Here’s the finalized shape of the rudder and the waterline drawn up on some scrap. It looks fairly big to me, actually it’s as wide as the centreboard, I don’t know if this serendipitous symmetry will have bearing on it’s function, or indeed if it will function. But I have faith.
I’ve often read that, when dealing with the design parameters of underwater foils, depth is more important than surface area in terms of lateral resistance and lift. This has been in relation to windsurfing, though, which is exclusively performance orientated. Obviously a planing board has little else but it’s fin in the water so it’s important that the small wetted surface is delivering the maximum lift and minimum drag.
The Trow is further towards the other end of the performance spectrum and this rudder with its large surface area should give plenty of grip in the turn so long as I don’t put the tiller too hard over.
The issue now is to work out how to make it a lifting rudder without it becoming too bulky. And, if possible, without interfering with the nice curve on the trailing edge. Lifting rudders have a pair of cheeks on either side of the blade, and the blade lifts up between them, modern ones have custom-made aluminium or stainless steel cheeks, homemade ones have ply.
One avenue I might take is that of leaving and returning to the beach under oars then shipping and unshipping the rudder in deeper water. It will be easier to handle the surf with oars, or at least I’ll be able to take the waves head on. I wont be going out in any large waves but I’ve seen other small boats come to grief in small shorebreak as, with their sails set they turn side on to the waves as the hapless crew struggles to lower a recalcitrant rudder. But again I don’t know if the Trow’s fine stern will permit my weight sufficiently far aft to ship the rudder.
I have a set of gudgeons and pintles purloined from a wrecked dinghy that allow the rudder to raise up when it touches the bottom. So I’ll try those as well.