Q: Why don’t we see winglets on the ends of rudder? Commercial airliners
(most) have winglets on the ends of the wings. Some keels have winglets
on both sides. My understanding is that this creates an "end plate"
effect to increase the lift from the foil.
Why not have winglets on the bottom of a rudder to create the same effect,
and allow for increased rudder lift (effectiveness) or lower rudder area
for the same lift?
A: For starters, you do sometimes. The early Jongert 24m yachts had big winglets
on the special low draft rudders. International 14’s working with Bieker’s
patent have an entirely different kind of lifting surface projecting laterally
from the rudder. I am certain to get a flood of mails about other yachts
which used wings on rudders. They are rare, but they are out there. You
do not see winglets on rudders very often because the rudder is there
to steer the boat. Not (only) trying to be coy! Rudders are designed to
work at a variable angle of attack, unlike keels and airplane wings. Much
of the time, the sailors are trimming the sails to prevent any rudder
angle at all. At such times, winglets would only add more drag to the
overall appendage package. Even when the rudder is supplying a significant
side force, such as sailing upwind, it does so by being at an angle to
the boat CL, usually while the boat is heeled. A winglet would not benefit
from this change in rudder angle, because it is oriented normal to the
End plates on foils are an attempt to encourage stronger pressure gradients
by preventing flow from one side of the blade to the other. If we have
a overloaded rudder where this would be desirable, it is generally more
efficient to add span, increasing aspect ratio, and thus increasing the
ratio of lift to drag. Area added via span does rotate across the flow
with rudder angle. Span is generally not a problem in rudders, as the
keel defines draft. And span also stays in the water when heel and trim
come into play. Winglets on keels are a marginal advantage in some boats.
Winglets on rudders are not even that good.