Bridgedeck centreboard why don't they work???

There is another video here:
https://www.youtube.com/watch?v=KH3Q-5vHLCs

 

That boat rides so well in that video!

That is a beautiful solution!
Beautiful!

Standing ovation!

 

Well there has been a huge amout of good information on this topic, but the basic answer to the question is that a surface piercing foil is just plain inefficient, (lifting foils excepted).

 

Such a blank condemnation ,...really open-minded thinking.

And why are lifting foils excepted?....they are often surface piercing as well, so should suffer some of those same 'losses' at the free surface??

 

your qualification is unnecessary given the OP's original request why
"single bridgedeck centerboard are not more use on cruising catamaran?"

 

My reply did not apply just to one type of catamaran, nor any particular multihull type. But to multi hulls sailingboats in general.
The late (and great) Edmond Bruce proved mathematically and by extensive water tank experimentation, that a low aspect ratio foil (IE: 1 to 1 ) with an end cap (Hull bottom), is the ideal.

Ref: A.Y.R.S. #82 "Design for Fast Sailing".

Lifting foils are mostly horizontal and fully immersed and therefore can be high aspect ratio and very efficient. Their vertical portions of course have to be surface piercing and should be high aspect with high AoA profiles.
Doug Lord's tip surfacing foils are good examples of this.

In order to enable a double berth in a Piver Nugget, I ditched the lifting daggerboard in favour of two surface piercing foils at the Aka hingepoints.
They were an abject failure.
I then fitted a Norman Cross low aspect ratio wooden keel, which transformed the boats performance, to a quick tacking, high pointing and pleasurable boat to sail.
Just my experience.

 

LAR keel brings joy and high pointing -- that's what I like to hear. Any design details that stick out?

 

Yes. A plain rectangular board, or non ballast keel will work well, but is a bit more exposed to damage and is less efficient than an elliptical plan form, but is fine for a cruising boat. The wetted area should be 2.2% of the projected sail area (fore Triangle and Main). The leading edge should be angled back, perhaps to 45deg to help shedding weed etc: I put a strip of half round metal bead on mine to protect the leading edge from hitting "things". Worked well for me on my cruising trimaran. (Piver Nugget).
For my first Buccaneer, which I configured for the N.Y./Bermuda race, I thought a fin keel would be best for the deep sea sailing.
Since water is (for our purposes) non compressable, I made the keel with a NACA 0006 supersonic section, in Cedar wood, with steel rods vertically epoxied inside. The planform was that of the Concords wing, suitably low aspect ratio.
As a LAR keel it was very successful, but I scrapped it the following year as it was fatally damaged by the rocky shallows of Toronto harbour. I replaced it with a lifting daggerboard, which worked just fine.

 

great detail, thanks for the info.
after watching a friend almost sink a Yamaha 24 foot mono by breaking a toilet discharge thru-hull-- not a wooden plug on board for love or money -- I have a bad feeling about holes in hulls under the WL. Dagger board cases are really really big holes of the worst kind. Lots of people use them but ---

 

Firstly I would like to say, good topic. I think if it can be made to work, the concept of a single centerboard on a cruising cat could be a good idea for the reasons Rob mentioned.

I have pondered the idea for a while but am still not totally sold on it, but remain open to the idea when considered in context of the whole design of the craft.

With the whole craft designed around matching certain ideas it could work in conjunction with a central pod which could house a single engine. The pod could also hold fuel and water tanks either side of the centreboard slot (which would only hold say half the chord when raised). When the CB is raised it would be say 300mm or less above the water, but it would present a fairly clean obstruction to oncoming waves and slice through them with little drag.

These 2 statements kind of relate to each other. With no boards to prevent leeway, most cats can still sail to windward with more slip. This means the hulls are acting as leeway preventers. A symmetrical board requires slip to be effective. Thereby relieving some of its duty to the hulls, so the board is not 100% responsible for leeway prevention. That means if the vessel is to be sailed with no slip, the board must do much more work as the board will be 100% responsible for leeway prevention.

So if an asymmetrical board is used with the idea of eliminating leeway totally, one must also consider the fact it is not getting any help from the hulls in leeway prevention. Therefore its not so simple to assume that a asym board can both be smaller AND prevent all hull leeway.

Do you have any pics? I see you are from deep water Sydney. I am from QLD and shallow draft and beach-ability are very high in my SOR. So your LAR keel would not have been a good option for me or many other people who like to beach thier boats just for the fun of it during everyday cruising in QLD. I think some properly designed daggerboards in the amas would have better served my SOR. But if I mainly sailed in deep water and was not interested in beaching, the LAR keel sounds like a perfectly acceptable option with less hassle.

 

No one can seriously say that a correctly designed high aspect foil has worse performance than a low aspect one over its intended operating range. Sure its possible and probably easy to design an ineffective high aspect foil. Seems like the design of high aspect foils is much more crucial than low aspect. So if you are going to design a foil without the required knowledge, a low aspect one will probably have a higher margin for error.

Look at cruising cats that are optioned with both LAR keels and correctly designed daggerboards. What has better performance in almost every situation?

 

Were I betting man...I would definitely say ____ .
What do you think, it is pretty obvious.

 

I conjunction with the 'central nacelle' I had suggested to contain those 2 asymmetric boards, I had also suggested it might incorporated a single drive retractable propulsion leg (at least on smaller vessels). Might appear something like this nacelle on this....

 

Sure would be nice to access your boards and your drive leg without hauling the vessel.

 

Yes exactly what i had in mind when I first ask about centerboard on a catamaran. Why don't they work?

I believe that if it is mounted in a pod under the bridgedeck it should work. But this configuration is pretty much close to a trimaran with a really small main hull.