[MHml] Cat Rudders Toed In?

[thomas deane]

and this is why the early polynesions put toe in on their canoe and ama. 
thomas deane
----- Original Message ----- 
From: "Glenn Brown" <gbrown at alumni.caltech.edu>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Friday, September 01, 2006 11:40 AM
Subject: Re: [MHml] Cat Rudders Toed In?


>>     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.
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