Foiling Rudder

[Alistair Edwards]

I would be grateful for any advice the foilerati are able to provide!

My new boat (a Big Issue 2) has a foiling rudder which is not a t foil. The wings are at the bottom of the rudder blade. As I understand it the t-foil rudder is particularly effective because it utilises the water flow coming up off the bottom of the hull.
Is there a major differential in terms of foil performance? If so might it be worth getting the rudder modified and repositioning the wings?
I haven't used the foiling rudder yet. Still getting used to sailing the boat with the dagger rudder.

[Alistair Edwards]

Anybody able to offer any advice?

[John m]

The foil position which people have had most success with is about 8 inches below the bottom of the transom.  I don't know why...   interesting article in the 2019 newsletter about using the t-foil by Graham Camm, but does not cover your question.

Gavin Willis and I think Tom Stewart have both tried wings at the bottom.

John

[Alistair Edwards]

Thanks for your input John.


From Graham's article I think the key point seems to be that as the boat speed approaches hull speed the water flow has a greater vertical component for the wings to work with. Presumably water which has passed under the rocker accelerates back and up towards the bottom of the transom?


I suppose this effect might be more pronounced on boats with more rocker?

[John m]

I think a boat with a t-foil can have more rocker as the wings provide lifting surfaces which would otherwise need to come from the hull shape.  The waterline outline of the dead cat bounce for example is like a tear drop when viewed in plan.

It may well be a factor that the sweet spot that people tend to be placing their foils works well with this flow of water close to the hull.

Another argument might be that the water is pretty disturbed close to the hull and putting the wings further down gives them less disturbance.  However they may feel less upwards force and therefore not provide as much lift for the same wing area?

Foils have been shown to work well with the final chapter which has more rocker than most, particularly in the final foot before the transom.

All well above my pay grade!