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Old 31-10-2016, 13:29   #31
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Re: Any Cats with a retractable centerboard?

Gday Uncivilised

I have cedar boards with multiple layers of uni running down each side. The unis taper out about 500mm up from the bottom of the board. Then the only laminate is the 600db covering the whole board. I then cut the bottom 300 mm off and then glued it back on with epoxy glue. No glass. I then quickly faired the join.

The loads on the tip are low enough for this to work well. I have sailed my boat about 10 000 miles on 5 trips up and down Australia's east coast - not a gale ridden place but it can get windy and the tips are still on after 16 years. This trick was told to me by a very clever builder - Shawn Arber who used it on lots of his designs.

I don't think you could do it on foam boards as easily as cedar. BTW I actually built the boards an extra 300mm longer as I thought I may be losing tips often. So I have two spare tips sitting in my shed that I have never needed. This method would work only when you hit something with the bottom of the board. As yet I haven't had any glue line issues. In fact it has one extra benefit on that if you do damage the bottom of the board then the water cannot cross the glue line and so the rest of the board is quarantined from water ingress. It is a lot easier to do than crash boxes or similar.

cheers

Phil
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Old 05-02-2017, 20:35   #32
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Re: Any Cats with a retractable centerboard?

This subject of a 'bridgedeck centerboard/daggerboard' has come up again over in this forum, with lots of participation...
Bridgedeck centreboard why don't they work??? - Boat Design ForumsIt has also been a subject I have held a long time interest in, particularly for cruising boats.

So let me first deal with a few quotes at the beginning of this thread that have questionably content.(my remarks in bold)
Quote:
Originally Posted by UNCIVILIZED View Post
Actually it is done, mostly in the larger open deck "beach cats", in the 20'-30' range.
Brian replied: not so

But from a structural perspective, it's tough to pull off in most boats, as the loads on such boards are huge. So that without a hull there to support them it doesn't make much sense. As with a board placed thusly you have to add so much structure to support them, that you're far better off placing the boards in the hulls.
Brian replied: May not be as difficult as you make it out to be.

Plus there's the obvious fact that such a board has zero end plate effect, as it's entirely surface piercing. So it's operating at at best 50% of the efficacy of such a board which enters the water underneath of a hull.
Brian replied: Where do you come up with this figure of 50% ??

In your description of such a hypothetical board, you make it sound as if a daggerboard or centerboard is losing something by not being, say in line with the mast. And I'm unclear as to why?
Brian replied: Perhaps there are some advantages to having a 'centralized board. Will see in the following discussions.

Also, it's not as if tri's with central boards go to weather better than do cats. As if you'll note, serious racing tri's have boards in their amas, as well as in the main hull. For use even when one ama & the main hull are in the water. So a big central board isn't some giant advantage. Even racing mono's have offset foils on each side.
Brian replied: You are looking primarily at racing multihulls that fly a hull. Most cruising types do not do this,...so maybe different rules?


BTW, the efficency of daggerboards is far far greater than that of centerboards. Even if you take out the drag which occurs due to the huge open slot of a centerboard's case. As opposed to a daggerboard which fills it's case 98% of it's volume/slot area. Their foils shapes are way superior, & generally much deeper. In addition to being much higher in aspect ratio. So, superior many times over, hydrodynamically.
Brian replied: SO MUCH SUPERIOR...foil shapes superior ???? Why can a pivoting dagger board shape be just as superior? ...and are symmetric shapes superior to asymmetric shapes??...lots of questions
Quote:
Originally Posted by 44'cruisingcat View Post
^^^ Pretty much nailed it. Structurally difficult to do, less effective, in a bridgedeck boat you'd lose space in the main living area..
Brian replied: Not necessarily so,..I can show you some examples

it would have to be a swing keel, because a daggerboard type would end up in the way of the boom.
Brian replied: Likely needs to be a 'swinging type', but Kelsall manged to do a few designed daggerboard types, and not interfere with the boom.
(and I might even be able to do one on my aftmast rigged design without interfering with ANY boom (none there).
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Old 05-02-2017, 20:53   #33
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Board Construction (experimental boards)

I'm getting a little ahead of myself here, but I did take note of your posting Phil, ....and it just might be a good manner to build my 'development boards'
Quote:
Originally Posted by brian eiland
I think what I would do Doug is just make a couple of my twin asymmetric boards out of wood to start out with...perhaps out of laminating up some good plywood. The two boards could be made out of one long length of asymmetric shaped 'board', then cut in half to get the two mirror images for either side.

This would allow one to experiment with several different foil shapes at a relatively cheap price, then either glass these wood boards over, or build molds and do a fancier job in glass, carbon, whatever. The prototypes being solid wood would preclude any fancy bearing fixture for the boards themselves,...just smooth bore holes to fit onto the UHMWPE bearing pins.

And its not like it would be a great problem to change out boards while experimenting,...no hauling of the vessel, and basically just one giant pivot pin mounting them to that flat plate nacelle beam.

KISS principle
Bridgedeck centreboard why don't they work??? - Page 11 - Boat Design Forums
Quote:
Originally Posted by catsketcher View Post
I have cedar boards with multiple layers of uni running down each side. The unis taper out about 500mm up from the bottom of the board. Then the only laminate is the 600db covering the whole board. I then cut the bottom 300 mm off and then glued it back on with epoxy glue. No glass. I then quickly faired the join.

The loads on the tip are low enough for this to work well. I have sailed my boat about 10 000 miles on 5 trips up and down Australia's east coast - not a gale ridden place but it can get windy and the tips are still on after 16 years. This trick was told to me by a very clever builder - Shawn Arber who used it on lots of his designs.

