Rudder Wobble(?)

bluefin

New Member
Just spent my first season sailing a J24! I bought a 2003 boat built in Argentina and we had a blast this summer on Lake Tahoe!

One issue I had that I'd like some advice on is the rudder had a distinct wobble at speed. The "wobble" was not a vibration. The tiller would move 2 to 3 inches back and forth approx. 3 or 4 times per second. This would only happen off the wind at or about hull speed. I checked the gudgeon straps and pins and there is no play whatsoever. I looked closely at both foils and could not see/feel any anomalies. I do not have class templates so the foils could be out of spec, but if they are, it would be so minor that I don't think it would cause the problem.

I faired a rudder on a Santana 35 once and did encounter a cavitation/vibration at the trailing edge. A pro told me to bevel one side of the trailing edge and that cured the problem. The J24 rudder trailing edge is so narrow that I don't think that is the issue either.

So as you can see, I'm at a loss as to the problem and solution. What I do know is this problem will become a much bigger problem if I don't figure out how to fix it. Any thoughts/advice would be much appreciated. Thanks...........
 

VinceH

Member
Wow! That sounds like a tough one. Certainly beyond my pay scale or experience.

If there's no loose hardware it sounds like it's some sort of hydrodynamic problem. Interesting that you don't see anything odd about the rudder shape. I think you need to consult with someone who has real experience with yacht design. Here's somebody who might be able to help you. I haven't had any contact with him, but it's apparent that he has experience working with the DIY crowd. Maybe he'd be willing to talk to you and offer you some suggestions.
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We'd be interested to know what you find out!
 

Dale Tanski

New Member
I have been think about this one since your posted. We used to race J22's and the "ridden hard and put away wet" boats sometimes suffered from a soft keel mount. Although J22's and J24's did not have an exceptionally deep keel sump, the older ones and perhaps the ones with higher flex cycles of the fiberglass resin layup tended to move more. New boats are unquestionable stiffer, you can tell by the ring when you thump them. Older boats not so much. You could be getting movement of the keel fin as it "swims" through the water due to a soft keep sump. This movement may indeed be the hydrodynamic problem the above writer eluded to. That movement or swimming would then be transferred back to the rudder as it sympathetically reacts. Far fetched but indeed possible.
The 24 has tabbed in wooden keel stringers that are designed to stiffen the sump spreading the load over the bottom of the hull. Perhaps the core material in those stringers are no longer with us and now the bottom of the hull is adding to the flex.
On a 22 it is most noticeable down hill. You could actually feel the boat hunt right to left without moving the rudder as the keel would "adjust" itself to the waves. Weird feeling.
Fin moan or hum is indeed a function of its fairing and it is slow. We fixed a J70 keel that had damage from trailering. It required repairing the trailing edge which required resurfacing the back half of the blade. We found several "shrinkage" areas where the layup shrunk while it cured. Those shrink spots caused depressions which I believe are worse than humps. The owner was astounded as he said the low frequency moan was no longer there. That boat went from mid fleet to upper 3rd in a heartbeat, same crew, same driver, same sails, better blades.
When it is hanging in the air, can you wiggle the keel side to side? Best way to do that is quick back and forth with your hand at the tip, actually a few fingers will do the trick if it is real soft. It will even continue "wiggling" after you stop inducing motion for several cycles.
Great post...
Dale
 

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