[Paddlewise] Rudder profile

There is an often repeated misconception that a rudder must be streamlined, 
"have an airfoil", "be profiled", et cetera.

This is not true, most of the time. When, say, a sailing canoe is properly 
trimmed there should be no load on the rudder (one of the beauties of
a yawl rig is that it is much easier to achive it with such a rig), as that 
means least drag from the rudder, no matter if it is "profiled", or a flat 
plate.

A thin plate has the least drag of all, when not loaded.

But during vigorous manouvering, a thick profiled rudder has an advantage,
as it is not so easily stalled, but in that respect a non-vertical rudder is 
better than a vertical, just as the Concorde flew very well at low speed with 
its highly swept wing, it just needed a lot of power.

So while a rudder might well be a flat plate with a rounded leading edge the
leeboards/dagger board/centre board should definitely be profiled, as that
is almost always sideways loaded, unless you're going straight down wind!

Depending on speed and the power needed, the optimum width of the rudder 
varies, as power is a function of the area and speed squared, but the 
narrower it is the easier it will be to stall, especially if it is long, as 
the sharpness of the stall is a function of the rudder's AR, aspect ratio 
(the length divided by its width). You see very few thin plate rudders of 
high aspect ratio, as they are very prone to flutter (pouch's leeboards are 
made of thin plate and while short a tubby buzz (flutter) a lot!

So at low speed a wide rudder is a real boost, while at high speed a
long, narrow rudder is good enough - US tugs have multiple rudders that are 
far wider than long (AR < 1!) and Americas Cup boats have long and narrow (AR 
> 4) :-)!

Yours,

Tord
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