Daggerboard Placement

Hello all...

Can anybody point out the cons of placing the daggerboard offset outboard from centerline so that it is more vertical and takes up less space in the hull? The exit will still be below the waterline but what kind of penalty can be expected if the board does not exit at the centerline.

Thanks
J

 

It is not clear what configuration you mean. To permanaly mount the centerboard off-center? Or to have two centerboard boxes, one on either side of the centerline?

If you are talking a sail boat, then I would think you would have different trim conditions on each tack direction, depending on if it is on the windward or leeward side of the centerline. Unweighted keel boats are sailed more or less with the centerboard strait up-and-down, using the crew's weight to counter act the heeling moment of the sail. there are many keel boats that use two keels, side by side on each side of the centerline.

Other than the trim condition there would not be a lot of difference since sailboat hulls are seldom symmetrically in the water. The smaller dingies use lee boards that are way off-center, they work just fine.

 

Hello Petros...

Sailing catamaran hulls with daggerboard in each ama. No centerboard. They would be outboard and slightly canted inboard so that the bottom of the board is closer to centerline than the top.

I suppose that I could set them completely vertical but I wanted to have a couple of degrees offset to get a little more lift.

Hmmm...there will be very little heel so I don't think trim will be a significant concern. Any other cons?

Thanks
J

 

Lots of small boats have been built with a leeboard outside on only one side of the boat. They work. I have a dinghy with the daggerboard offset so that is is just inside a side tank/seat. Works fine and allows a completely clear interior and moveable seat with two rowing stations to balance different passenger loads. Have built or seen dozens with centerboards place just outside the keelson. Have never experienced any difference from tack to tack with one this close to center.

There are several reasons for placing a CB off center. Does not interrupt the external keel so is stronger. Won't catch on a trailer when launching. Won't drag crap up into the CB trunk when beaching. Allows masts of mizzen to be placed on-center. That is a sail trim issue but the offset CB is not. Easier to build.

 

Thanks Tom...

I don't think I can consider leeboards for a 34 foot catamaran...but I get what you're saying.

It's starting to look like very little if any penalty will be associated with offsetting the daggerboards outboard. I'll wait a little longer to see if someone comes up with something scary or not and then incorporate the change.

Thanks
J

 

I think the reason to cant them on a cat is to get lift, reducing the wetted area of the hull. The disadvantage of this is that you get induced drag penalty of the lift, presumably it would be lower than reduced drag of the hull. But you still have this induced drag even when you are at the lower displacement speeds, so what you gain at the high end you lose a little in the lower speeds.

On a cat I see no reason for the dagger board to be centered on the hull. The only thing is you want to make sure of is where the board meets the hull it is always underwater. If this location is not below the surface when you are underway it could suck air down the low pressure side of the board and reduce its efficiency. You would also lose the "end plate" effect of the hull.

Any sailboat, cat, tri or monohull seldom does the water "see" a symmetrical hull anyway, so symmetry is not important.

 

Thanks Petros

This is good news. It will be below the surface. As far as the end plate goes...well, everything is one compromise or another. There will still be a plate...it will just be smaller. I'm going to incorporate the design offset outboard off the centerline.

Thanks
J

 

It may be beneficial to place the board to one side. If it is centered, an impact is harder on the board. If offset, the boat will tend more to swing around the board, lessening the shock.
Turning the boat will be slightly different between sides. It isn't necessarily perceptable, but it is there to some degree.
Symmetry is overrated.

Alan