Scarlattikvarten technically speaking
Dagger boards
Introduction
Dagger boards, pivoting centre boards or fixed keels are what we use to limit leeway. The efficiency
of these leeway preventers has a big impact on our ability to go to windward. On other courses there
is less need for leeway preventers and dead down-wind they are just drag. Well, they still help with
traction but only small areas are needed then. From this follows that daggers and pivoting boards have
a big advantage over fixed keels. Since they can be retracted there is no need for a compromise
between windward and downwind performance.
Below I will assume that 'optimum' windward performance is aimed for and that the boat is strong enough to handle the loads generated by the boards. This is far from always the case and if it is not both board 'size' and structural engineering must be adjusted accordingly.
Priorities
Here is my rant on what is important and what is merely icing on the cake when it comes to designing dagger boards.
- 1) Make them big enough!
- If the boards are too small they must work with a large Cl (lift coefficient). A large Cl means high induced drag. Not only is this inefficient but maneuvering will suffer. The boat will be slow out of tacks and every so often the boards will stall after a tack. (Down-load this document for more on board-size. Beware of the math.)
- 2) High aspect ratio is better!
- By increasing AR (aspect ratio) you can decrease drag more than by any other measure. Keep an eye on the structural aspect of this though. Handling might get tricky for really extreme ARs.
- 3) Plan-form:
- There is more to gain here than from optimising the section. But is it worth it? A fancy plan-form is definitely more time consuming to build than a simple rectangular one. Note that quite a few hot racers like Cogito, RC-27, Ventilo… make do with rectangular boards. All these have really high AR foils (low induced drag to start with) and Bill Roberts (RC27 etc.) always uses forward swept dagger boards, something which is supposed to decrease induced drag. Cogito and others have boards swept the other way which reduces board/hull interference and drag due to wavemaking. I guess the jury is still out on sweep angle. A pretty nice compromise if you want to do something about the plan-form is to keep the trailing edge straight and make the leading edge straight apart from the lowest part which follows a quarter circle arc. (A not too good photo of this will pop up if you click here.)
- 4) Choosing a wing section:
- Let me start with stating that it is important to use a proper wing
section. Just making something that looks like wing section is not a good idea. Such sections
are not only draggy but are also prone to stall and/or to ventilate. For rudders this can be dangerous
as you may loose control at the wrong moment.
But spending a lot of time and energy on 'optimising' the wing section should be a luxury activity
once all above have been ticked off. Good old NACA0012 isn't bad at all actually. At low speed
(low RE-numbers (Reynolds numbers) really) it is actually 80% laminar and pretty hard to beat according
to Mark Drela (of Daedalus, Decavitator and X-foil
fame). On the other hand it isn't harder to build
other sections that are less prone to ventilate and that are better at higher RE. Yes, laminar sections
are used (I use such sections for instance) but try to look for one that is insensitive to perturbations
in shape and without pressure (suction really) extremes. You will pay with higher section drag (in
the computer) but it will work better in reality. One more thing on this: There is no reason for going
thin (below 11%) from a drag point of view. You will only get structural problems while doing that.
What about asymmetric foil sections for dagger boards? Well, in my view this adds almost nothing to the performance of the boat but it does add extra work in each tack. I have tried to summarise the symmetric vs. asymmetric situation in this document.
Strength and stiffness
My view is that there are really no short-cuts available when it comes to structural engineering. Either you do it properly or you don't. Here are a mere eight pages on the structural engineering of dagger boards. It is not a complete tutorial but rather an introduction to some of it. Beware, I use some university level math.
"regardless of what you have been told, recreational use of mathematics is harmless" /Peter Olofsson
