Originally Posted by rdubuque
I am building a new rudder and am including a trim tab system mimicking the Autohelm model by Scanmar. So far it seems to be going quite well.
One question. Does anyone here have the dimensions of the actual aluminum
vane with the holes in it. It appears to be about 36" long- 12' across the top, and 5' across the bottom where it attaches to the mast. Want to get it as exact as possible. The ability to reef this vane seems to be a very useful feature.
Don't worry about the exact dimensions of the wind vane
... not at all critical. The important thing is that you must be able to counter balance the weight of the vane so that it balances with it in the vertical position. This position is for use in light airs, and you don't want to desensitize it. As wind
strengths increase, you tilt the vane more towards the horizontal. This both decreases the lever arm and increases the inbalance of the weights... both of which desensitize the system and make it more stable in the stronger winds. A bit of experimentation is required, but it works very well when you get the hang of things. Basically, if the boat is over responding to yaw, lower the vane a bit at a time until it maintains a steady heading.
All the above knowledge was gained on our previous boat where I built a aux rudder-trim tab system similar to the AutoHelm model sold now by Scanmar. Steered the boat (an ex IOR one-tonner) for around 50,000 miles.
Incidentally, it is important to minimize friction in both the bearings that support the wind vane itself and in the cable drives for the trim tab. Keeping the mass of the vane as low as possible helps to keep those bearing loads low, too. OUr vane would steer down to about 5 knots apparent wind on any point of sail, which is pretty good as those things go!
Good luck with it, mate!