Canted Centerboard

[billgow]

Anyone have any ideas or opinions about canted centerboards?

I'm thinking this might be a good way to save a little interior space and maybe get some lift at the same time. I'm concerned about durability, though. I uploaded a couple pictures I made with SketchUp to give you an idea of what I'm thinking about.



This would be for a 36' catamaran. In my drawing, the board is 1' 5" wide, 6' 8" long, 2 3/8" thick and ~28 degrees off center. I'm thinking I should go a little wider, maybe not quite as long and not such a steep angle. My criteria for this one was to keep it confined to the space between the bottom of the hull and the cabin sole (~18"). Increasing width and decreasing angle will mean a taller pocket but I think I can live with that by making the top of the pocket my bottom companionway step and extending it all the way to the outer hull wall.

I really don't want daggerboards running through my hulls and I don't want anything to do with fixed keels. I'm looking to maximize performance and still have something habitable.

[catsketcher]

Canted daggerboards are a good thing. In some early cats the boards used to run right through the interior. Eventually designers put them at the side of the hull and tilted them over. I would put the bottom exit of the case further to the side - My board exits the hull about 350mm from the centreline and the top is just inside the gunwale.

I would caution against going too far with the cant. Cats over here with excessive cant are pains to get into a pile berth with the boards up. Also I was pushing hard down on a friends board and then it suddenly popped down. I was pushing hard on the board top about 500mm off the gunwale edge. When it slammed down I toppled in the drink. It was a calm day on the lake so I didn't even lose my sunnies but I like the tops to be inside the gunwale even more after that experience.

cheers

Phil Thompson

[catsketcher]

Just saw the centreboards thing - UhOh. Centreboards do not have a good reputation on multis. I had one and the top leaked and the thing was a general pain in the bum. The Searunner one is okay but it is massive.

Your case looks very weak to me. I would urge you down the daggerboard route. To save your case make the daggerboard 300mm longer than you need. Cut 300mm off and put it in your spares locker. Then with the board at the correct length cut the bottom 300mm off again and then glue it back on.

This gives your board a weak point so it can break at the bottom and not at the case or hurt the case.

If you think you are in thin water pull the boards up. In a crusing cat you will have your boards up at anchor and any sailing deeper than a close reach. Then you sail nice and fast with no drag. The turbulence from your case will be substantial, the case will leak, blow the top off the board when at speed and get full of barnacles (happened to me)

On top of having a weaker case than a daggerboard you also end up with a much weaker hull due to the long slit near the centreline.

And as to my own boards. After 3 years cruising I never looked at them. The spare tips are somewhere in the shed now. Make the case strong by using unis around the back of the case and go the dagger!

[tspeer]

I would go with a daggerboard, too. The drag from the slot of a centerboard is a killer at multi speeds. The drag due to lift on the board is inversely proportional to the square of the depth of the board, so the limits on board length from a reasonably sized CB trunk also mean increased drag.

If you want to avoid much of a space incursion, put the daggerboard trunk against the inside of the skin. Or on the outside and fair it in. Then, at worst, you have a bump in the wall as the only compromise to the interior.

The offset daggerboard also has the advantage that the inside skin is the primary structure. In the event of a grounding, the secondary structure of the trunk can be damaged, but if the board is allowed to slide along the inner skin, hull integrity will not be compromised.

The daggerboard can be made as long as desired and still retract fully through the deck. The reduction in drag due to a longer board will far exceed anything you can hope to achieve by lifting the hull with a canted centerboard.

[billgow]

Originally Posted by tspeer

The drag due to lift on the board is inversely proportional to the square of the depth of the board...

I didn't even have to read to the end and I knew it had to be you! I should have stayed awake in physics class. I know you're right but it'll take me all afternoon to make sense out of it.

I'm going to delete that crazy file and forget the idea. On top of the drag issues, I know I'd have a problem with 6' of board being supported by only 2.5" of it in the slot. The stress would be huge and something would eventually come apart, I'm sure of it.

Did I see something like a retractable, thin centerboard on one of Ian Farrier's tris?

Thanks again!

[billgow]

Originally Posted by catsketcher

I had one and the top leaked and the thing was a general pain in the bum..... Your case looks very weak to me.

Tom and a couple others have shown me the light. I'm not going any further with this crazy idea. But, to address your concerns, I would have permanently enclosed the top of the slot and made it part of the cabin sole. There would have been a hatch to get to the pivoting pin for servicing from above, otherwise replacing a board would have been done from the bottom.

Thanks for your thoughts!

[tspeer]

Originally Posted by billgow

I didn't even have to read to the end and I knew it had to be you! I should have stayed awake in physics class. I know you're right but it'll take me all afternoon to make sense out of it.

It's not that complicated. Say the board is 6 ft deep. If you can make it just one foot deeper, you reduce the induced drag by a quarter. If you add another foot, the induced drag is almost cut in half from the original board. Since induced drag is the biggest source of drag from the board when going upwind, these are very significant savings.

The centerboard is tremendously limiting in this regard, while the daggerboard give you the freedom to see what the right length is.

Did I see something like a retractable, thin centerboard on one of Ian Farrier's tris?

The F24 MKI has a centerboard. It is offset to starboard from the center of the boat so the trunk is under the seat. There's only one seat in the saloon, and the galley counter is on the opposite side, with a drop leaf for a table.

The F24 MKII has the same underwater hull shape, with somewhat different topsides, and a daggerboard.