Overbalanced rudder with no balance area

Yes, pictures please!
This model of boat? MICROTONNER 19 - sailboatdata https://sailboatdata.com/sailboat/microtonner-19/


This type of rudder head? Kick-up rudder design options https://www.boatdesign.net/threads/kick-up-rudder-design-options.69575/#post-967070

 

Yes, quite like that but without the kick-up, just simply lifting daggerboard style. I've tried to find a pic online but the best i've got is just the stock:


You can imagine the blade slotting down into the aluminium stock. The blade is rectangular cross section at the head to fit the stock, mine develops into a NACA0012 section just below the stock.

 

Maybe it's not the rudder...?

 

Sounds like a directional stability problem, not a rudder balance problem. If the centre of pressure of hull and fixed appendage is ahead of the centre of gravity, then any small deviation from a straight course will be amplified by centrifugal force. That centrifugal force acting on the rudder gives you the extra moment on the tiller when you want to restore the course.

You could test this on a dead calm day by getting your boat up to speed on the engine, shutting it down (pull it up if it is an outboard), then letting go of the rudder. I expect you would see much the same effect.

If my guess is correct, then the only effect of either of these would be to increase the force on the tiller when you want to restore the course. If you want to focus on the rudder only, then give the rudder blade some balance and install some way to lock the tiller.

The Globe 19 class deals with problems arising from a (deliberately designed to be) small rudder by adding daggerboards on the transom. In your case, these could move the centre of pressure of hull and fixed appendages aft of the centre of gravity, and should keep your boat course stable without rudder input.

 

If the rudder feels overbalanced, forces are acting on the rudder, of course. There is nothing that can act on the rudder with the force you describe except water flow. There are two possibilities:
1-The rudder simply trails in the flow coming off the hull. A fixed skeg forward of the rudder should straighten out the flow at that point.
2-Circulation. Lanchester (iirc) demonstrated in the 1920's that circulation isn't fully formed until about 7 cord lengths of travel. Circulation around the hull as a whole places the rudder in the downwash, so it would be responding as you describe. Try using the rudder hard over, then returning it to center over very short, then progressively longer intervals to see if the effect grows. If it does, circulation. Turns out it is real. A real emergent property of lift developed by a foil that is. That is harder to fix, given that you don't want to add balance area.

Ironically the NACA 12 section gives a very steep lift • angle of attack curve before early stall. The flow from the hull doesn't have to be too far of axial at all for the rudder to experience meaningful forces.

 

It's quite likely the boat will spin rapidly, like a top (exaggerated to visualize it), and water will enter the rudder in the opposite face to if the boat were moving forward at a good speed.

On my old MiniTransat, I use an elastic cord for the rudder—it's convenient, simple, and effective.

 

My 2 cents is that the boat having a very wide stern makes it very sensitive to steering by heeling the boat. This is compounded by the vertical short chord centerboard well forward, and the tall rig. Once that boat heels over and starts spinning it’s going to want to keep spinning and the rudder will simply want to follow the direction that its pintles and gudgeons are headed. A boat like this is steered with the weight of skipper and crew and the job of the rudder is to follow with as little drag as possible.

 

My other 2 cents would be that you should always give any sailboat some weather helm so that the rudder will have a greater angle of attack than the hull because it is more efficient than the hull at generating lift. You can do this with mast rake or by sailing the boat with a little more heel.

From the picture I see now that the boat is not that wide at the stern on the waterline, but that depends a lot on your fore and aft trim. You can also play around with shifting crew weight further forward in light winds and further aft in stronger winds.

 

A cassette rudder has the "pivot" point in the middle of the cassette so.. if the rudder is straight it's overbalanced by design. Another factor might contribute with high aspect rudder is if it's not stiff enough and enables twist thus making it behave like you described.

 

Yes, i think this is most likely the case. in which case there isn't much i can do about that. I can't (wont)move keel or mast substantially.