I don't think you could do it on foam boards as easily as cedar. BTW I actually built the boards an extra 300mm longer as I thought I may be losing tips often. So I have two spare tips sitting in my shed that I have never needed. This method would work only when you hit something with the bottom of the board. As yet I haven't had any glue line issues. In fact it has one extra benefit on that if you do damage the bottom of the board then the water cannot cross the glue line and so the rest of the board is quarantined from water ingress. It is a lot easier to do than crash boxes or similar.
cheers
Phil
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Old 05-02-2017, 21:14   #34
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by Seaslug Caravan View Post
YES it can and has been done on many boats in the past.

The Stilleto 27 catamaran is one example of a larger vessel with this feature.
I am very familiar with these boats as I was once there largest healer.wholesaler. And I do know of the problems that the 27 experienced,...not overwhelming, but could be improved upon.

Quote:
Originally Posted by Seaslug Caravan View Post
In Australia many of the plywood home built OTB catamarans such as the Cunningham Quick Cat, the Arrow, and the Arafura catamaran all had a single central swing board.
Totally unfamiliar with any of these. Would you have any photos of their arrangements??

Quote:
Originally Posted by Seaslug Caravan View Post
Potentially a safer alternative to a hull mounted dagger board.
Possibly,..certainly for a cruising boat

Quote:
Originally Posted by Seaslug Caravan View Post
Some site the lack of hull endplate effects as being a negative for this feature, but I would like to note there is not much hull endplate action happening on the latest crop of foilers.
Good ponit!

Quote:
Originally Posted by Seaslug Caravan View Post
As for required structure I did see a 12+ meter catamaran launched a while back with a single central swing centerboard with the structural loads contained using a simple bracket and wires. All this was mounted under the high clearance (around 900mm) wing deck. The whole structure didn't appear to offer any more drag than what one would see from the common prodder stays and dolphin striker arangements seen on most modern racing catamarans.
Would you know anything more about this boat,...designer, builder,...photos??
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Old 05-02-2017, 21:19   #35
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by Dulcesuenos View Post
Probably the coolest Catamaran I have stepped foot on. 78' with retractable centerboard.. Sail Trekker - The Catamaran
Cat's Meow
How about a LARGER version of that daggerboard arrangement. Here is Kelsall's 78 footer with a central daggerboard, and a central rudder,..in a full length nacelle.
Sail Trekker - The Catamaran
Not sure why the website is titled Sail Trekker when the boat's name is "Cat's Meow".

This vessel has been thru St Augustine a few times recently, and I got a tour onboard on one occasion. I specifically wanted to ask the captain/owner about the effectiveness of this board arrangement. Unfortunately it appears as though the captain and his wife have a liitle trouble handling this size vessel on their own, and thus seldom sail it under full sail from what I understand. She does have quite a BIG rig.
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Old 05-02-2017, 21:25   #36
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by Cheechako View Post
Most people think bridgedeck clearance to the water is important on cats. This interferes with putting a centerline daggerboard as the unsupported length of the board gets pretty long....
Not as bad as you might suppose, particularly if you want to keep the pivot point of your hull mounted centerboards above the waterline (safety...no leaking...etc)
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Old 05-02-2017, 21:36   #37
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by DerekKelsall View Post
Sailtrekker was built in the Isle of Man as "Mannanan". The central dagger board is angled back, so come out ahead of the mast, taking no space from the accommodation. We designed a number of cats this way. A nacelle runs from the forestay chainplate (cantilever - no bridle), supports the dagger board and runs back to take a single rudder. It is the good old design compromise but I would repeat this any time on a cruiser, particularly as the simplest way to be able to reduce the draft to the minimum for beaching (kick back , lifting rudder). A couple of 45 footers have kickback boards as well as lifting.

For area, we measure only the area of board below the hull depth. We tried spoilers but saw no advantage. With single rudder, I advise using trim boards in the transom steps. Small but takes a lot of the load off the rudder in difficult steering conditions.

Tinkerbelle is a Kelly 45 owned by same owner for 40 years. he and his family have owned a number of cats. The single board and single rudder and features he would want to repeat is replacing Tinkerbelle.
Happy boating,

Derek Kelsall.
I agree with you Derek,...particularly for a cruising boat that does not plan on flying any hulls.

A few other 'believers'...
Quote:
Originally Posted by rob denney
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.
DynaRig MotorSailer
Quote:
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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Old 05-02-2017, 22:18   #38
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Re: Any Cats with a retractable centerboard?

Interesting cat and interesting discussion. I guess, this cat is the one of its kind, though.

Quote:
Originally Posted by beiland View Post
Cat's Meow
How about a LARGER version of that daggerboard arrangement. Here is Kelsall's 78 footer with a central daggerboard, and a central rudder,..in a full length nacelle.
Sail Trekker - The Catamaran
Not sure why the website is titled Sail Trekker when the boat's name is "Cat's Meow".

This vessel has been thru St Augustine a few times recently, and I got a tour onboard on one occasion. I specifically wanted to ask the captain/owner about the effectiveness of this board arrangement. Unfortunately it appears as though the captain and his wife have a liitle trouble handling this size vessel on their own, and thus seldom sail it under full sail from what I understand. She does have quite a BIG rig.
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Old 06-02-2017, 00:17   #39
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Re: Any Cats with a retractable centerboard?

Perhaps this is repetitious, but just a little different wording,...

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.


something like this...

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