Cruising Catamaran with Center Board

[gonewalkabout]

Under the heading of 'there are no new ideas' ...
I am in the initial designs phase for a 36 to 40 ft. sail catamaran strictly for cruising.
I am up against the dilemma of daggerboards vs keel/skeg. I like the idea of a shallow draft for beaching and repairs in areas that don't have facilities but don't like the 'internal space' that goes with them. I can't seem to find a definitive answer as to how much performance hit I'll get going to a slightly deeper keel and doing away with the daggerboards.
SO ... is there such a thing as / am I insane for thinking that a single center board for a cat? It seems to me that the weight of 2 daggerboards, sleeves and lifting rigs would leave me lots of room. The idea is to hang a hinged 'gimble' under the bridge (possible as a part of the mast step. This would transmit forces up the mast and into the rigging. The gimble would allow the board to be 'pulled' and fold flat against the underside of the bridge (and, with careful shaping) progressively reduce the depth of the board.
The obvious problem would be the forces involved but I have lots of weight allowance to move to aluminum and carbon. Some other pros / cons I thought of:
- by moving the board inside the hulls the 'tripping effect would be reduced.
- carefully shaped the board wetted area could be reduced gradually.

On the con side ... I'm not sure a single board might not create a 'pivot' that would create all kind of issues.

I'm think I might build a simple prototype just to see and the link below is my 'musings' this far. Any feedback would be appreciated and if you think I'm just smoking crack feel free to say so!!!
http://www.walkaboutcanuck.com/Saili...er%20Board.htm

[Chris Ostlind]

Well, yes, you can do this as a solution. It does have some very real issues for which you will need to find secondary solutions, such as: 1. A surface piercing foil will tend to ventilate at the types of speeds you may see on this boat, robbing lift and causing the boat to fall off upwind. The immediate solution is to make the foil deeper to compensate for the loss in efficiency, but that feeds directly into the next item on the list... 2. There will be a significant side load on the structure of the foil and its mounting system which will have to be overcome with heavier build techniques and some very good planning/engineering.

If you are not planning to fly the hull on this bad boy, perhaps you could consider a single dagger/centerboard? You can pick the hull in which to place it where it will intrude the least in your interior space planning. There are several cats running around with this system already and they have no problems with upwind performance. You will be sailing the boat more conservatively than a speed burner, but the drawing you show does not indicate that hauling ass is a criteria near the top of the list.

[brian eiland]

Have a look here to a suggestion I made in 2003:
Big Cat, alt CB's & sail rigs

or here:
http://www.boatdesign.net/forums/sho...3&postcount=30
http://www.boatdesign.net/forums/sho...8&postcount=36

Then look at "ASYMMETRIC CENTERBOARDS" here:
http://www.boatdesign.net/forums/sho...9&postcount=16

...and finally on my latest DynaRig catamaran design:
http://www.runningtideyachts.com/dyn...enterboard.php
http://www.runningtideyachts.com/dynarig/

Obviously I think this makes sense

[eponodyne]

I've been thinking about the same sort of thing for my mad-scientist kite-powered catamaran (yet to be built, but blistering fast inside my head). Not a cruiser, but an all-out speed machine; hopefully capable of longer passages; we'll see.

Anyhoo. I like the idea of lower wetted area given by a single daggerboard-- but didn't want to give up the option of asymmetrical boards for better windward capabilities and beign able to haul up the leeward board for balance. My shadetree-engineering solution was actually kind of simple:

Since the hulls will be asymmetrical, with their inner surfaces flat, I reckoned it would be a simple matter to move each daggerboard inboard a foot or so and mount them inside trunks built ONto the flat of the hulls, rather than inside the hulls; stbd board mounted on the stbd hull's port side, and vice-versa. That way I lose no interior volume, and since the hull itself doesn't need a slot cut into it, its integrity is maintained. There might also be a slight strength advantage in this method as well.

[Pericles]

eponodyne,

Internal leeboards eh? Up and down or rotate?

If you are kite powered, why dagger boards at all? It's all downwind.

Pericles

[eponodyne]

Originally Posted by Pericles

eponodyne,

Internal leeboards eh?

External daggerboards, as stated above.

Up and down or rotate?

Up and down, by definition.

If you are kite powered, why dagger boards at all? It's all downwind.

