Bridgedeck centreboard why don't they work???

for general comment

One problem is the wake created by the board. looking at a board as a miniature hull, the max speed for a 75 cm chord board is just 2 knots (approx.), at this speed the bow wave should collapse and reform smoothly behind the TE (assuming it is sharp etc). At greater speeds the bow wave depression behind the bow wave becomes so deep (volume of positively displaced water - bow wave - must equal the volume of negatively displaced water - bow wave depression) that a reverse breaking wave forms close to the TE. This is also seen with submerged flat transoms. This tumbling wave creates a lot turbulence and air entrapment. Shown very clearly in the simulation video.
We usually suppress bow waves by using very sharp LEs but they are far too good at stalling - a strong trigger for ventilation.
So, a basic contradiction. For the board to work it must develop lift. i.e low pressure. Reliable lift generation - over a range of AoA - demands a soft LE but soft LE will generate strong bow waves which are a strong trigger for ventilation.

 

Doug, that isn't really a catamaran, but a trimaran with a skinny central hull.

(not that there is anything wrong with that).

 

Leading Edge Shapes

Lets see if I can repost this basic idea with a little more clarification as to how I was arriving at this thought.

I see that I termed it a shallow fence in one case and a mellow fence in another instance. At any rate it might look something like one of these


First off I was trying to 'interrupt' that long continuous straight-line leading edge of the centerboard,..particularly at its upper most water surface piercing point.

I also began to think back about the numerous bulbous bow discussions, photos, and videos I had seen. That video that UponStands posted shows how dramatic the bow wave of a ship can be subdued by a certain 'sickle shape' (Look at minute 3:49 of that video https://www.youtube.com/watch?v=Xb21gLDgY98)

Some more IMAGES of bulb bows

Here is another video example that's kind of fun to look back at...
Bulbous Bow - Japan's super battleship "Yamato"

Another bow video

Even the newest Zumwalt destroyer has a shallow bulb down below the waterline


What I was envisioning was a sort of extension of the bulb bow into a short mellow fence at the bottom of my scallop cut at the leading edge of the centerboard.

 

All discussions on bulbous bows to suppress bow waves say that a lot of computer modelling is necessary to match bulb configuration with hull shape and design speed range. Get it wrong and all you end up with is extra drag. Certainly those ships spend majority of time steaming at their design drafts.
remember that cmharwood emphasized that in his tests with the thin nosed profile the ventilation started at the TE but that if a regular foil section were used the nose section would not be a critical site for ventilation.
but we are chasing our tails here. we need
speed range; both forward and lee, gives us AoA
Heave range;

 

cmhardwood says
""Kill that flow separation, kill the ventilation,"
OK
one way is shown below
the foil is through drilled to create small vent holes in the suction area of the foil up either side of the waterline. The high pressure side feeds the low pressure side to disrupt/prevent flow separation just under the waterline. The four small green dots in the figure below. Foil moves right to left.

This will destroy the flow separation where it is critical to do so with minimal physical impact to the foil.

 

That appears to be a variation on those slotted windsurfing skegs I brought up back here...?
Windsurfer Skeg Experiments

 

I do agree that the bulbous bow can be quite challenging to get correct, and therefore of questionable use. But in many cases they do cut down on tendency of the free surface of the water to climb up the foils's LE.

I was not trying to indicate that we might put such a large protuberance on our centerboard foil, but rather the possibility that they might influence the shaping of the forward portion of that 'mellow fence' I had suggested at the bottom edge of my scallop cut-out on the LE,....maybe impede , in a gentle manner, that ventilation front moving down the leading edge of the foil,...as opposed to a 'hard fence'?

BTW, I do not think Casey said "that if a regular foil section were used the nose section would not be a critical site for ventilation" .
But rather this is what I read, ...."I'd wager that by choosing a medium-to-thick airfoil section (to delay stall) near the free surface would help you out."

 

Leeway Reduction

After all isn't this one of the major jobs we have for our 'centerboard(s)'. It's not as straight forward as one would suppose,...just providing a foil.

I started to seek out some explanations/discussions that might have occurred on these forums, but I ran across this first,...pretty good info even while it is written for a heeling monohull.

Keels and Rudders: How they work and why they stop working

 

nope. and a quick check shows no one is using the slot. too much drag

 

umm, read it all, what part was "not straight forward"? seems like basic sailing 101.
the part that was missing was the interesting part for catamarans, the role of the centerboard as a pivot around which the hull tacks. Very important as modern commercial cat hulls are so smooth with no chine anywhere near the water.

 

Backbone on AC 50 catamaran

Check out the latest 'backbone rib' on this new America's Cup 50
https://www.youtube.com/watch?v=qkq0nEh3xfI

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

 

...just happened across this
https://www.youtube.com/watch?v=Jb5MvZ-vm14

 

thanks for the link
I saw that video some time ago but its timely
correct me if I'm wrong but believe the FP Lavessi 40 is a mini keel boat.
so, point 1 the keels appear to be working well.
point 2, looking at the sea state between the hulls, I do not want to put anything even 6 inches beneath the bridgedeck. would really slow the boat to a crawl. Something at the static WL would be beaten all to hell.
May have to accept some loss in downwind speed with the constant drag of the keels but when the sea turns ugly they are so reassuring.

 

You may have a point there about the mini-keels, but in reality there is not enough information given to determine the extent of their leeway reduction.

Here's a good shot of the vessel's mini-keels.


In a cruising cat I would be real tempted to utilize these over hull penetrating boards.

But there could be problems in real shallow waters or coral head strewn waters,....or what other obstacle might have inflicted this damage to the same type of vessel??
https://www.foxsmarina.com/portfolio-items/fountaine-pajot-lavezzi-40-catamaran/

 

Another point of view by Gregor Tarjan

Multihull Keels and Daggerboards
...one of his claims..