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  #46  
Old 01-12-2017, 01:09 PM
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brian eiland brian eiland is offline
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Split Asymmetric Centerboad(s) Under Bridgedeck

Quote:
Originally Posted by valery gaulin View Post
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.
Quote:
Originally Posted by rob denney View Post
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.
Perhaps have a look here....
Quote:
http://www.runningtideyachts.com/dynarig/
MISC ITEMS of note:

There is a nacelle structure down the centerline of the vessel that acts as a bottom truss member, acts as a wave splitter, and provides a mounting for two asymmetric centerboards, thus eliminating any daggerboard or centerboard penetrations into the main hulls. And everything,…cables, bearings, boards are all above the load waterline…serviceable in remote areas.
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  #47  
Old 01-12-2017, 01:23 PM
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brian eiland brian eiland is offline
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Here are a few other refences of postings where I have addressed this subject....

Quote:
Originally Posted by brian eiland View Post
Have a look here to a suggestion I made in 2003:
Big Cat, alt CB's & sail rigs

or here:
http://www.boatdesign.net/forums/sho...3&postcount=30
http://www.boatdesign.net/forums/sho...8&postcount=36

Then look at "ASYMMETRIC CENTERBOARDS" here:
http://www.boatdesign.net/forums/sho...9&postcount=16

...and finally on my latest DynaRig catamaran design:
http://www.runningtideyachts.com/dyn...enterboard.php
http://www.runningtideyachts.com/dynarig/

Obviously I think this makes sense
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.
Attached Thumbnails
Bridgedeck centreboard why don't they work???-asymmetric-cbs.jpg  Bridgedeck centreboard why don't they work???-central-cb-arrangemet-ps640.jpg  Bridgedeck centreboard why don't they work???-wave-splitting-nacelle-ps640.jpg  

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  #48  
Old 01-12-2017, 01:33 PM
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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.
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  #49  
Old 01-12-2017, 03:05 PM
Stumble Stumble is offline
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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.
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  #50  
Old 01-12-2017, 06:03 PM
UpOnStands UpOnStands is offline
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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.
Attached Thumbnails
Bridgedeck centreboard why don't they work???-screen-shot-2017-01-13-9.02.49-am.jpg  
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  #51  
Old 01-12-2017, 06:14 PM
upchurchmr upchurchmr is offline
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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.
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  #52  
Old 01-12-2017, 06:31 PM
UpOnStands UpOnStands is offline
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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.
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  #53  
Old 01-12-2017, 07:03 PM
upchurchmr upchurchmr is offline
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All true (I assume the loads are good).
It does not mean it cannot be adequately designed and built.
At a weight cost.
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  #54  
Old 01-12-2017, 07:08 PM
UpOnStands UpOnStands is offline
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just checked on Stiletto web site.
latest design, Stiletto X-Series, has in-hull boards.
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  #55  
Old 01-12-2017, 07:43 PM
upchurchmr upchurchmr is offline
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Thanks, I only knew about the original boats, and owners modifications.
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  #56  
Old 01-13-2017, 07:44 AM
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Stiletto correction

It was the original Stiletto 27' that had the central board. The 23' had boards in each hull.
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  #57  
Old 01-13-2017, 08:01 AM
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Quote:
Originally Posted by brian eiland View Post
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.
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.
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  #58  
Old 01-13-2017, 08:55 AM
upchurchmr upchurchmr is offline
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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

Quote:
Originally Posted by brian eiland View Post
It was the original Stiletto 27' that had the central board. The 23' had boards in each hull.
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  #59  
Old 01-19-2017, 10:24 PM
RAraujo RAraujo is offline
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Have a look at this...

https://www.youtube.com/watch?v=ZggxS6ZrVOw
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  #60  
Old 01-19-2017, 11:01 PM
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Happy Feet bi-pod rig, etc

Quote:
Originally Posted by RAraujo View Post
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/t...-feet/?p=13403
Quote:
Posted 17 June 2010
Happy Feet is a bi plane rig cat developed by Arnie Duckworth, and the centre pod goes side to side and carries a foiling centreboard and I think the rudder as well.

In theory you can lift the winward hull in the light, and then lift the leeward hull in the heavy stuff.

It has burned through a fair few sets of foils I think because the loads much be huge - Arnie is an American I think that basically grew up or lived a long time in NZ with various odd boats like that Beale cat rigged boat. He also has a foiling power cat up here, and basically could best be described as a legend having brought WEST system and Adhesive technoligies to NZ and Aussie and THailand along with all the advanced lamination techniques and so on that goes with it.

His house, everything about the guy is 'out there' and so no wonder he has a boat that is not exactly normal like the Pescott cats.

The traditional multi fleet have an issue with these biplane rigged boats. There are basically two groups; Arnie and the Yachting Siam boat builders who are keen on biplane rigs. THe issue is how much the 2nd rig should be rated at; when it was 55% (because the theory is downwind the 2nd rig is doing nothing) the biplaners cleaned up. When it was 75% then the traditional ones cleaned up.

In a blow the biplanes are damn quick, it's in the light (IMHO having watched them) particularly downwind that they suffer big time as they don't for the most part carry kites. I don't know how quick the pescotts are but they are quicker on line than the biplanes to date and most of the time, although apparently when it is lit up the Happy feet stomped all over them as those boots were made for walking, er , stomping in some faces.

Or something to that effect.

I'm not sure what Arnie's next 'mad invention' will be or if he will continue to persavere with the happy feet. Since the Seacart showed up, all the other multis look, for want of a better word, pretty average. That tri is f&*king quick.
PS: Appears as though the central foils (rudder and front foil) are cooperating pretty well with free surface of water.
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Bridgedeck centreboard why don't they work???-web-happy_fleet-catamaran.jpg  
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