Catamaran C/board Position?

Hi was just wondering if anyone could help me i have a 28 foot racing cat i have designed and am unsure of where the exact fore/aft position of the boards in relation to the CE.. i.e. lead length to achieve a balanced helm should be. i currently have a lead length of 6% with the CE aft of the CLR.. could anyone shed some light on this subject and will anything change when flying a hull?

cheers

bob

 

The general rule-of-thumb for catamarans is to put the center of effort of the sailplan directly above and in vertical line with the center of area of the daggerboards--that is, no lead forward or aft. The fact that the daggerboards are offset to leeward makes up for the proper balance.

Eric

 

need input on design

I too am looking for info on where to put the daggerboards. I have a 27' home built cat that has never had much lateral resistance. Im looking to increase her performance, but not sure which method is most suitable. My thoughts are to fasten dagger boards to the hulls on rotating plates so they may be swung up or down. In trying to maintain her nice lines I would rather put the boards on the inboard face of each hull. Does this create a problem with lift? Can the windward board be used only, as the hull would provide the likely necessary suport? Should the boards be syemtrical or asymetrical? Any other information that you think may be beneficial to a novice would be appreciated.

 

In an ideal world, the best place for daggerboards or centerboards is in the middle of each hull, on centerline. The leeward board is usually the one that is used on any given tack. The boards can be asymmetrical and even toed inward about 2 degrees each, leading edge pointing inward toward the boat centerline. The boards can also be canted inboard (tip closer to the boat centerline) about 5-10 degrees or so to give additional upward lift.

Daggerboards should really not be put on the side of the hull; it's a very bad place to put them--they don't created near the amount of lift for a given draft that they should when mounted that way, and you pick up a lot of wave and wake drag from the board piercing the water surface.

Eric

 

cheers mate thats a big help!

 

no perfect world here

I understand the ideal situation, but short of that will the additional drag outweight the benefit of being able to point a little better. If not is there any huge dissadvantage to fastening leeboards to the inside as opposed to the outside face? I will reconsider trying to center the boards, but Im really trying to avoid surgery below the waterline.
Thanks

 

Cat daggerboards

Thanks to Eric W Sponberg for your guidance.

Could I ask for your thoughts on 'rule-of-thumb' for optimum area for cat daggerboards. I have, at present, rectangular boxes in each elliptical hull - about 40" X 4" , and full depth of about 6 feet. These are canted in about 8-10 degrees, and I'll also go for an asymmetric profile, once I find a shape I can readily construct and repair.

:?:

 

Bilbo,

I think there is a lot of variance on how much area you need for daggerboards, depending on the designer and what the boat is to be used for. A brief inspection of a variety of designs looks like the daggerboard should comprise between say 20% and 40% of the total underwater profile per hull. And that is a really rough estimate. Others visiting this forum may have better advice.

For Capt. Mayhem,
Any daggerboard that is attached to the side of the hull will always have more drag and less pointing ability than a daggerboard that goes through the bottom of the hull. If you must, it is better to put the daggerboard on the outside of the hull since it is slightly more submerged on that side than on the inside. This is not too big a deal when you are sailing upright, but if it is a small boat in which you intend to fly a hull, then there would be a significant difference between the inside and outside location, with the outside location, to my mind, being preferred. This assumes that the structure holding the daggerboard has some fairing in front of it to prevent excessive drag on the more submerged side. If you want to put the daggerboard on the inside, then make it somewhat longer to compensate for it being somewhat higher out of the water as the boat heels.

Eric

 

What about boats with a single daggerboard in one hull, as in some Shuttleworth designs?

Ray

 

I find it unusual that a catamaran would have only one daggerboard, and I am not all that familiar with all the details of Shuttleworth's designs. But I would suggest that you put it where he specifies--he would know best.

Eric

 

That would certainly be the best idea, were I building a Shuttleworth design.

Apparently this is not a terribly unusual approach, at least from my research. Thomas Firth Jones puts single daggerboards in his cats, and if I recall correctly, the original MacGregor 36 had a single board. The Mainecat 30 has a single board.

Anyway, Jones' cats seem to sail well enough, and he reports no unusual helm problems, or differing helm on either tack. This would seem to suggest that two boards are not necessary to provide a balanced helm.

Here's what Shuttleworth had to say about the design of a 70' cat:

In order to determine the optimum underwater configuration, a series of tank tests were carried out at the Wolfson Unit in Southampton, England, with the assistance of Andrew Claughton and John Flewitt, both of whom have extensive experience in tank testing, from Flying boats to Americas Cup contenders. Tests were conducted for various angles of heel and yaw, in flat water and in waves, to check optimum bridgedeck clearance, keel configuration, and balance. All tests were recorded on video.

We tested:

* Deep fixed keels
* Shallow fixed keels
* Winged keels
* One daggerboard (downwind and upwind)
* Two daggerboards.
* 3 different hull forms

The results of the tank tests were fed into a Velocity Prediction Programme (VPP) and the polar performance curves were derived for the boat in various wind speeds. The results showed that a single daggerboard was the best option. However as a compromise we decided to use two shallow keels to protect the rudders and propellers, and then to have one daggerboard in one hull to give extra lift upwind when needed. The shallow keels are also an efficient way of increasing the buoyancy of the hulls, thereby increasing the payload capacity.

 

Eric,
As I understood from your posts above overall clr must be aft ce? I get results like that If I place the dagerboard so it is in one vertical line with CE. Do I understood you corectly?

 

To Rayaldridge,

Sounds like Shuttleworth did some homework to arrive at his answer. I have been to the Wolfson Unit myself and have worked with Andrew Claughton and know they produce reliable results. So this is good to know.

To Tamkvaitis,

Yes, you understand correctly.

Eric