External Daggerboards

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It works at about 7 degrees in the Moth class.....

picture by Thierry Martinez:

 

Fanie, are you making the leeboards symmetric or asymmetric?

Altough a bit bigger; Alexander's Multihull Design did design a catamaran, inpired on vikingships, with leeboards and outside rudders but I do not know anything about its performance.

http://www.amdmultihulls.nl/
Look under gallery II and catamaran Stormvogel.

I must have some better pictures somewhere... I think.

Cheers,

Hielan

 

exterior dboars, aka leeboards

it can work, you might want to install a cavitation plate.

 

I can see many benefits of leeboards to a multihull, and almost no downsides.

Wave resistance should not be a significant factor since the fin keel is very short in relation to the hull speed of the boat.

Ventilation might cause a loss of efficiency but I expect this would be easily offset by the longer keel for a given draft (starting from the waterline), increased efficiency of an asymmetrical section, and a fairer hull.

On a multihull we have more options for placing the keel away from the hydrodynamic influence of the hull. For cruising we spend a lot of time running downwind with the keel hauled up and the fairer hull could really pay off then.

Of course, there are the oft mentioned benefits of easier maintenance and repair, a stronger and/or lighter hull and more cabin/storage volume.

The only downsides I can see are aesthetics and engineering challenges of hanging the keel outside the hull.

Anders.

 

The SYZ catamaran foiler uses external surface piercing foils that also provide lateral resistance: http://www.boatdesign.net/forums/multihulls/syz-catamaran-foiler-24428.html

 

Since I'm not a racing fanietic it should work well enough. I have to watch the width since the hulls would make close to trailable width. But a solution would be to have vertical dents in the hull side where the boards sheeth in.

There is basically one advantage and that is nothing goes through the hull, you cannot make water even if you smash a board at high speed.

If one lose "1"kn, that's not the end of the world either.

Thanks all !

 

And who would want to dock this thing? Doesn't look like much of an example for the real world.

 

That's the idea Dough !

Thank you, nice find. For mine I was thinking a bit different setup, but I'll figure it out.

LOL, anything is possible !

 

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First thing that comes to mind , huh? Well, the foils fully retract ,as I understand it.
Heres their new website: https://www.syzfoiler.com/?lang=en You may be right ,though, I can't find a single picture with the foils fully retracted.....

 

Hi Fanie
just a thought, would it be possible to install them on the inboard side of the hull ? they wont look as unsightly that way.

Al

 

Dialdan, I doubt it would be the same, the outside will be better in some weather. Unsightly ? Perhaps... don't plan to be in between others too much.

Perhaps one can make the boards to fold up by swinging on a shaft in a housing. If you do strike something it will fold back and up. Just throwing some ideas around.

 

For a cat that never lifts a hull, a single fold up centerboard might be worth considering.

If you find that it does ventillate (suck air during turns of high load), adding a fence just below the surface is an easy fix.

More effort in one really good board is usually less effort than two average boards and is likely to guve better results.

Initial lee/weather helm trim can be less than perfect on a one-off boat. The centerboard gives you another tool that can be handy for an adjustment. Just design in provisions for small adjustment of the angle of the "full down position".

 

They fold, lift clear. Sorry about image size, can't seem to copy full size ... but you can see them lifted clear of water surface.

 

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Thanks Gary-I couldn't find an image that showed them retracted....

 

Not so much wave drag as increased induced drag for leeboards, I think. I believe the hull can double the span efficiency of the board. Thus lightly loaded rudders won't probably suffer as much as leeboards.
I think it was discussed in the thread "foiler design" or similar a few years back.