Patric A wrote:

I've created a (very poor) sketch and uploaded it to "my photos" showing the trim tab on my current rudder.

Patric A wrote:

No love? Seems my questions go down like lead balloons.

Pete could answer this more fully, but doesn't seem to want to contribute to these fora.  We had problems with the servo-pendulum in the Falklands, because just about every time we went sailing, we got kelp caught around the paddle and the break-away did just that.  So we decided to fit a trim tab so that it wouldn't catch.  If my memory serves, we used the same paddle (maybe scarfed on an extra piece) and the reason that we placed it as we did was to have it vertical.  We used information from Bill Belcher's book to make the decisions.  The trim tab worked better than well - it was the best self-steering gear that we had on Badger, with the added advantage that it was easy to override in moments of stress and controllable with a 'Brown Stick' which meant that one effectively had down-below steering: great for dodging lumps of  ice, fishing boats and steamers!

I have added a sketch of the basic trim tab design used on Arion in my photo album "Arion details". (here) If you follow this basic design you can increase or decrease the size of the trim tab and vane until you get the desired result, but to give you a basic ratio, my trim tab is 100mm by 1.1m and the vane is 900mm high by 600mm wide.  Make the shaft of the trim tab as close to vertical as possible, which will dictate how long the bottom pintle bracket should be.  The whole thing can be varied to suit, as long as you get the linkages between vane and trim tab sorted out, as shown in the sketch.

Thanks, surprisingly there seem to be no photod online showing Taleisin's trim tab but I did find a sketch from Serrafyn in the "Self Sufficient Sailor", where it's perpendicular to the waterline.

The fitting instructions for the Autosteer unit mention that:

"If the trailing edge of the rudder is not straight chose a position that places the Trim Tab towards a more vertical situation but at the same time does not have too long a bottom Pintle Bracket. The Pintle Brackets may be shortened and /or bent to suit the rudder."

It would be nice to get it right instead of handing over a rather substantial amount of cash only to find out that I'll need a new bracket. The online information from Sarana (they have a downloadable pdf) and the book mentioned earlier require an engineering degree to decide on size and shape of trim tabs. 

I'll have to find out what Hydra Engineering mean exactly  by "not too long a bottom pintle bracket" and why. 

Arion has a trim tab (servo rudder) on the back of the rudder that works really well.  The rudder is about 400mm wide at the top, 600mm wide at the bottom and 1.2m deep.  The trim tab is 1.1m deep and 100mm wide, the top of it is close the trailing edge of the rudder but the bottom is about 400mm back.  This was done to increase leverage.  I didn't do any maths, just copied the trim tab used by the Pardeys on Taleisin.  The upper works of the vane were copied from early Hasler vanes, as fitted to the Hiscock's Wanderer 111, the vane being offset on its own bracket and linked to the trim tab with a differential linkage, to prevent oversteering.  I also link an electronic autopilot into the trim tab for motoring, motor-sailing or light winds.