Rudder Balance advice please

[Alan Saunders]

I wondered about 'rudder-judder', far more annoying/tiring than an a slightly under/overbalanced rudder.

[Dalslandia]

I wrote before the crash, to cure rudder-judder, it need a rounder leading edge, an simple Schilling rudder will also improve the rudder efficiency, can be made with a wrap like a ice cream cone profile, 15-25% thick (to the blade length (chord)) welded into the original blade at 60% chord.

 

the balance should be 15-20% on small boats, sailboats often have a 17% balance in front of stock.

make it 25% and it will be neutral at small angles, but might get revered forces at big angles.

33% and you have to fight it all the time.

[Alan Saunders]

The previous owner of my boat had an inch or so removed from the forward end of the rudder. It runs nicely in a straight line with little effort and no judder. Turning, there is considerable 'feedback' even when going astern. No tendency to go over-centre to maximum rudder. I guess the previous owner got it as 'right' as it will ever be?

 

Alan

[tosher]

Thank you all for the helpfull suggestions and advice and having concidered them all I have decided to carry out the following mods. Cut 1 inch off the leading edge of the balance blade and weld on an extension to the trailing edge of the rudder blade to make it sguare, not angled forward as it is at present. This will reduce the balance ratio from 31% to 22%. If I find this has not reduced it enough I can easily grind some more off the balance blade at the next blacking. Below is a picture off my over balanced rudder at it's last blacking. I do have a little bit of tiller "wobble" at the moment and wondered how cutting 1 inch off the leading edge will affect this (reduce it I hope) as it will increase the distance from the prop to the balance blade from 11 to 12 inches???

[Rose Narrowboats]

Personally, I'd weld a bit it on the bottom edge as well - looking at that pic a fair percentage of your prop wash is going straight under the rudder blade. At that point you probably won't need to take any of the balance plate, and the boat will turn very nicely at low speed.

 

I've never understood why Liverpool Boats et al felt the need to build a rudder that stuck out further underwater where you can't see it. If you haven't already, check the blade to stock bolts and if there's any play whatsoever change them now - they wear very quickly once they start to move, and make sure the pinch bolt is tightened by an ape.

[Dalslandia]

measuring at the pic, if it is perpendicular to camera, your balance is 2,5 cm of 11,5 cm (in the pic on my screen) that is 21,7 % balance or so, can not be 31%

do you mean balance is 31% of the rear part? so 31% of 131% = 23,7% balance

 

But it is still more then 20 or 17% (of total length) as recommended

 

measure the whole blade, measure the balance in front of centre of stock. (balance/whole blade= %)

not square shape so measure at mid height.

Edited June 5, 2015 by Dalslandia

[tosher]

Well chaps thanks again for your invaluable contribution, it seems I have being doing the calcs all wrong. I have been using the balance blade as a percentage of the rudder blade (and NOT the whole blade) to calculate the balance ratio. What a numpty! So starting again using the few measurements I have I calculate that my balance blade is about 23% of the whole blade and to reduce that to 20% I will need to add about 70 sq. ins. to the rudder blade and leave the balance blade as is it is now. If I square off the trailing edge of the rudder + add a 1.5 inch strip to the base of it that should do the trick and will also cover the point that Rose Narrowboats made about some prop wash missing the rudder. When the boat comes out of the water next month I will do some proper measuring, take some pics and post the results. Many thanks again for all your help. tosher.