prop kick

[Tezza]

My Beneteau First 345 has a heavy pull on the tiller to port under power which is quite pronounced. Everything seems in line and the pull is tiresome. Under sail the rudder is well balanced. Any idea what causes this?

Tezza

 

[FWB]

It's the vortex off the prop. Same thing with light aircraft, you have to apply rudder with power.

 

[oldharry]

Its called prop walk, and will be found on most single prop installations to some degree. Very useful in manoevering as it can be used to shunt the boat sideways by using short bursts of power. Bit of pain the rest of the time, but you can either experiment with different types of prop (quite expensive both for props and hauling out to change it) or like the rest of us - live with it.

 

[Csail]

You could build a trim tab.

 

[sailorman]

wot RPM do you get flat out & is there much black smoke.
if low rpm it could be "Over-Propped" ie too coarse a pitch .
my Sons Cinder 22 had the very same problem, i altered the prop & 1/2 the prob

P.S.

The Cinder 22 is for Sale with new Yanmar 1 GM

 

[FWB]

Prop Walk is the effect of the prop pulling the boat sideways, esp astern at low revs. What is being experienced here is the vortex from the prop pushing on one side of the rudder. Which side depends upon the direction of rotation.
In light aircraft with US engines you apply left rudder, with a British Engine its right rudder---could be other way round 40 years since I learned to fly. But the principle is the same.

 

[Oldhombre]

OK, two different effects here. Everyone talking about prop walk or prop kick which is a sideways force from the bottom of the prop. OP doesnt mention prop walk at all. Apples and pears.

I'll get my coat.

 

[FWB]

Please read my posts. I mentioned prop walk, and the subject of the topic, which is caused by the vortex from the prop. When going astern the rudder is of no concern with the effect of prop walk.

 

[flaming]

[ QUOTE ]
[ QUOTE ]
Prop Walk is the effect of the prop pulling the boat sideways, esp astern at low revs. What is being experienced here is the vortex from the prop pushing on one side of the rudder.

[/ QUOTE ]

Sorry but that is a load of ********. Prop kick is not caused by effects on the rudder it is caused by the bottom of the prop having more effect sideways than the top of the prop. Even with no rudder you still get sideways prop kick and it is always in the direction which is "kicked" by the bottom of the prop. You actually contradict yourself by suggesting the vortex off the prop hits the rudder when going astern.

[/ QUOTE ]

Er, I think he defined prop walk, then said that is something different, which it is.....

 

[Tranona]

Suspect your prop is fairly close to your rudder and is probably at at least 10 degrees down angle, maybe more. What you are experiencing is the effect of water not flowing square to the rudder. You probably don't have much balance area forward of the stock either. To see the difference, try a boat with a saildrive. Not only is the prop much further forward but is parallel to the water line, reducing or eliminating this effect.

You can still get propwalk which as others have said is nothing to do with the rudder, but its effect is less because the prop is nearer the pivot point of the boat. What you lose is the instant "bite" when you go into gear because of the slight delay before the water hits the rudder.

 

[Eygthene]

Dylan,

My Jeanneau has exactly the same problem as the poster describes and again only felt under power, not under sail. Whilst she is ashore, I have carefully studied the alignment of the fin keel, the slightly balanced rudder, the two bladed folding prop and the p bracket and all seem to be perfectly aligned.

Yet my previous boat - a quarter tonner - which was practically identical underneath but a bit smaller, had no unbalanced sideways force on the rudder when motoring or sailing. If anything the two bladed folding prop was rather closer to the slightly balanced rudder.

One poster has suggested a trim tab. Any other solutions?

 

[sailorman]

its the proximity of the prop / rudder & more than likely to be over propped.
my Sons Cinder as i said had this problem. rather than alter the pitch i reduced the Dia by 25m/m now no prob & engine not overloaded.

as she was if one let go the tiller she would shart turn to port & almost launched you into orbit /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[michael_w]

On my boat the engine beds, stern tube and the P bracket are slightly offset to ensure she runs straight with minimum of rudder. The other advantage is you can slide the shaft past the skeg. Otherwise, you'd have to take the engine out.

 

[sailorman]

They learnt that after the 37 /forums/images/graemlins/mad.gif /forums/images/graemlins/mad.gif

 

[Eygthene]

Michael,

That would be the answer, and I think single engined aircraft used to be offset a bit like that to help them fly straight. I seem to remember that we had to do the same to get our models to fly straight when aeromodelling was still a popular hobby. I am not too keen to start moving the engine beds and sterntube and p bracket, though I expect it would cure it, if the correction were the just the right amount.

Sailorman,

Did I gather that the later Jeanneaus have the engine and shaft assembly offset at an angle to the centreline for this reason?

 

[Twister_Ken]

Presumably on a plane it's not prop kick, but torque reaction?

 

[sailorman]

re-read my replies therein lies a real possibility of a solution /forums/images/graemlins/ooo.gif
Bin there got a Tee Shirt /forums/images/graemlins/wink.gif

 

[Tezza]

Many thanks for all of the contributions. The kick is present at all speeds but increses with revs. Tried swapping the feathering prop with original fixed prop but no improvement. By rigging a simple servo line using a winch we take the tedium out of the pull but was iterested in what causes it. Still confused as to why vortex shedding hits one side of the prop.

 

[FWB]

" Still confused as to why vortex shedding hits one side of the prop."

Try to imagine the vortex leaving the prop. Follow the start of it, it goes round and around in a corkscrew until hit hits the rudder. Facing aft... if it's clockwise it will hit the stbd side, anticlock it will hit the port side.
Yes I know you could say ahah it misses the rudder first time and hits it on the other side. Same result different direction. It's the principle that's important.

 

[nimbusgb]

The flow off the propeller has a corkscrew motion. It tends to hit the rudder blade on one side. This both pushes the blade in the opposite direction and tends to reduce the lift that the blade generates. This needs counteracting by the application of opposite rudder.