Assymetric daggerboards

[danielro]

Hello, anyone can explain me the working principle and how them are used (the assym. daggerboards) in association with the canting keel. ¿Why are they called Assymetric?
Thanks.

[SailDesign]

Danielro,
THey are called asymmetric because they are, and thee is usually (always?) one on each side of the boat. The priciple is that if you have a more efficent lifting surface, cambered, toed-in, etc., then you can use a smaller one with less drag overall.
The downside is that you have to tack the boards as well as the keel and the sails. Not for round-the-buoys racing.
Steve

[danielro]

Thanks Steve, i thought that the term asymmetric referred to the only one dagger with a profile that was placed with certain angle inside the case to improve the lift, by putting a wedge or due to form of the case, really in the most of minis 650 i can see only one dagger in front of the mast but i dont know if it's used the shift of the profile to achieve the best incidence angle or the dagger fit exactly inside the case.
Sorry for my english, i hope that you can understand me.
Regards.

[SailDesign]

If you see only one hole, chances are it is not asymmetrical. Especially if that hole is on centreline.

And your English is waaaaaayyyy better than my Spanish. ;-)

Steve

[Chris Krumm]

Has there been any work done with single daggerboards on the centerline that have, say, a +/- 4 degree variation in AOA?

I would think since the best place to carry bending loads in a foil is at it's thickest point, you could make a vertical, sliding bearing here that could allow this small rotation in the daggerboard trunk while still being plenty strong and allow the board to retract.

Sealing the varying gap between the board and the hull bottom might be tricky, but I'll bet the whole system wouldn't be any worse to engineer than a canting keel.

[jonathan]

Does anybody have any idea as to the typical toe-in angle for twin daggerbords, on the kind of wide hull form one might encouter on a Mini, or an Open type boat ?
Thanks in advance.

[SailDesign]

jonathan,
We used 2% toe-in on the Wind of Change 40s. Coupled with a mildly asymmetrical foil (NACA 6412 if memory serves) and sized the foil not so much for required lift, but for required strength at that lift. I'm sure there is some leeway, but it is only noticable at low speed. Above about 8 knots it is just not discernible.
Steve

On the 505 class and some other classes a "gybing " board is legal: it changes angle of incidence automaticaly from tack to tack and can be neutralized downwind.
Twin foils, as in CBTF, may provide the best solution for a canting keel because both foils can be turned a small amount to weather to zero out leeway and can be adjusted for the characteristics of the boat in every condition. Being able to do this allows the canting keel strut to be designed with less wetted surface and a shorter, thicker chord section since it is no longer developing lateral resistance. The hull is also able to go thru the water straight rather than at a slight crabbing angle common for most boats.Also ,if the foils are properly placed a reduction in wavemaking drag can be expected as well.
If the complexity of a daggerboard or twin foils is too much hassle another solution for a canting keel has recently been tried on a 52 footer in Australia called Bondi Tram: she uses wings similar to IACC wings but specifically designed to generate lateral resistance when the keel is canted.Not as high performance as twin foils but somewhat less complicated with no interior disruptions...

[jonathan]

Thanks for your answer, steve. I guess that the toe-in angle also depends on L/B ratio. The "heeled centerline" (for lack of better term) is at more of angle for wider boats, isn't it?

Doug, what about the fact that offset boards can be set at an angle, and thus be vertical when the boat is heeling? Wouldn't that give this arrangement an advantage over CTBF?

[danielro]

Thanks for all the answers,
I suposse that the main question is obtain the equilibrium between strenght, drag and lift, always talking about a close hauling boat. I'm thinking in a mini,... i've read that the canting keel angled doesn't give too much to the overall lift neccesary to sail against the wind so that the lift must be supplied by the dagger, and if the asymmetric dagger is more efficient i suppose that i need less board area that if i use a center dagger, but really the difference will be so important to justify the added weight (two trunks, two boards), the complexity of the building and in the handling.
...Other interesting question, the longitudinal position, and if we must consider that the rudder, normally in a mini we have two, create some lift.
Regards

Jonathan, the CBTF boat can be designed to sail at a more or less specific angle-say 15 degrees. With the twin foils at that angle I don't think the daggerboard, even if it could be kept vertical, would have any advantage especially since the CBTF boat can precisely control the angle of incidence of the twin foils.
I haven't looked into the application of CBTF to a boat with a real wide transom and don't have any half way intelligent speculation on that--maybe CBTFco could help you-contact Bill or Bruce.