Bridgedeck centreboard why don't they work???

Yes you can add many tiny effects to the calculation. I even skipped the downward force in the sails. It's very rough. But adding all the details will not remove the fact that the centerboard increased the heeling moment by a few percents.

Dough, because of the 10° angle of heel, the board will get a lifting force. This force doesn't change the heeling moment if the board is in the hull, since it's acting up through the hull. So it's only reducing hydro drag from the hull a small amount. "Semi foiling". But if the board is in the center, this upward force will also increase heeling moment, apart from the "semi foiling" effect.

 

Bridgedeck Centerboard

Thats where design comes into it. For purposes of the illustration both HA's are the same.

 

Bjn,

You need to tell us where you are centering the moment calculation otherwise you have inconsistent analysis.

 

Bridgedeck Centerboard

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Thats a good point I hadn't considered-thanks for pointing it out.

 

I centered the moment in the hull in both calculation.

If I center moment in center of boat, result is the same, only the number of factors in the two calculations are switched.

 

Bridgedeck Centerboard

I revised the sketch to show the vertical lift you pointed out:

 

steady state (say upwind at 50 degrees) the lift component seems rather small but if the boat yaws, the board may develop significantly higher forces (greater lift and greater drag) so board position could make a difference.

 

Good point!

 

Bridgedeck Centerboard

For the purposes of a daysailing or cruising boat like Len Surtees 9m I think the advantages, as pointed out so well by Rob, far outweigh the disadvantages of a board in the center of the boat.
I know on the little windsurfing cat I did it was the best solution.

 

Cruising cat with 500 sq feet total sail area with total board area of 20 sq feet (est). Its underwing clearance is 26"
OP mentioned open bridgedeck so lets jack the clearance up to 32".
Centerboard would be 36" wide by [32"+92"(to yield 20sq ft)+6" (support bearing/structure)]
so that 3 foot by 11 foot board has to be swung up under and into the bridge deck? Seems to create a whole lot of structural design problems.
Another point. If it swings, not sure you could use just half as the submerged active area would be far aft creating balance problems?

 

For the board in the center, the board Clr is higher, therefore HA is shorter, or is that too small for you to deal with?

 

It might have significance, depending on dimensions and ratios. But then you need the full equation. Feel free to do it. My point was simple, and therefore I made a simple/rough calculation to show my point.

 

4% of sail area is a lot. I envisaged the support being above the bridgedeck floor, so you would have a lot more than 6".
For sure the balance changes when it swings, It also changes when you lift the board vertically. Possibly a problem for conventional big rocker cats, less so for low rocker. Nowhere near the size of the problem when a daggerboard hits a log at 20 knots.

Low aspect ratio keels are an option, but add draft and drag downwind and are unsatisfactory upwind unless the boat has a low performance rig.

 

20 knots? you proa guys really know how to party!

yeah, the 20 sq foot is a direct summation of 2 boards. If you can assume that only one board would be used at a time the area of the center board drops a lot but you still have to allow for some surface water variation so not exactly half.

the 6 inch bury is centerline offset from bottom of the underwing? The more you bury it, the sooner the board hits the underwing. interesting design problem.

 

Let's say we center the moment equation in the Clr of Lee board. And align the coordinate system with the boat. Then we look at the center board. At zero heel the new Clr is in line with the force, thus there is no change in heeling arm. With heel, the force will act closer to the tip of the centerboard. Which makes the heeling arm longer.