Effects of Moving Dagger Board on Catamaran? Performance? Safety? Structural?

Hello everyone,

Lots of questions, I know. Sorry about that.

The galley design thread really got me thinking I'd like to move that the dagger board trunk from the center of the hull to - anywhere else.

Issues I can see are:

1) Performance: How does moving a dagger board further outboard by a foot or so affect performance of a catamaran designed to go 18 knots?

2) Extra Force: Moving the board off the centerline means more board will be sticking out of the hull unsupported. This will change loading on the board and also on the trunk that holds it I assume. Potential ramifications?

Here is a the diagram I had originally put up in the galley layout thread. I added two blue boxes to represent where I'd like to move the dagger boards to. Notice the designed location for the boards is just inboard of the blue boxes, in the very center of the galley in the starboard hull.

Any thoughts? Any tips? Any ideas why this could not work or would be a bad idea?

My designer tends to ignore questions like this, so I have to find answers elsewhere.

 

I doubt performance would be effected very much, if at all. Putting boards off center and even angled, as mentioned in the galley thread, is pretty common.

The other considerations are just structural engineering and in some ways easier to deal with than a board trunk in the center of the hull, easier to tie into a shelf or counter for additional strength. I would bet that Kurt has plenty of safety margin in the board strength.

Of course, I would recommend you run this by him for his thoughts.

Good luck, Steve

 

Agreed. I am definitely over my head here so I will just observe this thread.

 

Well, Kurt's a busy guy and he tends to ignore questions like this. He's currently ignoring this one, so I'm out to get opinions of others for the moment.

 

I am over my head but I don't see a problem as long as you keep alignment true and adequate lateral bracing.

 

That's what I was thinking too. Thanks, Hoyt.

A couple extra opinions would be nice. Where is everybody from the Reserve Buoyancy and Hard vs Round chine thread?

 

Hi CB, most of Lock Crowthers cats do this- proven on a lot of boats, could even be better from a structural point as the case exit is away from the hull C/L & area that the vessel will ground/dry out on. The loads of the board are taken mostly by the hull bottom- on my Egan design the core is cut out for a fair perimeter around the case & many layers of 856gm quad are replacing it. You gotta make sure if you've got outboard cant on them that when the're up they dont hang outboard of your gunnel/sponson/belting for snagging piles at wharves etc. Your lifting tackle can also be incorperated into a custom rail/guard/stanchion, keeps your side decks clearer & safer for your pax also.
Thats the only way I want my daggerboards, regards from Jeff.

 

No problem IMHO. You can also consider to move only the other dagger board,no need for that on the port side hull.

 

Check similar threads. http://www.boatdesign.net/forums/multihulls/lock-crowther-biography-30229.html

In post 12 the centerview end profile is shown, but the whole hull(ama) is tilted, with the daggerboard remaining centrally placed.

I think the board should still exit the hull at the hulls lowest point(at the half-hull seam, but could still lean inboard above hull.

 

The vessel represented there is a tri, & they are fixed mini keels. On the foam sandwich Crowther cats I have had involvement with, the designs 85/95 the hulls where canted out 8 degrees from memory & had a laminated timber keel member on the "notional" canted C/L, the daggers where pushed outboard as far as practicable, also displayed on production boats with uncanted hulls such as the Crowther C10 amongst others, contemporary designers use this strategy also such as Shionning, Grainger etc. Regards from Jeff.

 

Wow, excellent advice, everyone. Thank you.

I think I'll move them. I'll send Kurt an email saying, "I'm moving them to this location. If I don't hear back from you, I'll assume it's approved."

Seems to be a common and safe thing to do.

The boards may have another foot (.3 meters) at the most, exposed to the sea's bending forces. It will probably be less than that.

It does seem worth it to have a functional galley.

These trunks are secured to the hull with layers of biaxial tape and all sorts of additional laminations I don't have in front of me right now. As a bonus, I can put a bulkhead in right where the board is and have it span from the board trunk to the outboard side of the hull. That'll hold it in place for sure. The bulkhead in that location should get me a nice, 12ft long, open galley, which is what I'm after.

Thanks so much for the input. This thread has helped a lot. Now I can look back at the galley design thread and give some more thought to those layouts without the trunk right in the center.

 

I would definitely move the galley board but have it canted so that the exit through the hull bottom is not too high up in the bilge as this means a big uneven opening.
Also agree with Teddy I would leave the port one as per plan.

Also consider a single board, port hull only in your case if 5 deg loss in pointing ability is not an issue for you.

 

Hey Cat,

For whatever it's worth, I was on a 42' charter catamaran today (The Gato Verde) and was looking at how she was laid out. She didn't have daggerboard boxes, but the dimensions inside the hulls was roughly comparable otherwise. The walkway through the hulls was outboard of the shower which left about a foot of room at the sole and it was tight, but not overly uncomfortable. My point is, with the wells designed as they are you can walk around them, but a walkway or storage is about all those spaces could be used for. This boat had the galley on the bridge deck, which I know you are trying to avoid. My overall impression of the boat was this it was uncomfortably small pretty much everywhere, for whatever that's worth. I'm 6' 1" and there was nowhere I could fully stand up.

 

CB, another thought is to swap the galley and aft cabin positions with each other. I haven't tried to draw it, but think it would allow you to leave the daggerboard trunnk in the original position. Yes, you would have two sets of stairs, but it could be workable?

Cheers - Brian

 

CT: That is a day charter cat, so it has smaller hulls not designed to carry a lot of weight or be lived in. Big difference.