Rotating Dagger boards on a cruising catamaran

[Becaris]

I'm busy working on my design for a 60' cruising cat and I have been reading the benefits/issues between using shallow keels or dagger boards. There is plenty of discussion on that comparison, so this isn't the question.

For my own reasons I have settled on using dagger boards. Now I am looking at how I want them to work. One of the issues with dagger boards is that the cases take up space in the hulls, and this is space I don't want to lose.

I have been considering rotating dagger boards, the cases would be horizontal and stay below the level of the floor. I have quite a bit of space in this 60' cat below the floor, but I would like to know how wide and deep I would need to make the dagger boards when fully rotated down for optimal performance.

Also... I can't seem to find anyone making rotating dagger boards, though I have found large mono hulls using rotating keels. If they can rotate a heavy keel up and down, why can't we rotate much lighter dagger boards?

[CatBuilder]

Here is a point of reference for you regarding the size of a dagger board, relative to a hull:

http://www.multihulldesigns.com/images/45bdcatSAIL.gif

I don't see any reason why this wouldn't work. Added complexity, but really nice idea for gaining interior space.

Possible disadvantage would be actively using the boards while sailing. Would you need to go below to operate them?

How much space do you have below that cabin sole in the 60 footer? I know I might have about a foot of space below the cabin sole in my 45 footer, but my dagger board is 3' from leading edge to trailing edge.

[Becaris]

I have about 33" below the cabin sole.

[CatBuilder]

Originally Posted by Becaris

I have about 33" below the cabin sole.

Hmmm.. my boards are 36" on the 45' boat. If you have the same relative depth of boards as I do, I'd be willing to bet you can't fit your boards under the cabin sole when they are on edge.

Alternatively, you could have very long (deep) boards to get the same surface area for less distance from leading edge to trailing edge. Then, on fold up, your problem might be you have no bilge at all left for tankage and may also impact a lot of bulkheads.

Still a good thought, I think.

It'll be interesting to see where the thread goes.

[cavalier mk2]

I think you are talking about centerboards. If you use a shallow keels for beaching they can also extend the trunk below the hull for more room.

[Becaris]

My boat doesn't have shallow keels. I was out at the boat yard a few days ago and measured 36' from floor to bottom of bilge, and more like 37" including through the hull. The boat drafts about 25", so I could get the pivot point above waterline. Of course, these should probably be called centerboards if I'm rotating them, but they aren't in the center of the boat. I guess they are sort of in the center of each hull .

Anyway... I'm leaning toward doing something like this, and wonder if anyone has done this before on a big cruising cat? I can find plenty of examples on small beach cats, but nothing in the larger size boat. Now, there are plenty of examples of mono's with rotating keels. I figure, if they can do it with 2000 lb. keels, I should be able to do it with 30 lb. carbon fiber centerboards, and with much less machinery. However, I can't seem to find any examples of this done on other big cats, so I guess I'll have to have it all custom made.

I also want to make them so that they can kick up if I strike something, which would be another advantage to doing it this way over typical dagger boards. I need to come up with a very simple mechanism to accomplish this, as well as how to raise and lower them into position, and avoid 'leaks' as much as possible. Any suggestions?

[Becaris]

Here is a picture of where I would be putting the rotating centerboards (with it up and down in the diagram).

[redreuben]

My suggestion would be add simplicity, just use one board under the bridge deck in a "v" or gull wing style pod, plenty off space for kick up, easy to service, no big long slots in hulls. You could either take pod down to waterline, hence gull wing suggestion or put a fence on top of board at waterline, because space is less off an issue in centre location you can add a bit more depth to counter any ventilation issues.
RR

[rberrey]

It would be nice if someone would come up with a telescoping dagger board. Rick

[redreuben]

I think you just did !

[Howaya]

There is a brief description of centerboards, with pictures, in Chris White's book The Cruising Multihull This may offer some ideas and issues of concern for you.

From experience whilst racing a Stiletto 27 in light winds, I wouldn't recommend Redrueben's idea to place a board in a pod between the hulls; that arrangement will not allow your boat to point as high as boards which extend from hulls. You might do some research about the Tom Cat 9.7 design to confirm or deny this.

Generally speaking, I think pivoting centerboards from the hulls will offer you a good solution, especially if the pivot is above the waterline. Being able to refine the location of CLR is an advantage over daggerboards. Gemini cats function this way and point fairly well.

[Gary Baigent]

Becaris, I'd stick with true daggerboards but ... set them and the cases close to the hull interior sides; you're not going to lose much interior space with that setup. They can even be angled slightly inwards to align more with the slightly flared hull shape, like in the photograph, giving you more space plus getting a little lift from the angle. Agreed, this is a much smaller boat than yours but the same thing applies. I know you're not thinking of foil assist - but you get a little so might as well use it. Daggers, although they'll have to be long ones in your case, are much superior to centreboards, better shape, (yours could be asymmetric for even better windward performance), much easier to clean and maintain. Centreboards are bastards for getting marine growth in the cases, plus you have to have a very long slot which causes drag, noise ... and slowness.

[Bruce Woods]

For inspiration check out the swing boards on the olympic tornado or hobie 17.
Nothing slow or inefficient about either boat.
The light weight sharpie also has a swing board . A vessel noted for its windward performance.

Regards

[Gary Baigent]

For sure, Bruce, the 6 metre Tornado has efficient swing boards with double flap gaskets covering the slot which is roughly, less than half a metre distance but ... we're talking about a 60 foot boat here, and that slot/double gasket is going to be loooong ... and gaskets, don't care what you say, are still draggy and a pain to maintain and, most of all, they don't compare to the simple, efficient and zero drag of a daggerboard exit.
Also you won't see a modern, high performance version of the 40 year old Tornado, carrying centreboards.

[catsketcher]

I don't really think centreboards are better in a cruising boat than dagger boards. I was on a tri the other day with centreboards and we couldn't get the board down due to a very small amount of growth in the case. I wanted to get the top off and look into the case but the owner said it was a pain to get watertight. Then there is the pin and the control lines too.

The idea is that you will hit objects from front on. Many times you will not. If going slowly you also hit with a sideways component. On top of this most of the time you will not have the boards down at all - going downwind needs no boards at all.

So go the daggerboard. The boards can be made close fit and be made totally silent and easy to operate.

cheers

Phil