Bridgedeck centreboard why don't they work???

Designer

You may have missed the link in post #5........

 

You are right i missed that link. Looks cool but there is not very many video on how it really works.

Thank you

 

You either need to study some engineering/ hydrodynamics or trust those who have, or just go ahead and build one and prove us all wrong.

Or you could try and research the older Stiletto 23 catamaran.
It was always built with a center hung centerboard.
Some owners modified the boat and installed hull mounted boards, with a distinct increase in performance.

Or you could look for boats which have installed hull mounted boards, but moved them closer to the side of an individual hull. This provided all the benefits of being in the hull, but left room in the hull, and also avoided getting dirt/ rocks jammed into the board slot when beaching the boat.

If you are concerned with cruisers, I would suggest you ignore all the cutting edge racers Doug Lord just published. Talk about cats and dogs.

 

Goodness me.

Ok I'll throw some wood on the fire...

A cruising cat does not foil. Whole different thing. valery gaulin foiling might look appealing but it doesn't work well in a seaway, or with any sort of load. Great for racing boats, not cruising.

Leeboards cavitate. Boards slung from a raised bridgedeck cavitate worse. You lose maybe 1/3 of the board area. You can of course design around that.

Big and heavy as will be the attachment be it daggerboard or centerboard case. Weight is a tremendous enemy of the multihull.

Most trimarans still position the board in the main hull. They are a completely different problem.

People mostly position dagger/center boards along the side of the hull now days. Minimal impact on accommodations, simpler, cheaper and lighter to make.

When you have 2 boards you have more options. Put both half down when running to track better, while minimizing draft and drag. Use them to trim rudder weight in different conditions.

So basically lots of reasons to do it the common way and few to do it the way you've asked about. Yes it will work, yes you can solve the problems. The question is is it worth the trouble ?

I like unusual boats, but some things are just trouble and not enough reason to bother IMO...about as sensible as building a wharram...

 

A central centreboard has the following advantages:
1) Kicks up in a collision or grounding. Even if it none of the other advantages applied, this should make it the option of choice.
2) Less structure to support it on a bridgedeck cat than required for two in hull boards.
3) ~half the weight or half the tip losses of 2 in hull boards, depending on whether the in hulls are symmetric or not.
4) Able to be lifted out of the water without leaving a drag creating hole in the hull.
5) No antifouling of difficult to get at slots in the hull.
6) No need to beef the hull up to resist impact damage.
7) The foil is working in clear water, rather than the turbulent layer next to the hull. Frank Bethwaite showed that the added drag from this was significant.
8) The turbulent flow off the board does not negatively affect the rudder, which it does with boards mounted centrally/leeward of centre. see Bethwaite for this, as well.
9) Able to adjust the clr by raking the board aft. Especially handy if the first sail reduction is to reduce the headsail area. Or forwards if you have a big reacher on a prod.
10) if the casing is designed correctly, the location of the board can be easily changed.

Sucking air is usually a function of poor foil shape, finish, balance or rake. If it is none of these, it is too much speed and easily fixed with a fence just below the water surface.

Derek Kelsall has used them on some of his designs and says they work well. There is no obvious reason why they wouldn't.

Bottom line on performance is that I doubt you will be able to tell the difference on a 30' cruising cat. You will notice a difference in build time, antifouling, stres in shallow water, repairing damaged boards and cases and internal space in the hulls.

UOS
Good analogy, which should be considered when designing the support structure. Done properly, this will be just as strong as in hull boards, but without all the extra material required to stop the dagger boards from slicing your hull open when you hit something at high speed.
Which is not to say that the loads are less on bridge deck cases. They are higher, but pretty easy to design for as you need no fore and aft strength if the board kicks up and is held down by a line or fuse.

Valery,
Your thinking is on the right track. Forget the racing stuff, design something that does what you want it to do. And do not think that because nobody does something that it won't work. Sail boat design is far too mired in myths, fads and people too lazy to try new stuff for this to be true.

 

sorry Rob, there are benefits for sure but to my mind the difficulty of accurately aligning and mounting the board re the boat and the difficulty of eliminating all slack and slop in the bearing, and keeping it slop free over longish voyages, make it too tricky.
Day excursions and home at night? not a problem.
LAR keel for me, better for sleeping on deep water.

 

Bridgedeck board

Rob, excellent , detailed post. Thanks!

 

Bridgedeck Centerboard

==========================
Why don't you e-mail Len or go to the website and call him:

lensurtees@hotmail.com

Here's a link to his youtube channel: https://www.youtube.com/channel/UC2KLebV2fzZp4EkfGc8i_pw

 

What comes to my mind is that with heel, part of the force on the board is pointing upwards, which will act to tip the boat over. The lever is big if the board is in the center (or WW hull). Lever is tiny if the board is in the LW hull.

So the heeling moment is (slightly, I would say after thinking about it for a while) larger with a central or WW board compared to a LW board.

 

@Rob Dennney: It seams that you just have read my mind. Thank you so much for your answer.

@guzzis3: I know that lifting foil are not for cruising catamaran or trimaran. I probably did not express myself in clear mannner.

Well with all your feedback my conclusion is that it is a possible solution bridgdeck centerboard but is not popular, probably because the engineering solution is not trivial.

I think that best would be to built myself a model to prove to myself the pros and cons. of such a solution. A model might come when i am done with my other project!

Always finish a project first before starting a new one!!! I need to keep reminding myself this quote...

Thank you so much all for your input.

PS: @Rob Denney: I have alot of respect for your proa design. Love your work and the solution you found to make proa more accessible.

 

With a some assumptions I find that the heeling moment is roughly 5% higher at 10° heel on a Tornado cat if the board is mounted in center instead of in the lee hull.

So probably not competitive in a race against other Tornados. But almost no difference in a cruiser. Maybe even better, if you consider situations where people would lift LW and drop WW board, as stated in the thread.

 

Bridgedeck Centerboard

Off the top of my head that doesn't seem right. Can you show your calculations?

 

Board in lee hull
Ce - Clr = 4m
Mhl = 4m * F * cos(10°)

Board in center
Half of boat width = 1,5m
Mhc = 4m * F * cos(10°) + 1,5m * F * sin(10°)

Mhc / Mhl = 1,066

6% difference

 

I'm not sure that's right-I don't see the beam entering into the calculation? If the CE on both are the same height above the CLR, why would the Heeling Arms and resultant Heeling Moment be the same?
Righting moment might be slightly better on the two foil boat because of the extra weight.

 

But if your centerline board is at a 10 degree angle, the board surface has been pulled from the water - some amount.
So the height above the Clr is no longer 4m.

Offsetting effects - how much I don't want to guess.

Also your hull has an effect on CLr.