Technical protest

Rockatross, don't think it's class legal, so it's a blade you can buy that will fit a Merlin stock.

Obviously it's added to blade, so it's a contrivance. You can't buy a Merlin rudder with a fence.

Lost train of thoughts now   Think it was in relation to adjustable wing and fence in relation to Merlin ruling.

Fence is a performance enhancing device like, 

Originally posted by JimC I'm not and have never been an official measurer for any class, but I have been involved in contributing to class rules for Cherubs and ICs. So take that for what its worth. If I were looking at a Merlin I would say that a fence becomes a foil when it has any lifting capability, and most especially if there is any control capability of changing the angle of incidence. So if its a small structure not extending beyond the likely boundary area of the flow round the blade, with a symmetrical section oriented as close as possible to the likely local water flow its a fence. [snip] [later] I've just looked at the rudder designs posted on the Merlin forum. I note - structure which doesn't appear to extend beyond the boundary layer - very low aspect ratio so very high drag if generating lift - small total area So unless there is some gear on the boat for changing the angle of incidence then I'd be quite happy with those - again for what little my opinion is worth.

In practical terms there will be a gap between a fence that is bigger than it needs be to do the job, and a foil that's too small and too low aspect ratio to do anything significant. The difference will be obvious. But in any case any control to accurately change the angle of incidence of the foil on the water would be a dead giveaway.