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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #21  
Old 06-11-2008
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Quote:
Originally Posted by JB2 View Post
AAaaarrrggghhhh...it seems most of the folks here are far smarter and muy better at close quarters boat handling than I will ever be!
No, most boats with single props exhibit the effects of prop walk, especially at low speed. I think the trick is to learn to use the effect.
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  #22  
Old 06-11-2008
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Quote:
Originally Posted by sailingdeacon View Post
Another clue that may be key: even with the strong pull to port, the rudder is aligned straight foreward and aft. To me this implies a right hand paddle wheel effect rather than an imbalanced rudder under power.
Is your prop-shaft angled downwards or do you have a sail drive with a nearly horizontal shaft? The more the prop-shaft is angled downwards, the more torque component gets into the vertical. Mine's at 12 degrees down, so sin(12deg)=0.2 or 20% of the torque applied to the prop is trying to yaw the boat instead of trying to roll it. However, this effect is not rudder dependent, except that the rudder must counter it. As you see a reversal of the force on the rudder, I think it you have and overbalanced rudder. If that's the case, extending the trailing edge of the rudder might help.

However, if you have a downward sloping prop-shaft, and a rudder that changes shape with depth, then the spiral of the prop-wash will apply differential lift and un-balance a balanced rudder.

Worth 0.2 cents?
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  #23  
Old 06-12-2008
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my question is on a similar track, I have a spindrift 22 with an evinrude 8.8 longshaft. the motor is mounted on the port side, and when under power it pulls port. Is there something I could do to lessen this? It makes impossible under power to do much else but steer, even for a moment. I finally bought a boat after many years of dreaming...and I have quite a learning curve aheat, but aint it sweet on the water. thanks.
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  #24  
Old 06-13-2008
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Quote:
Originally Posted by Brucesrq View Post
my question is on a similar track, I have a spindrift 22 with an evinrude 8.8 longshaft. the motor is mounted on the port side, and when under power it pulls port. Is there something I could do to lessen this? It makes impossible under power to do much else but steer, even for a moment. I finally bought a boat after many years of dreaming...and I have quite a learning curve aheat, but aint it sweet on the water. thanks.

Brucesrq,

Can't you just turn the outboard slightly to counteract the pull to port and lock it in this position?
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  #25  
Old 06-13-2008
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I was thinking about this last night and I think that the pitch is to great.

If the pitch is trying to push more water than the hull speed, then the extra force that can't move the water would go to moving the aft starboard causing you to go to port.

Just a thought.

Chris
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  #26  
Old 06-13-2008
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Deacon;

This is almost like a party game...I love it.
Question: Is the boat actually trying to yaw, or is it mostly rudder pressure? In other words is your wheel centered or nearly so when going straight, but with such pull that if you release the wheel it turns on it's own, or do you have to apply some starboard rudder just to keep on a heading?

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Old 06-14-2008
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I agree with EO32: There has to be a paddle wheel effect if the pitch is too great. for sure if there was no pitch the prop would be like a paddle wheel with definite yaw. See the photos at the bottom here, which might indicate a paddle wheel effect more than other props.

Feetup: Yes it is trying to yaw... I have to hold the wheel under some force to keep the WHEEL from spinning to to port When the wheel is released under high power, the wheel turns on its own quickly to port and the boat immediately turns hard left..

Unfortunately I can't tell for absolute certain the straight ahead rudder position based on the wheel position. Remember that with a wheel it is hard to know exactly how centered the rudder is based on the wheel position.

HOWEVER, after much recent careful observation under power and coasting, if, under speed, I take it out of gear and hold the wheel in the same position, the boat very slightly go to port.. If I turn slightly to starboard the boat goes straight. (I think I have been misled due to thinking I knew where the wheel-rudder position was )

This amazes me. I think this is saying that the boat needs a slight starboard rudder position to go straight while NOT under power. THe hull/rudder is causing the boat to yaw left. Yet when under power with this prop, the rudder needs to be straight ahead, while the boat is trying to go to even port more because of the high pitch of the prop and higher speeds. Jeepers! Wild!

In other words it seems I have to turn slightly to starboard when going from power to coasting. And when going from coasting to power I have to turn slightly to port (apparently to get a straight ahead rudder).

If on the other hand, we were to assume that while not under power (coasting) the rudder is straight ahead, then under power as the boat yaws left, the rudder is turned to port in order to go straight. This makes no sense since the boat is trying to go to port already.

Here are photos of the prop. (for a limited time).. The prop size appears larger due to some photo distortion. In other words it appears larger than 15" diameter, but it is not. The rudder is aft out of the photo.

Right now I think the boat/rudder is a subtle problem but made considerable by the two blade efficient prop. I think there is a good chance the reduced pitch will help at least. My thinking is that a reduction in pitch of an inch will reduce the yaw or paddlewheel effect proportionately more.

whew!
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  #28  
Old 06-15-2008
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Quote:
Originally Posted by sailingdeacon View Post
I agree with EO32: There has to be a paddle wheel effect if the pitch is too great. for sure if there was no pitch the prop would be like a paddle wheel with definite yaw. See the photos at the bottom here, which might indicate a paddle wheel effect more than other props.

....

Right now I think the boat/rudder is a subtle problem but made considerable by the two blade efficient prop. I think there is a good chance the reduced pitch will help at least. My thinking is that a reduction in pitch of an inch will reduce the yaw or paddlewheel effect proportionately more.

whew!
Just to chuck something else into the mix:

I noticed exactly the same thing on a Catalina 28 we took out recently for a spin. What the owner told me lined up with Idiens posts - that it was caused by the wash from the top blade striking the rudder side-on and has absolutely nothing to do with rudder design.

Apparently, this effect is significantly reduced by installing a small skeg immediately aft of the propeller - exactly like the ones fitted to outboards for the same reason - to straighten the flow.

If this is indeed the case, reducing the pitch should reduce the effect to a tolerable level - but will not completely eliminate it.

Good luck!
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