the M12 will plane. :: Dave Scobie -------------------------------------------- On Mon, 12/23/13, Howard Audsley <haudsley@tranquility.net> wrote: Jared: I've not seen the M12 boat or rudder in person.......only pictures. Having said that, from the looks of it, it is a "skinny" blade and I would think a 10% foil shape would be adequate. The M17 rudder is 12 inches, fore and aft, at the tip and is 1 1/2 inches wide, so is about a 12% shape. It has been described as a "lifting foil", and generally operates at speeds under 5.5 knots and can be loaded up pretty heavy, depending on sail trim and conditions. It probably benefits from the extra lift the fatter shape offers. The M12 may have the potential to plane, in which case, a much thinner blade will work and at speed, may even work better. If the boat can plane, an 8% blade may be just what you want. Since you seem to know about NACA foils, you probably know how to build one, but if not, I'd offer these crib notes for advice. This seems to be a constant chord blade (vs. one like the M17, which tapers), so is relatively easy to build. Once you get your blank ready to shape, lay it flat on a surface, then using something about half the width of your blank, scribe a centerline down the leading and trailing edge. Flip it over and do it again. The single or double lines will mark your exact centerline. These are "sacred". You don't want to shape inside those lines. Front edge is rounded of course. Trailing edge does not come to a point, but is planed square. For a blade this size and for this boat, no more than 1/8" flat shoulder on the trailing edge and maybe even less. On the blade, fair leading and trailing edges and symmetry are the goals. Since this is a constant chord shape, best way to achieve this is to draw out the full outline of your foil shape on a thin scrap of plywood, then remove the inside, leaving the outside as a pattern. If you leave it as wide as the foil shape, the bottom edge can register on the same flat edge the blanks rests upon. This way you know when you have gone far enough. The pattern can then slide up and down the foil to find the high and low spots. This way you get a good shape and perfect symmetry from side to side. Good luck! Post some pictures when you get it finished. Howard On Dec 20, 2013, at 4:06 PM, jaredprindle wrote:
I'm aiming this question to Jerry since he was the designer but anyone...feel free. I'm making a new rudder for my M12. The old one is great but I'm making a fold up one and I'd still like to keep the original. I like learning these things so that's part of the reason too. It's fun messing with splinters and fumes. Anyway, I was going to make something along the lines of a Naca 0012 as that seems to be a popular foil - not to racey but not a pig either. Measuring the original rudder it doesn't look naca anything but the thickness to chord length ratio seems really small...about 8.5%. Literature suggests that this rudder may stale alot without constant attention. I never felt that way about it while using it so I guess I'm just looking for some clarification or understanding. Any thoughts on this? Considering the wood I'm using is a little thinner then what I wanted my rudder may end up closer to a 10% ratio. Again thoughts? Ok, I know, I know that all this detail really doesn't matter since there's no M12 class to be racing against and my dad has said it's pretty hard to screw up a rudder. I'm happy to know the tolerances are fairly forgiving but I'd still like to understand the theory for doing it right and even understand what and why behind the original rudder design. Thanks Jared