Bridgedeck centreboard why don't they work???

Split Asymmetric Centerboad(s) Under Bridgedeck

Perhaps have a look here....

 

Here are a few other refences of postings where I have addressed this subject....

ASYMMETRIC CENTERBOARDS:
Superior tacking, leeway reduction, and balance could be attainable with optional nacelle-mounted centerboards.

An edge-on flat plate is located down the centerline of the vessel acting as a rib to strengthen the fore-to-aft rigidity of the vessel (weaker characteristic of catamaran). A tow bundle of carbon fiber (kevlar, PBO) is laid along the bottom edge to produce a ‘bottom truss structure’. On either side of this plate/nacelle two asymmetrical centerboards are mounted with their flat sides up against the nacelle, and rotate on oversize diameter bearings. Only one board at a time is lowered, possibly linked together such that the act of lifting one automatically lowers the other. Both could be rigged to 'kick up' upon hitting any solid object, or shallow cruising.

Several advantages to an asymmetrical shaped centerboard;

· Requires less surface-area (smaller board) to develop a leeway reducing force
· The boat itself does not have to be sailed at a skewed angle of attack to develop the 'board's lift' (leeway reducing force)…resulting in less leeway.


Drag forces are on the centerline of the vessel, producing minimal turning moments about the center of the vessel…improves the tacking capabilities

Front of this nacelle/plate could be configured to act as wave splitter, attacking the formation of those peaky waves under the tramp areas that eventually slap the bridge deck underside….slice those waves down a bit. A fairing could be added to nacelle.

Maintenance of this board system, particularly in remote cruising areas is much improved. No need to haul-out to repair CB problems, or bottom paint hull-mounted trunks and boards. Everything,…cables, bearings, and boards is all above the load waterline.

Eliminating board trunks in the two hulls results in greater watertight integrity, and reduces initial building cost.

 

This centerline mounting may also improve the tacking capabilities of the vessel as it allows the 'clean' hulls to slip a little while pivoting about the central board.

And how about the maintenance factor, particularly in remote cruising areas. No need to haul-out the vessel to repair kick-up CB problems, or even bottom painting problems. Everything, including the cables, bearings, and boards is all above the load waterline. The initial building cost should be less by eliminating the trunks in two hulls, and the watertight integrity is much better.

The twin boards might have to be made a little bit longer as they operate with a 'free-surface' end, but then they are asymmetric so they can be correspondingly shorter. I would further suggest that surplus helicopter blades are prime candidate sources for both CB blades and rudder blades....high tech, extremely strong carbon fiber fabrications that have a prescribed limited life span aboard aircraft, but are perfectly happy for our use.

 

Ne of the big issues I see is the additional stiffness a centerline board needs to have. With the trunk in the hulls you can use the exit plate as a structural member (much like the partners on a mast). By putting a centerboard in the middle you now have a much longer unsupported span that needs to be contended with.

Assuming you want a working spad 4' deep on the hull mounted boards you go 4' and are done thanks to the end plate effect. If you want the same working area on a centerline board at a minimum you need (bridgedeck clearence)+4'+4'*.33=total length.

The 4*.33 is to account for lot area due to ventilation and heel. So assuming 3' of clearance the board needs to be 34+1.32=8.32' long.

To handle the torque of this longer arm the foil needs to be much larger, the structure more robust. Assuming the same side load the foil will need to be more than twice as long, and more than twice as stiff. Not to mention the hull structure needed to contain these forces needs to be pretty substantial as well.

In short, I just don't think it buys you much. Sure it's one board instead of two, but that board is bigger, heavier, and more expensive than the two smaller boards combined.

 

Help me out here, the force paths seem far from optimum.
The key weakness is the poor structural resistance to twist in the board. The board itself can as stiff as hell but the really really large torque forces have to be resisted with "zero" distortion as any movement will directly alter the angle of the board re. the water flow and thus increase/decrease the loading.
I believe the board will make the boat very jerky as it "flutters" about -- the board is stiff enough it is the lateral/torque forces that are the problem. These forces are roughly orthogonal to the wave splitter board, its weakest orientation.

 

That just means you have to be real good in building the board supporting structure, which also generally means the weight goes up since there is not a "natural" supporting load path in a catamaran.

One of the things missing is the weight of such structure and the weight of the bigger board on centerline in Bjn's analysis. That weight will oppose any heeling moment due to the centerline board position.

Lets just assume anyone putting the board in a center position will do the work to support it well enough so that flutter and free motion will be controlled. We don't need to assume poor workmanship/design.

I never heard any such complaints from the one owner of a Stilleto 23 that I knew.

 

not a question of "it can't be done" of course it can. We now have foiling AC boats - but the cost is out of sight.
As boat size falls all the loads relative to the structure fall dramatically, so small boats are quite possible.
Holme Designs (?) has a spread sheet for cat design. He finds for a 12 m cruiser that the load placed on a dagger board is 570 kg at 12 knots. Easy to build for 570 kg steady force but now imagine this fluctuating at the end of a 4-5 foot lever arm.
In-hull mounting resists the load right at the hull surface so the lever arm is the shortest possible.

 

All true (I assume the loads are good).
It does not mean it cannot be adequately designed and built.
At a weight cost.

 

just checked on Stiletto web site.
latest design, Stiletto X-Series, has in-hull boards.

 

Thanks, I only knew about the original boats, and owners modifications.

 

Stiletto correction

It was the original Stiletto 27' that had the central board. The 23' had boards in each hull.

 

Did you notice this?

And did you notice the athwartships support 'wires' (sprectra?) in the illustration here? Did you notice that the nacelle support wires are anchored in a main bulkhead of the vessel??
http://www.boatdesign.net/forums/attachments/multihulls/111459d1484248887-bridgedeck-centreboard-why-dont-they-work-central-cb-arrangemet-ps640.jpg

And I specifically chose a really large diameter bearing for the board(s) to operate on (1 foot in dia).

It will be not much more difficult to build these 'boards' than to built good rudder blades.

BTW I am very familiar with the Stiletto arrangement and some of its problems, as I was one of their largest dealers. I suspect with modern materials and design the 'whole structure' could be built much lighter than trying to built with the SS metal structure of the Stiletto.

 

Brian,

Thanks for the correction. I can see why I might of confused everyone.
YES, I meant the Stilleto 27 - the original boat in the series.

Marc

 

Have a look at this...

https://www.youtube.com/watch?v=ZggxS6ZrVOw

 

Happy Feet bi-pod rig, etc

That was interesting. I was unaware of that vessel even while I live in Thailand part time, (but up north away from the water )

I did find this on one link
http://crew.org.nz/forum/index.php/topic/873-happy-feet/?p=13403

PS: Appears as though the central foils (rudder and front foil) are cooperating pretty well with free surface of water.