Pericles

Not necessarily. I can work to windward fairly well on my kiteboard; 6 points maybe. If I hang the kite right on the edge of a stall, I can "slingshot" myself around it and work to windward a little bit higher, but it's pretty tiring.

[brian eiland]

Originally Posted by Pericles

If you are kite powered, why dagger boards at all? It's all downwind.

Even on a 'beam reach' with the kite, a 'board' would be advantageous in reducing sideways drift.

Fashioned right it might even provide for some addition 'lift' to the intended 'windward destination'.

Originally Posted by eponodyne

Since the hulls will be asymmetrical, with their inner surfaces flat, I reckoned it would be a simple matter....

I would NOT be so set on asymmetric hulls, they are not that efficient, and they loose a lot of volume for their immersion. If you did go this route, I would have a look at the Prindle 18 asymmetric hull, and notice in particular, the manner in which the keel lines 'toe-in'.....one of the very best asymmetrical designs in my opinion.

[gonewalkabout]

First ... I've only just joined this forum and let me say the speed of response and the technical detail and knowledge is really something compared to most.

In no particular order:
- Yup ... this is meant to be a strictly cruising, blue-water, liveaboard. It also will sail solo most of the time. Safety is my first issue always. The first thing that hit me in this design is to reduce the 'tripping effect' that seems to be the biggest contributor to going turtle. (having said that I can be persuaded that this is an issue for racers and the old days only). I'd assume a single board would have the same issue (at least 50% of the time )
- The surface piecing issue ... The idea is to go fairly deep. The foil would be below the DWL. The 'rudder post / center board post' (I'm not sure what you call it) would be shaped. BUT (just adding more discussion) what would be the trade offs of putting a chine/deep knuckle above the DWL of the board to deflect the internal wave and reduce pounding?
- I like the asymmetric board idea ... I'll have to look at the links everyone provided.

The one thing I mentioned no one has this commented on is 'hanging' this rig off the underside of the mast step? This would serve a couple of purposes ... it would transmit the side loading already mentioned as a big issue into the rigging (but I have no idea to what effect) and would allow for a fairly sturdy gimble arrangement given the weight I've recovered from the existing boards/sleeves/tack and it 'middle of boat'.

Thanks again all.

[eponodyne]

Brian,

Since right now my only investment is in skull-sweat, my current thinking is to take one of John Hacker's designs, one of the narrower ones (currently thinking White Hat) and split it down the middle, separating the hulls by about 5 feet and putting a fairly small pod in between them on a bridgedeck where I and one crewman can sit in tandem. Mount the kite controls on a semicircular traveller (sourced from McMaster-Carr) so that distance from the kite controls to the load point is always constant, yet the point of greatest strain on a reach can be all the way to leeward, only about a foot and a half or so above the waterline (reduced heeling moment). This might also be advantageous in a capsize (Although my mad seamanship skyllz would of course never allow this to happen) in that the kite could be re-launched and used to hoist a hull straight up out of the water, then simply pulled over with the breeze.

If I'm very careful, I think I can reduce the scantlings and planking to get the weight down considerably and still maintain a good margin of strength. I don't know enough about foils to start putzing with them on a more than very limited basis, so a planing hull seems to be my better bet. And obviously this isn't designed for open ocean.

We shall see what we shall see. Evenutally, bye-em-bye, ev'y t'ing gwine be irie.

[Trevlyns]

Chris White’s book “The Cruising Multihull” features a very interesting 29 footer (his design) with a single daggerboard and rudder but I can’t seem to find it on his website. Try contacting Chris through his website.

[tspeer]

Cruising cats have successfully used a single board mounted in one hull. The asymmetry is really not a problem.

The issue of internal space can be solved by making one side of the board trunk the exterior skin of the hull. The the only hit to the space inside is a somewhat narrower width to the accommodations, but no obstruction to work around. If you use a daggerboard, then you can arrange the trunk so that in the event of a collision, if the board breaks through the aft end of the trunk, it will be outside the watertight skin and not degrade the integrity of the hull.

The drag due to side force of a board or keel is inversely proportional to the square of its depth. So the performance is worth it to go as deep as you can stand. A modest increase in depth can make a significant difference when going to weather.

[rwatson]

Derek Kelsall has been designing single centrboard cats for years with a great deal of success. Look up Hydroptère for a real successfull affort of a multihull with 'daggerboards' :-)