Re: [Paddlewise] Rudder types?

Philip wrote;

(SNIP)

>
>Anyway, in the last several years Feathercraft and similar thin, tall surf
>rudders have become pretty standard, especially on single kayaks. My
>understanding of their advantages:

>1) They submerge below surface turbulence to grip the "solid" water
beneath.

Surface effects do reduce effectiveness. I don't think turbulence is a
significant problem with most sea kayaks that have fairly shallow slopes to
the buttucks. The problem of air entrainment (ventilation) exists for both
types and t would seem more logical to try to reduce this with baldes that
have more area donw low and a ticker airfoil section (NACA 0009 is commonly
used).


>2) They can retract fully onto the rear deck to avoid catching following
>seas when paddling, or following paddlers when portaging, or following
cars
>when driving.

True and seems worthwhile to me.

>3) By being smaller, they have less drag than wider, shorter rudders.

Not sure why they are smaller. Seems to me that a low aspect ratio rudder
could conceivably be smaller due to the greater turning moment generated. I
think Dagger has a boat with a small low aspect ratio rudder  (Sitka?) and
from reports I have heard it works well.
>
>The case I've heard for wider rudders:
>1) They offer more turning power for tighter turns, especially important
in
>doubles where paddles are used more for propulsion than control.

Probably the case although I am puzzled by why tandems have to be so
difficult to turn.

>2) They can be cranked to a greater "angle of attack" before they stall to
>cease being rudders and start being brakes.

This depends upon the section shape and edge condition so much that I don't
think it is a fair generalization.


>3) By effectively increasing the waterline length of the hull, they create
>less drag than thin rudders (a direct contradiction of point 3 in the
>previous paragraph).

Simply not true unless the rudder has unusually huge volume. I have not yet
seen a rudder that had enough volume and was properly faired into the hull
to do this. Even when the rudder is faired into the hull (ala Dagger) the
flow around and over the edges disrupts the wavemaking. I think to get any
effect one would have to have an effective seal between hull and rudder.

I guess what I am saying is that ruddes on most kayaks are far from beingas
good as they could be and that a propely designed rudder could serve the
needs of tandem as well as solo boats. A nice simple treatment of this
topic can be  found in Ross Garrett's "The Symmetry of Sailing"

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/






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