Trimaran:relocation of daggerboard

I have a 30ft Hartley trimaran, 1980's, epoxy/ply

The original design did not, I believe, have a daggerboard. In conjunction with Harltey by post, the builder planned and added a daggerboard case in the main hull, right in the middle of the saloon. The mast is stepped onto the top of the case. Apparently the windward ability is much improved ('scuse the pun).

This arrangement, while efficient, makes for cramped camping. My question to all is around the repositioning of this daggerboard. ideas I've had are: centreboard case instead (not ideal), replace daggerboard with two smaller cases and boards in the amas (like this idea), remove daggerboard entirely and replace with nothing, or perhaps with short fixed blades on the bottom of the amas.

Any thoughts? Anyone done this?

Thanks,

Danny Bishop
Wellington, NZ

 

Boards

Seems like the least hassle might be the fixed boards under the amas. Have you considered a single low aspect fixed "keel"?
Of the options you mentioned I'd think the best performance would come from daggerboards in each ama. That is,in essence,the way it is done on the Rave hydrofoil; no central board at all.

 

I was sailing on a (friends) trimaran with its (a-symmetric) daggerboards in the amas. His tri wasn't a Hartley design.
I liked the arrangement, providing at least some more space in the main hull, until my friend balanced over the net to raise the windward board out of the ama. It went well, but it didn't look very safe. I would definitely design a remote pulley system for the daggerboards (up and down).
Also, I do assume that your amas are completely enclosed. Glassing in a pre-fabricated box for your daggerboards will only be possible until you cut some holes into the decks. Note, with your head upside down, working in an enclosed chamber filled with styrene vapor, you'll require some type of breathing system.

 

Ama Mounted Foils

If you put the boards in the amas, not only will you have some interesting retro-fitting in front of you, but you will also have to look at the added loadings that will be created at the ama/aka juncture.

Virtually all of the lateral forces will put a premium on the existing joints. When retro-fitted in this fashion, it's not uncommon for triamrans to experience connection failures. These failures usually happen at the least opportune time, as in when you're out smokin' along in large swells and strong winds. The result is shear panic until the boat is brought over on the other side and things get lashed together for the trip home.

See connection failure report on Cheekee Monkee http://www.cheekeemonkee.com/documents/Round_the_County_2004.pdf

Still, don't be bummed. Just get it properly engineered and get on with it.

Or, allow for Doug's suggestions to do a nice hard keel fin on the amas and forget about crawling inside the amas with a bucket of resin on a hot day.

Chris Ostlind

 

If you go with twin boards in the amas and you want to preserve your windward performance, keep them the same depth as your existing board. Short boards will not go to windward well.

Daggerboards in the amas also provide less roll damping. When the boat rolls, the boards are moving more up and down instead of side to side.

Instead of getting rid of the daggerboard, you might consider reshaping it. The Farrier tri's have a daggerboard going through the cabin, but it's not as big an obstacle as you'd think, because the board is swept aft. This puts the daggerboard farther forward in the cabin, while putting the area of the board back under the mainsail.

A similar swept case could open up your cabin. You might still be able to use the daggerboard case to support the mast. If your current daggerboard is vertical, I would guess that it comes up behind the mast. A swept daggerboard case could have the opening on deck ahead of the mast, and the opening through the hull at or just behind the mast. This will move your board forward by at least the chord of the board, which could make a big difference to your interior.

I'd draw a line down your current board one quarter of the way back from the leading edge, and rotate the board about the middle of the exposed part of this line when sweeping it. And I'd try to keep the depth the same, which might require a bit longer board.

Another option is to offset the daggerboard case to one side. This won't have an appreciable effect on the performance, and can give you at least one generous sided berth beside it.