work/thrust (eg. surface piercing drives, etc.). The prop has better "bite" in the less turbulent water away from the hull. This has been my experience too... This is true. That's one of the causes of prop walk, not the only one though. That's why a boat with the prop further from the hull will have less prop walk. But that's not what the question was. The question is why a small change in the shaft angle (say only 10 or 20 degrees) will have a dramatic effect on the amount of prop walk. To have that large an effect with such a small change you have to have a large total force and the one you mentioned above creates a very small force. But when the shaft is completely horizontal, it's the main propellor related force causing prop walk. Tilt the shaft down a little and it becomes overwhelmed by the torque force. Correct Craft & Century inboards: RH prop, walks to port in reverse, favors left turns in forward... (the opposite of your explanation if taken to the extreme of a 90 degree propshaft angle). But the prop shaft is not 90 degrees. If it were, the boat would turn opposite the direction of the prop since it's the only prop related force being applied. Also, not all boats with a RH prop will walk the same way. It depends on the specific configuration of the boat including shaft angle, type of prop, how far the prop is from the hull, etc. My boat is a single screw inboard with a right hand prop and it backs to starboard with a pretty severe case of prop walk. Mastercraft inboard: LH prop, walks to starboard in reverse, favors right turns in forward. (again, the opposite of your explanation if taken to the extreme of a 90 degree propshaft angle). Same comments as above. The rotational force of the shaft/prop is transferred primarily to the attitude of the boat on a single screw craft. Exactly what I said. i.e., the torque from the prop will primarily cause the boat to list when the shaft is at or near horizontal. As you increase the downward angle (or upward angle for that matter although I don't know of many boats with an upward pointing shaft) the rotation force from the shaft/prop will more and more cause the boat to rotate and less and less cause the attitude (list) to change. You can realize just how much rotational force from the shaft/prop there is since it's big enough to cause a noticable list. Apply the same size force from the paddle wheel affect in the direction to cause list and you probably won't notice a thing. I'll agree that on most boats, the rotational force from the torque of the prop/shaft is not the main contributor to prop walk and is a much greater contributor to list. But what we're talking about here is what forces that contribute to prop walk change as the shaft angle changes. And the torque effect certainly does. Steve |
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#6
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Angle of prop shaft - theoretical question.
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#7
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Angle of prop shaft - theoretical question.
Another P.S. The change in direction of applied force from the torque of the prop/shaft is only one part of the theory. The rest is the change in the amout of paddle wheel effect as the angle changes as well as a change in the amout of available thrust in the proper direction to counteract the prop walk as the shaft angle changes. I'm always willing to adjust the theory.  Steve |
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#8
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Angle of prop shaft - theoretical question.
steve, bone up on "P-torque" or "P-thrust" or more accurately "asymetrical
thrust". It is a common problem for aircraft with "conventional" landing gear (meaning two wheels up front and one on the tail), and has been fully understood since the early days of WWII. Another P.S. The change in direction of applied force from the torque of the prop/shaft is only one part of the theory. The rest is the change in the amout of paddle wheel effect as the angle changes as well as a change in the amout of available thrust in the proper direction to counteract the prop walk as the shaft angle changes. I'm always willing to adjust the theory. Steve |
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#9
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Angle of prop shaft - theoretical question.
gene, you are one horse**** pilot, if you are any pilot at all, for not being
able to tell the difference between conventional gear and trikes. Let me inform you a tad. when the aircraft is rolling with the tail down, the angle of attack one side of the prop circle is greater than on the other side, because of the forward movement of the craft. The side of the prop with great a of a developes more thrust and pulls the aircraft forward faster than the other side of the prop. than means the aircraft pulls to one side. This is often (erroneously) referred to a P-Torque, because it feels like the engine is torqueing the aircraft to the side. In fact, it is P-Thrust, or asymetrical thrust that is causing the aircraft to veer. be carefull, gene. Remember what the FAA taught you, "Safety is no accident". You need to do some boning up on your pilot skills, gene, or maybe hang up your googles. (JAXAshby) wrote: steve, bone up on "P-torque" or "P-thrust" or more accurately "asymetrical thrust". It is a common problem for aircraft with "conventional" landing gear (meaning two wheels up front and one on the tail), and has been fully understood since the early days of WWII. So..... what conundrum of JaxWorld prevents this from affecting tricycle gear aircraft, as well?? (Sure takes a lot of right rudder to keep every aircraft, conventional or tricycle, I have flown on the center line.) You *do* realize, of course, that most of the forces felt in this regard are from propeller swirl impinging upon the vertical stabilizer? And, that said, given the arm of a 16" boat screw vs a 8 foot or better aircraft propeller, it makes the effect of P-factor minimal, at best, in a nautical scenario (unless, of course, you would like to include things like WWII battleship propellers, just to prove the laboratory experience in deference to real life). -- 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 |
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