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Old 03-04-2008, 13:43   #1
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Daggerboard design

Are there any guidelines or suggestions as to the sizing of daggerboards for a cat, the pointers I'm looking for are:

Area
Chord
Aspect ratio
Depth
Profile (symmetrical)
Placement fore/aft and offset from centreline

Also any experience with trunk design and lifting/dropping rigging would be appreciated.

Regards

Alan
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Old 03-04-2008, 14:25   #2
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Hi Alan - not being a naval architect, I cannot speak to most of these considerations, but I bet somebody else will!

I can offer you a bit of info on a couple items.

Off setting the boards from centerline seems common to keep the trunks from being in the middle of the living spaces. The Catana boards are placed outboard such that the outboard side of the trunks coincides with the outboard edge of the hulls. It seems they could be "justified" to the inboard sides as well. This also has the advantage of not having the opening for the boards right at the bottom of the hulls if the boat can be beached. I've seen some boards angled a bit off vertical athwartships apparently to make the trunks conform to the outboard angle of the hulls.

Designing the trunks to be stronger than the boards seems intuitive - sacrifice the board instead of the hull in the event of collision.

The lifting mechanisms I've seen are remarkably similar, although I certainly haven't seen them all. A line attached in some manner low on the board runs vertically up the trunk in a slot, around a block below deck which is routed through a chase to a winch aft. Rope clutch to hold it up. To raise the board higher or remove it you have to lift it from the top. Drop by gravity. The Catana boards have holes in the bottom so they flood with water so they won't float in the trunks.

Hope this helps,
Dave
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Old 03-04-2008, 14:49   #3
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Much the same, except the downhaul/uphaul systems I've seen were entirely above the deck.

In general, high aspect ratio's are more efficient. (Long with narrow chord) And most designers I've had anything to do with seem to use NACA 10 or 11 sections.
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Old 07-04-2008, 10:52   #4
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Alan,

Concerning area, I have read that "underwater appendages" by which it take it to mean daggerboards and rudders, should be 2.5% of sail area on a cat. Can't remember where I read it but the numbers stuck in my head and seem to reflect those of my own boat. Mine are offset outboard of centerline and are NACA 10 sections with a chord of 70cm. Depth is 170cm but I will be extending this just a bit. Hope this helps.

Mike
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Old 07-04-2008, 15:14   #5
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Quote:
Originally Posted by mikereed100 View Post
Alan,

Concerning area, I have read that "underwater appendages" by which it take it to mean daggerboards and rudders, should be 2.5% of sail area on a cat. Can't remember where I read it but the numbers stuck in my head and seem to reflect those of my own boat. Mine are offset outboard of centerline and are NACA 10 sections with a chord of 70cm. Depth is 170cm but I will be extending this just a bit. Hope this helps.

Mike

Thanks Mike,

I just did a quick calculation for my design.

Daggerboards: 2.125 x 0.7 x 2pc = 3.655 m2
Rudders : 0.85 x 0.4 x 2 pc = 0.68 m2
Vestigal keels: 1.3 x 0.3 x 2pc = 0.75 m2
TOTAL = 5.085 m2

Sail area: Mains and masts: 162 m2

Ratio is then 3.13% with boards fully down

Even without the vestigal keel areas I'll be above the 2,5%

Anybody have comments to these figures?

Regards

Alan
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