Bridgedeck centreboard why don't they work???

UOS,
Well in the video the ventilation seems to descend from the surface and then continues to be propagated by the bottom leading edge, which in use would be at much greater depth and hence pressure.
So Im not sure what the lesson is here ?
Is it the parallel shape ? Should the board sweep back and end in a point like a rudder ?

 

for us, the lesson is that ventilation is very much a probability based event.
as the foil is developing lift it is developing negative pressure across the "top-surface" of the board - the fat side. The water wants to separate from the top surface and if some chance event puts an opening to the air into the region of low pressure, and the Froude number is greater than 1.6, then ventilation is triggered and becomes self sustaining.
what speeds through the water are we looking at when tacking over +/- 50 degrees? 6 k max? = ~3m/s. look at cmharwood's graph and 3m/s is just over the boundary but this graph is for his rather odd laboratory foil shape and he suggests that a regular foil with fuller nose etc might be less likely to ventilate than indicated.
The centerboard is locked to the hull orientation unlike the rudder so the changes in board AoA are far more gentle (unless you fall off the side of a wave)
So, a pair of regular asymmetric foils - reversed for port and starboard tacking - and a short section of profile across the water surface that develops little or no lift (suppresses the onset of ventilation) get us very close to something doable.
I believe that the asymmetrical boards could dramatically improve tacking performance as argued here http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-21.html#post798149
How to engineer it is another question. as is cost.

 

could watch this stuff for hours -- simulated flow around a regular symmetrical aerofoil
https://www.youtube.com/watch?v=Xb21gLDgY98

 

UOS,
Yep. I get the advantages of asymmetrical boards but I'm yet to be convinced the extra work, weight and complexity is worth it, remember one of the big pluses of a central board is simplicity.
I'm still thinking one symmetrical kick back board that can be canted for and aft with perhaps tubicles if they are a clear benefit is the way to go for simplicity.

Preventing Ventilation from the surface.
I have a pool in the backyard with the return line from the filter mounted just a bit too high. At night, you can hear the return water sucking air and gurgling like a drain !
Its really annoying !
One hot still night, kept awake by the noise I grabbed the rubber doormat and draped it over the pool edge and put a brick on it, that was 2 years ago, works perfectly.
Do you see where I'm going with this ?
Maybe a strip(s) of rubber suspended under the boat in way of the board might act as a flexible movable "fence" riding the water surface and cutting off the air supply ?

 

yep

Cool music too !

So by validated they mean the video is a representation of a real life experiment/investigation ? Or is it all mathematical modelling ?

 

one segment shows data plots projected onto the foil indicating measured water surface points. I assume that is what they mean by validated. The bow wave is well modeled but not the "return" wave, not sure what to call it.
the other thing, no mention of speed or dimensions. and AoA is zero?

 

no one will know the cost/benefit balance until it gets done.
as to the rubber on surface, wont work. Andrew Mason had his fins mounted to the underside of his speed board -- far larger and more effective end fence - still the fins ventilated -- of course he was doing 35+ knots. http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-21.html#post798136
he tried fences and they cut ventilation but were too draggy. he finally was successful with 1 inch of non-lifting profile between the board and the top of the lifting part of the fin.

 

At 35kts the board would be a pump not a fence !

 

Leading Edge Shapes

I had not taken proper notice of the very fine leading edge you had chosen for your testing. Quite naturally this would have made your foils much more prone to ventilation.

Tom Speer had this observation on surface piercing hydrofoils...

Both Tom and I have had similar discussions about very fine entry airfoils (foils at the leading edge of headsail ) being a real problem area

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-11.html#post92052

 

Asymmetric vs Symmetric Boards, leading edge pressure peaks

I found this previous posting on this subject thread needed repeating....

...from the attached PDF on that same posting (in case someone missed it)...

It was for this reason that I chose to have my centerboard foils to be ALL asymmetric in shape from top to bottom.



Now take this asymmetric shape and give it a more elliptical rounded leading edge, and cut the foot and the head off of the board something like I shown in the sketch here...

 

I wonder about those 'pretty pictures' that the computers generate .

Anyway the follow up video that came up just after the one you referenced was a hi-tech test tank at Univ of Iowa. I think that is the one that our fellow contributor Casey Harwood is now working at??
https://www.youtube.com/watch?v=T7bYl0jKt70

 

Perhaps that multiple scalloped board I displayed in the first sketch I posted could be modified to a single big scallop at the leading edge waterline region,


.....with a mellow fence like the ones on this skeg



This might purposely mis-align the board's leading edge juncture at the waterline with the rest of the leading edge down deeper,...upsetting the direct path that ventilation might take down a leading edge.

 

Transom Hung Rudders (are surface piercing foils)

Perhaps we can also learn a few tricks from transom hung rudders

 

If air doesn't travel back upstream generally, is there some critical angle at which it ceases to propagate? If so then making an 'entry area' that pierces the surface with a knuckle forward to the actual leading edge of the board (at that critical angle), should prevent flatwater ventilation from propagating down the board's leading edge.

It would also balance out a transom hung rudder a bit which is never a bad thing. Though it might increase vortex generation at the corner.

 

Outboard motors are designed to operate bolted to light boats that closely follow the sea surface. Blue water cruisers use really deep V hulls and props way under the boat.

Transom hung rudders? The ones I have seen are as straight and plain as a Quaker. But rudders have a different design task from centerboards.
A single small scallop in the LE works only if the ocean remains perfectly flat. Something with vertical extent of 40-60 cm is needed. And most that under the static WL.
edit: this of course means that your uniform profile is no longer uniform. sorry about that.
The ventilation problem is most severe when the board lifts out of the water. Less area lift = higher loading on the board remaining in the water = higher probability of ventilation.
One design challenge with regard to centerboards is that we need them most in the worst possible conditions. When the seas are very rough, being able to tack reliably and efficiently is essential. Otherwise its motor time