[MHml] Cat Rudders Toed In?

[Glenn Brown]

>     Our new to us 35' Cat (designer unknown) appears to have 
 > both rudders toed in about half a degree from parallel.
 >  Is this a normal thing with cats or a builder's mistake?

That amount of toe-in is not at all unusual for small performance cats, 
where toe-in in the 0 to 1 degree range is considered acceptable.  I 
think 0.5 degrees is what the Olympic Tornado sailors found optimal for 
that boat by experiment, though the theory behind it is not 100% clear. 
  In general, toe in reduces the angle of attack of the windward foil, 
reducing its lift and increasing the AoA and load on the leeward foil. 
Running less lift on the windward rudder with toe-in reduces helm 
perturbations as that foil enters and leaves chop, and reduces the 
tendency to round up when the hull lifts.

For laminar foils, toe-in can also reduce total rudder drag if parallel 
rudders both run outside the low-drag "bucket" (at an AoA of more than 
about 3 degrees), and if toe in allows the windward rudder to enter the 
low-drag bucket.  In this case, toe-in can cause the drag on the 
less-loaded windward foil to drop more than the drag on the more-loaded 
leeward foil increases.  However, loading the rudders to such a high 
angle of attack only pays if you have relatively inefficient forward 
foils, such as most low aspect ratio centerboards and keels.

--Glenn

P.S.: Your NACA 0012 foils are not laminar flow foils.