Bridgedeck centreboard why don't they work???

putting the ribs on the board would seem to prevent raking the board.
How about this? boat moving from right to left.
The forward tensioning ropes would act on the red vortex breaker hinged at the front of the foil.
Changing the rake of the board by 5 fwd to 10 aft would change the angle of the tensioning ropes only slightly and thus the angle of vortex breaker relative to the water surface would stay constant. Depth would decrease slightly.

 

You'd get too much turbulence from the ropes. Articulating fences have to be strong and fit the foil section which is problematic for adjustment. A internal control rod is a possibility but again one end has to pivot while the control end moves while keeping the water from going through a gap and not breaking.

 

external rod acting on the back edge of the vortex breaker so any disturbance is after the foil?

 

Yes. External rod as part of the back edge of the foil, i.e. use the section in the rod. The lower end could telescope into a lower edge/step and control from above. as one example. How complicated is answered by how many fences.

 

I'll generate a new sketch in a few days when I return from a trip up to Laos. This is not really that difficult.

I'm hoping he joins in and answers some of your questions here.

In the meantime I've found a few other postings on this forum concerning leeway & leeway forces.

 

If you design the boat correctly in the first place (that means getting the centers reasonable close to correct), it should not be necessary to rake the boards. How many cruising cats with daggerboards allow you to rake the boards?

 

I am hopeful that one macro fence at the top could replace 5 or 6 riblets spread along the foil.
The foiling trimaran pictured needs to have many riblets as the depth of immersion changes so much.

 

That is a problem with the central board idea. But isn't debris collection a problem for hull mounted boards as well??

 

Not to the degree you think because the bows/hulls shoulder the bulk of the small stuff out of the way. The big stuff you should be missing, we try not to bump anything.

 

This is true, but one of the neat things about a centerboard is changing the CoLR to suit different sail combinations.

The main benefit of rake would be if a forward set did cut down on the air interference.

 

The problem is going to be waves with that approach or it will have to be deep down which reduces the effectiveness of the upper section.

A nose rod would work too, again part of the section.

 

Re; Adjustable fence.

My understanding of fences is that like T foil rudders the join to the board should have a radius to ward off cavitation.
That would make adjusting the fence difficult, neigh impossible.

 

I find the multiple riblets solution rather pleasing.

On the central board idea I might suggest we only need 2-3 as we are not foiling up in the air. And as mentioned they are likely only needed on the 'outside/curved shape' of the twin asymmetric boards that I am suggesting.

And I am still thinking a little forward rake to the leading edge of these boards when they are in the fully down position.

And additional attention to the leading edge shape of these asymmetric boards will lessen the larger pressure differences, as has been referenced before (thus lessening the tendencies for the start of the air migration down the board.

 

I was thinking of something like a neoprene seal to add the fat so to speak and cut out the pressure losses.

Then again I think my tracked/deck trunked cassette with fixed fences might be easier and cheaper but less intriguing. What ever it is has to beat the weight of 2 pieces of ply and some timber because that is all a 40' boat needs in weight for a dagger.

 

something like this?