Bridgedeck centreboard why don't they work?

From what I can discern at this time it appears there are at least 2 things we should definitely consider:
1) utilize some forward rake to the board.
2) try to minimize any large pressure differences at the leading edge of the board (foil) where it meets the water. As had been mentioned before this seems to suggest an asymmetric shape that creates less pressure difference at its leading edge. And some careful shaping of this leading edge
This might also suggest a board with its max thickness 50% or better aft.

I've asked that hydrodynamicist if he has every encountered this concept of whale tubercles on leading edges of foils. We'll see what comes of it??

 

Andrew Mason, many-time world record holder for windsurfer speed records found best approach to reduce ventilation was a very short section of zero lift profile at the water surface and an asymmetrical profile below. http://www.boatdesign.net/forums/sailboats/best-shape-surface-piercing-foil-3991-2.html#post26571
Roughly 1 inch of zero lift profile for what, a 5-7 inch chord fin. For a 75 cm chord centerboard that's 25 cm, so 25 cm below WL and all the way up to the support
His work was on a fin attached to the bottom of a board but everyone found the fins were ventilating badly. Our centerboards have no such end plate hence a little extra (10cm) to suppress ventilation.

 

That could be a vote for Brian's twin boards. The top of the boards foil could be set at a zero angle of attack at the intended leeway, then the shape would twist below to an asymmetrical shape.

 

here is one image -- foil is upside down . the bottom section is symmetrical for the waterline while top is an NACA 2415 section.
missing here is the parallel sided section for the support bearing.
As the support board separates the two foils we do not need to use flat bottomed foils which apparently have very high drag.

 

Not a good idea, IMO.
The abrupt change of airfoil section will create a vortex along the edges of the board, which will:
1) increase the drag of the centreboard
2) create a low-pressure area which might induce the ventilation instead of preventing it.
All the changes of board geometry have to be smooth and gradual.

 

Tubercles on Humpback Whales' Flippers, Bio Inspired Tech

Tubercles on Humpback Whales' Flippers: Application of Bio-Inspired Technology

https://academic.oup.com/icb/article/51/1/203/636829/The-Tubercles-on-Humpback-Whales-Flippers

...this material was more fun reading..

 

I expected the real version would be blended. Maybe hard to draw on the software?

 

If that is the case, then ok. I assumed that the idea was exactly as drawn.

 

voila, blended section. not hard, just one more step

 

That looks much better

I'm not sure how far it should be blended over.

 

I thought he said his fins worked exceptional well??

 

now for something else to consider

traditionally cats are thought to be not so good at tacking.
this paired asymmetrical arrangement might solve a lot of the problems.
Assume boat is tacking 315 degrees to wind. traditional centerboard/daggerboard generates resistance only when the boat slips, i.e. has significant leeway. Now we want to tack. As we come through 0 degrees the s.board does nothing but slow us down. As we move through stbd we still get no value from the s.board until the sails generate enough heel force to reestablish leeway.
The asymmetric boards work differently.
As we swing from 315 thru to 360 we still have decent fwd speed so the a.board is generating lift in the direction we want the boat to turn. As it is asymmetric, we can use a smaller profile for the same lift, less drag. Reaching 20 degrees(?) the a.board is still generating lift to stbd. At which point we raise it and drop the other a.board. Falling away we are now generating resistance on the opposite tack without having to wait for leeway based on the boat's fwd speed. Hopefully with more boat speed in hand the sails are filling more efficiently so speed is recovered more quickly.

 

yes, he tried fences and they did not work.
the short section of zero lift next to the waterline worked exceptionally well.

 

Why are you (we) looking for a board that is both symmetrical in one portion and asymmetrical in the other??

 

I believe he said the fences worked well also, they just produced more drag?