Bridgedeck centreboard why don't they work?

[valery gaulin]

I was wondering why that single bridgedeck centerboard are not more use on cruising catamaran?

I have try to find the answer and they is no clear answer. On Sir Woods website I have read that bridgedeck centerboard don't really work and that the Prouts brother have tried them with no success.

I can't find a definite answer to the problem of the single bridgedeck centerboard. Is a structure problem? A cavitation problem? Can someone with experience tell me what is wrong with this type of setup?

Thank you for the answer to this issue.

 

[Mr Efficiency]

Well, I suppose you need a much larger area and longer centreboard, with all the structural problems that go with that, and possibly variable loads as waves pass by. But I really am guessing.

 

[Doug Lord]

Hydroptere.ch

Don't know how well it worked but this boat has set a record or two-but it's not a cruising boat:

 

[PAR]

It can work, but there are issues to contend with, as with all decisions in yacht design. Because of a multihull's configuration, you pull the centrally located board out of the water, as the boat takes on some heel, decreasing it's effectiveness, just at the point you'd prefer it to be most effective. Without a hull acting as an endplate, further losses can be expected. It can be done, but there are more effective approaches to these issues, which is why you don't see it that much.

 

[outside the box]

Don't know how well it worked but this boat has set a record or two-but it's not a cruising boat:

Len Surtees did it on his 9 mtr but the set of design compromises were not to everybody's liking. It was quoted to work well http://www.surteesmultihulls.com/9m-trailerable-catamaran.html.

Team Ezifold Yachts Ltd

 

[Doug Lord]

Thanks for reminding me about that-I think I even talked to Les about it a few years ago. I used a board in the center on a little cat windsurfer I designed and produced-worked ok.

click-

 

[Emerson White]

It can work, but there are issues to contend with, as with all decisions in yacht design. Because of a multihull's configuration, you pull the centrally located board out of the water, as the boat takes on some heel, decreasing it's effectiveness, just at the point you'd prefer it to be most effective. Without a hull acting as an endplate, further losses can be expected. It can be done, but there are more effective approaches to these issues, which is why you don't see it that much.

It seems to me when the cat is starting to heel significantly is precisely when you want the centerboard to stop being so effective. As your cat rises out of the water you want to start making leeway rather than holding the water while it goes over. Most cats drop the windward daggerboard and raise the leeward.

 

[upchurchmr]

Leeway vs sailing it over the side are two different things.
You can't make enough leeway to protect yourself when you get a wind puff.

I never in my life raised the leeward daggerboard.

 

[Emerson White]

It's a balance of forces issue. There are gusts that will be big enough to topple a cat even with all the weight on the windward hull and no daggerboard down. Decreasing the windward lateral force from the daggerboard at the bottom of the system is going to raise the leeward force at the top needed by a gust to topple the cat. Gusts of the higher strength are less frequent, so you decrease the frequency of mishaps.

 

[upchurchmr]

Draw me a diagram, It doesn't make sense.

I understand the concept of balance of forces - I just cant match up anything to your explanation.

 

[Corley]

If you want to allow sideslip so the boat to surf sideways you can just leave the windward board partway down, same effect.

 

[Ilan Voyager]

There are several reasons explaining that bridgedeck boards have been tried and forgotten;
The first one is that does not work well. A lot of people have tried it, nobody got good solid results.
Second it's an engineering complication, with multiples issues of weight, vibrations and strength.
Third hydro-dynamically it's a disaster with a board vibrating and twisting while sucking air on the surface of the water. It obliges to use plain symmetric thick profiles which are far from the best.

A board protruding from a hull is in good green water, and interacts with the hull's flow for a better efficiency. It can use any kind of profile you want suiting to your needs. It can be auto-orientable, self regulating. It's "small", and the engineering problem is a breeze to solve.
And it's placed at the good place ie with the hull which is counteracting the effects of the sails, so every thing is aligned. Hull board rudder in the same line, thats better for controlling the boat...For tacking also.

And I remind that at least on a good multi you have a fast acting throttle; the sheet and the track to control mainsail's power. Like on a good motorcycle you control the slide with the throttle and you want that the tires have the maximal grip. All who know how to ride a bike or sail a sport cat will agree with me.

The trimarans have had since years the boards and rudders on the amas. For good and similar reasons ; finally it's simpler, easier, safer and efficient. And you have not a board in the way of the boom. That was a plague on the first trimarans designs. Why to have this problem on a catamaran?

 

[Doug Lord]

air

I've heard the "sucking air theory" of why a board in the center is bad for a cat. But it simply isn't the problem some make it out to be-especially on a cruising boat or a daysailer. Boards in the center can be designed and engineered to work well. Some designers have gotten excellent results with such a configuration.
At their best speeds the fastest boats in the world don't have the hull acting as an endplate:

 

[UpOnStands]

Try the pencil test.
take a regular 15-16 cm pencil
wrap your fist around the pencil and draw a circle
next, hold the top of the pencil with just 3 fingertips and try to draw the same circle.
Now, imagine drawing circles continuously for a few hours.
Your fist models the regular daggerboard case in the hull arrangement.
Your finger tips give an idea of the greater loads imposed by the center-of-the bridgedeck mount.
Its possible but do you want to?
Use just one daggerboard in port or stbd hull if you want simplicity.

 

[valery gaulin]

This amaze me!!! It is possible to do a really good foil that lift an entire catamaran, trimaran, but very difficult to do a pivoting centerboard from the bridgedeck of a catamaran only to prevent leeway!!!

The only reason i ask is because it seams to reduce things that can go wrong with a single centerboard above the waterline hung from the bridgedeck of a cruising catamaran. Anyway on a cruising catamaran there is no hull that should be flying were the efficiency of the centerboard would be lost.

But you are all right in your answer, it is true that i can't find a designer that successfully incorporated this bridgedeck centerboard on a cruising catamaran even the small one i bthe 30foot range, this is what i am interested in.

Thank you all for your reply.