Diagonal daggerboards

[weys]

At the moment as some as you know I am designing a big cat for my study.

I would like to have diagonal daggerboards , with that i mean daggerboard that don't go straigth up thru the cabin area in front view the dagger would be like this

\-/

what you see is the left hulldaggerboard wingdeck en right hulldaggerboard.
Does any cataran used this and what are the disadvantages ?

one negative things that it produces less lift ?

anybody who has an idea please let me know!!

[sigurd]

Why do you have two daggerboards?

[sigurd]

ps. check out techniques avancees and the "little americas cup" thread over at sailing anarchy where banana boards are discussed a bit, if you have time.

Formula for loss of horizontal lift and gain of vertical lift is presented here, in "sailboats", in a thread called something like "upwind foiling best of all" not too long ago. Seems like not much horiz lift is lost with moderate angles. but the one to windward would suck down the hull, maybe that is a significant penalty?

[alan white]

Originally Posted by sigurd

Why do you have two daggerboards?

Yes, why do you have two? Is this related to pod shape and space considerations??

[sigurd]

Btw a kickable centerboard is safer and only takes space in the bottom but then less bury. long slit=some drag.

[catsketcher]

Hello Weys

Most cats in Australia have daggerboards as you have indicated. Lock Crowther was a fan of them and these inward canting daggerboards featured on many of his designs. Although on my little folding cat I have one daggerboard there are good reasons for two inward (at the bottom) canting daggerboards.

The first is to reduce waste of interior room. They are much easier to design accomodation around. There is a little issue about not being able to get into marine pens with pilings with the boards up so don't cant them so much that they extend beyond the hulls Bmax when up.

The second good reason for canting is so that transverse capsize problems are reduced. One of Australia's best multi racers, Paul Nudd, used to race a Seawind 24 which has very canted daggerboards. He found that when pushed hard the boat leant over, the leeward board became far less effective and skated to leeward whilst the windward board dug in and pulled the windward hull back down. Draw it and think about moving sideways.

So go ahead. Look at a Seawind 24 and other cats and learn from them.

cheers

Phil Thompson

[Trevlyns]

My understanding is that only one (windward) is used at a time. Because of this it can be asymmetric in section shape, generating more lift to weather.

[tspeer]

Originally Posted by Trevlyns

My understanding is that only one (windward) is used at a time. Because of this it can be asymmetric in section shape, generating more lift to weather.

You may have some misconceptions on both counts. I suspect that if they use one at a time, it is the leeward board because it reduces the displacement instead of increasing it and the board is in the right place to balance the helm.

Asymmetric sections do not generate more lift to weather. The lift produced by the board is equal to the loading from the sail trim, and leeway adjusts to ensure this is so. Asymmetric sections can make the board more resistant to stalling, and center the minimum profile drag over the design operating condition for slightly better performance.

[Richard Woods]

I think it is a rare monohull that can sail to windward with its keel vertical, even so, monohull sailors claim their boats sail to windward better than multihulls do.

So in practise there is no problem using angled boards. I use them on most of my designs as by doing so there is more room for accommodation in the hulls. But keep the angle to under 15deg, 5 -10 deg is better.

I have also found that angled boards do not bang so much, so are less noisy and allow one to sleep at night. (I crossed the Atlantic last year on a catamaran with vertical boards, the banging drove us mad). Read a report of that trip on

www.sailingcatamarans.com

Hope this helps