Optimum Dagger Board Use

[44'cruisingcat]

Originally Posted by MidLandOne

Regarding using one board or two I am just throwing this in the pot from a consideration of the foils point of view - there may be other things going on.

The drag from a foil increases as the square of the lift from the foil so one is (remember, just considering the foils) often better off to spread the lift over two foils rather than just one. That is two foils giving the same lift as one have half the drag, all else being equal.

Hence, for mono hulls, why it is good to spread the lift between a spade rudder and the fin keel, and is a reason why there have been a lot of goes at trying multiple foils under monos eg as has been tried on IACC boats. That last is an example of other things going on because other performance advantages (eg manoeuvrability) have been lost that out weigh any decrease in drag. Monos generally (not always) have the disadvantage that the foils are in line so to give the full advantage they have to be far enough apart for the foils to be outside of the turbulence of the forwards ones (not possible), a disadvantage cats do not have with their dagger boards.

So, just with that limited view I would have thought that in the case that one fully dropped dagger board gave the lift needed then one is better off to use the two boards only part dropped enough to give the same total lift but much less drag.

I wonder if anyone can comment as to how much that fits real life and what else might be going on?

Roger Shuttleworth has done tank testing, and has found one larger board to be superior to two smaller ones.

You can get the same lift, with just over half the drag.

For racing boats, which will fly a hull regularly, you obviously need two boards.