Bridgedeck centreboard why don't they work???

That solution looks much lighter, simpler, cheaper and easier to build than 2 boards plus cases and the extra interior room would be nice.

Can we say the myth is busted that the bridgedeck centerboard does not work? Its simply untrue and just has pros and cons like everything.

 

Its also feasible to connect waterstays to the board itself as long as they terminate exactly at the athwartships plane of the pivot pin. This might be a good idea in cases where the board pivots well above the waterline to make its cantilever shorter.

 

Kelsall Lima 34 cat

this has wheel not tiller but does have a nacelle -- the "For Sale" description lists LAR keels.
http://www.multihull.nl/multihulls/used-multihulls/78151.Lima34/78151.Lima34.fotos.html

 

Nobody said "the bridgedeck centerboard does not work."
They just said its inefficient compared with through hull daggers, or LAR keels.

Any surface piercing blade providing sideways resistance will have pressure on its leeway resisting side which will produce a surface wave and turbulence, which in turn means DRAG..

The extra complication and weight of the means to support it, against the conderable sideways force it needs to exert to do the job, helps make it inefficient.

As Clive Cessna said,"Simplicate and add less weight".

 

I'm thinking cavitation more than anything.

Its already a shallow draft boat with a tough time getting the boards to bite on good deep strong water.

I've tried and seen lee-boards on a sailing canoe and they basically sucked. You could see energy sucking swirls as soon as you tried to make the sails do some work.

 

Oldsailor, did you not see the posts where a poster actually built a very simple arrangement? It looks like it would have been cheaper and lighter than 2 x boards and cases.

One large board has one tip loss so its not immediately obvious that the loss of efficiency at the waterline is much worse than the 2 tip losses and interference drag where it 2 boards would usually comes out of the hull. I think to make this claim requires more than an assertion. I'm not saying one is worse than the other. But I am not the one making any claims.

One objective claim I can make is that two in-hull mounted daggerboards has the following extra resistance in areas the single surface piercer does not.

One extra tip loss to reduce lift and create drag. DRAG Loss of lift
Two hull to board junctions which cause drag but increase lift. DRAG DRAG LIFT LIFT
Two holes in the hull which always produce drag. DRAG DRAG

The surface piercer has drag at the waters surface and loss of lift. DRAG

I have no idea how all these 5 extra sources of drag compare. I would not like to make a claim because its seems more than a little complicated.

 

Ventilation of the Board

Here are a few recent posting, but I'm sure there are many more that can be found. You guys are making it sound as though any surface piercing foil that doesn't have a 'hat' on it just can't be effective at all,...rubbish:!:

 

Ventilation research

This would probable be an interesting paper,...

Ventilated cavities on a surface-piercing hydrofoil at moderate Froude numbers: cavity formation, elimination and stability

 

You are thinking something like me. I am imagining the idea of attaching at least one set of these 'waterstays' to either centerpoint-end of that big 1 foot diameter bearing I had suggested.

 

ventilation
https://www.youtube.com/watch?v=zJttVDFbYJY

scary stuff as they accelerate from 10 knots to 11.25 knots - see 37 s mark
https://www.youtube.com/watch?v=0J0jaMIRMR0

 

Quantifying it is beyond me, but it's pretty clear experimentally that two boards and the associated drag are still better than a single larger board. For proof see the entire A-Class Catamaran fleet prior to the introduction of C boards. Despite a fully open development class, where a center board would have been perfectly legal, no one adopted a surface piercing centerboard.

You find the same result if you look at the C-Class, and the Formula 40 Cats.

Experimental data from a number of very innovative classes with a lot of money and development tossed at them, and everyone selected twin daggerboard in the hull.

 

You don't have to sell me on the fact that twin daggers in the hulls are more efficient,...I agree

But when you consider all of the other benefits of the 'central board(s)', no hull penetrations, etc, concept for cruising boats, I think the alternatives weight in significantly for alternatives.

 

I've seen a number of such videos, but most of them appear to be test on foils involved with hi-speed foiling (power) vessels,...not exactly applicable to our 6-12 knot sailing/cruising vessels?

 

This thread relates to cruising cats. Cruising cats spend much more time sailing downwind where the dagger case slot holes will create drag all the time. This is totally the opposite to racing where upwind VMG wins races. So lets say an arbitrary speed loss of 3% upwind VMG with the surface piercer but the downwind gains are 2%, the surface piercer will be slower around a race course but faster over all for the average cruiser. Please feel free to change my arbitrary figures. Whats fast for a racer is not automatically fast for a cruiser.

 

believe that they are as our boards are far less smooth and far fatter, both of which would encourage ventilation.
Note that the foil in first test in the second video has zero angle of incidence?