asymmetric daggers

I suspect that you will get a bigger boost to your windward performance by making the boards longer than by changing the section shape. The section shape determines the profile drag, which is a comparatively small portion of the total drag, and the differences between two decent sections (be they symmetrical or asymmetrical) are even smaller. But the drag due to lift on the daggerboards is much bigger and is inversely proportional to the square of the depth of the board. So even a modest increase in length can result in a significant reduction in drag.

 

when you cut off the trailing edge, do not round the corners off, but rather leave the little flat on the trailing edge as square edged as you can. It will have less drag and affect the upstream flow on the surface much less than if you round off the trialing edge. Cutting it at a 45 deg angle as Mr. Woods suggest is also an excellent suggestion.

 

reducing the the thickness by multiplying all of the coordinates by 0.9 should work fine, this is done all the time. It will likely results in lower drag section as well and a bit more stall resistance. Not likely the properties will change enough to affect you. But as you know, it will have to be heavier with more reinforcement to keep the same strength.

One thing to consider is that is a laminar flow foil, its desirable properties depend on have a clean and smooth surface to maintain laminar flow over the forward part of the foil. If you routinely keep it clean and polished (as the C cat racers do), this should not be an issue. IF it is not practical to keep the surface clean most of the time, it will behave very poorly and the non-laminar characteristics can be unpredictable. You would be better off to choose a foil shape that performs well in the normally turbulent flow conditions found on the underside of most boat hulls.

There are a few laminar flow foil shapes that are specially design to also perform well in both laminar and turbulent flow fields, they compromise the performance somewhat is both flow conditions, but are reasonably good performers in either mode. You will have to find one of these relatively new technology designs that are specif to your application, it may not be worth the trouble since the designers that can develop you these shapes charge you for the service.

Personally, since I hate doing a lot of boat maintenance (and I have never owned a boat as large as yours), I would choose a foil that was designed to perform well in turbulent flow conditions. They are much more tolerant shapes of surface irregularities and imperfect surface finish and foil shape. You will not get that extra little bit of drag reduction of the laminar flow section when it is clean, but it will perform reliably and predictably in much more realistic flow conditions. For recreational use, not an all out racing machine, the non-laminar flow foils are the way to go. the good old NACA sections will work very well for this kind of application.

 

As promised many months ago I enclose a photo of the completed asymmetric dagger alongside the old one it replaces. Last Sunday was the 1st chance to test it in fresh winds against other high pointing boats and I was marginally pleased. The improvement in pointing ability is not as much as I had hoped for but there is a discernible improvement and we could hold the same height as a Beneteau First 35 while sailing at 3 knots greater boat speed. The big bonuses come from improved weight (19kg against 85 kg), the ability to lift it clear of the water, and the ease with which we can raise or lower it.
Since it was entirely experimental I only built 1 dagger and now I am ready to build the second one with a few tweaks. Hopefully it will be a lot faster build.

 

Good work. What section did you go with?

 

Asymmetric foils are definitely superior; have put them on half a dozen multihulls and they do perform.
Catsailor, on your second foil build, why not try an elliptical tip - meaning make the foil longer/deeper - and then you can compare the difference on each tack. Think you will find the deeper board is the way to go.

 

Looks like the new one is over 30% deeper than the original. Do narrow boards stall in very light winds?

 

It is deeper but a little difficult to say how much - because the anti fouling on the old board appears to stop before the total immersed depth is reached - meaning that the packing blocks at bottom are higher on the board. The owner can probably correct me there. Actually, if I'm seeing this correctly, there must be gaps around the old board at hull exit point - that will definitely hurt windward performance.
A deeper board of high aspect ratio, imo, is at its best in light airs; when the breeze is up, you can reduce depth, retain high pointing, reduce loads.

 

In answer to various questions
I got scared at construction time and went with the simplest shape I could find the N10-il which I set at 2 degrees AOA. As it turned out I could have been more adventurous as the vacuum bagging easily pulled the carbon into the shape.
The working portion of the dagger is the same depth as the previous one giving me a draft of 1.8 metres. Since the daggers are the same length as each other and when fully lowered come flush with the deck it follows that the projected portion will be the same. I was keen to keep maximum draft to 1.8 metres as one of the races I do every year has a sandbar at one point which comes up to about 2 metres at low tide.

Gary is correct and there are gaps around the board at exit point. Next time I lift the boat I will fair in the slot around the new shape.

I will need more time on the water to find out how much performance we have gained or lost in various wind speeds and what is the optimum depth for the dagger to be set.

 

Before we put this topic to bed I would like to get some input for the down and dirty, recreation not race set.

My first question is conceptual -my idea of the difference between cutting edge race foils and the profiles that would be best for recreation is the distance from leading edge to maximum thickness. Racers are much more optimistic about how far they can maintain laminar flow. Is this the right way to think about this? The other thing I expect from a less aggressive (optimistic?) profile is a higher angle of attack and or lower speed to stall.

What profile would be best for an asymmetric daggerboard in turbulence or low maintenance?

Would taper and or sweep help a high aspect ratio foil like they do on low aspect keels?

What is the best crude way to end the board? I consider elliptical tip too difficult to do right and suspect approximations are detrimental.

 

Okay, I'll have a stab: if you were wanting the simplest asymmetrical, low maintenance, build shape and finish, then an ogival cross section would be the way to go. Ogival is completely flat on the leeward side and a segment of a semi-circle on the lifting side. This foil shape is what the Grogono brothers used on their pioneering Icarus cats - and their foils worked fine. Also David Keiper had the same foil design on again pioneering ocean crossing tri foiler Williwaw.
The sharp leading edge; need to soften it to reduce stalling; angle the foil and case at a couple of degrees. Keep the boards vertical, or close to. Cut the tips flat, aligned with water flow.