prop kick

[earlybird]

Rigging a relieving line shows that the OP has felt the need to go to a fairly extreme length to get over the problem. Since his Beneteau has a fairly typical underwater geometry, why don't many,many other owners feel the same need? The prop vortex argument would surely apply to them too. Is it possible that the boat in question has a rudder that is slightly more "balanced" than other examples, e.g. due to the stock being bent slightly forward? This would amplify the vortex effect but might have no effect under sail when the vortex is absent.

 

[Eygthene]

Dylan, Nimbus, Earlybird

Thanks for an excellent set of replies. I think your explanations are convincing. Yes it is curious that the effect is so significant on one craft when absent on another, it could indeed be due to small differences in the fractional area of rudder forward of the stock. I like the point about the vortex affecting the lift. I can see it might have a stronger or weaker effect on one side of the rudder where the vortex is deeper in the water than on the other side where it is nearer the surface.
Also on my quarter tonner, where the effect is reasonably balanced when going straight ahead under motor, there is a sort of "servo" effect if I start to turn, when the tiller pulls very strongly to increase the turn. This would certainly be explained by the effect of the vortex on the lift, as the effect of the flow against the blade would itself tend to straighten the rudder against the turn.

Sailorman,

Thanks for getting me to look again at your posts. I can see that reducing the prop would reduce the flow past the rudder and significantly reduce the problem - you achieved a 50% reduction on the Cinder with a small reduction in diameter without reducing pitch.
I have only travelled 3 miles with the Jeanneau and maybe 1 mile of that was under power. That was on the sea trial before purchase earlier last month. I didn't notice any signs of over-propping, but we were only doing <5 kts at 2000 rpm - dont know gearbox ratio at the moment - will examine the gearbox plate later this week. Engine is Yanmar 2GM20 and prop is folding 2 bladed 14 x 10. She is going back into the water next weekend so I will have a chance to look for signs of overpropping - black smoke, engine revs etc. My quarter tonner's folding two blade prop is 12 x 8 with a 1GM10 and 2.2:1 reduction gearbox. i think she is a little underpropped as I get full revs with ease and whilst the speed is about 5.6 kts in flat water, it quickly drops in any waves.