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Old 26-10-2016, 22:54   #1
tms
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Any Cats with a retractable centerboard?

I'm probably missing something obvious, but couldn't a Cat add a retractable centerboard under the bridgedeck. Instead of daggerboards in the hulls, like a Catana, a single long-slender retractable centerboard directly under the center of effort used exclusively for pointing, much like racing monohulls?

It could be 8 feet in the water and narrow. It might add some benefits of a Trimaran.
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Old 26-10-2016, 23:15   #2
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Re: Any Cats with a retractable centerboard?

Actually it is done, mostly in the larger open deck "beach cats", in the 20'-30' range. 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.

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.

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?

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.


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.
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Old 26-10-2016, 23:20   #3
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Re: Any Cats with a retractable centerboard?

^^^ Pretty much nailed it. Structurally difficult to do, less effective, in a bridgedeck boat you'd lose space in the main living area...it would have to be a swing keel, because a daggerboard type would end up in the way of the boom.
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Old 27-10-2016, 00:56   #4
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by tms View Post
I'm probably missing something obvious, but couldn't a Cat add a retractable centerboard under the bridgedeck. Instead of daggerboards in the hulls, like a Catana, a single long-slender retractable centreboard directly under the center of effort used exclusively for pointing, much like racing monohulls?

It could be 8 feet in the water and narrow. It might add some benefits of a Trimaran.
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.

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.

Potentially a safer alternative to a hull mounted dagger board.

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.

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.
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Old 27-10-2016, 01:18   #5
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Re: Any Cats with a retractable centerboard?

Putting them in the hulls has an interior space advantage. The center of the bridge deck is usually the center of the salon. Last place you want to lose room for a big structure that splits the salon in half.


Down in the hull, there is often a wall or cabinet face where you can hide them only taking up a small sliver of space.


Otherwise the other issues described still hold true....less efficient, harder to make structurally secure, etc...
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Old 27-10-2016, 01:54   #6
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Re: Any Cats with a retractable centerboard?

Probably the coolest Catamaran I have stepped foot on. 78' with retractable centerboard.. http://www.sailtrekker.com/
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Old 27-10-2016, 05:31   #7
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Re: Any Cats with a retractable centerboard?

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Probably the coolest Catamaran I have stepped foot on. 78' with retractable centerboard.. Sail Trekker - The Catamaran
It's actually a daggerboard, not a pivoting centerboard, but Cat's Meow is a truly impressive vessel.

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Old 27-10-2016, 08:24   #8
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Re: Any Cats with a retractable centerboard?

The Tomcat 32 has a bridge deck centerboard.

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Old 27-10-2016, 08:36   #9
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by tms View Post
I'm probably missing something obvious, but couldn't a Cat add a retractable centerboard under the bridgedeck. Instead of daggerboards in the hulls, like a Catana, a single long-slender retractable centerboard directly under the center of effort used exclusively for pointing, much like racing monohulls? It could be 8 feet in the water and narrow. It might add some benefits of a Trimaran.
I doubt you'll find a "centerboard" mounted in the middle of a cruising cat. In Europe, Outremers -with side hulls centerboards- are looked at as one of the best compromise between comfort & sailing performances. I include a picture of a 38/43 on the second hand market: catamaran-outremer.com/fr/Occasions
catamaran-outremer.com/fr/Les-Catamarans-Outremer/Modele/Outremer-45#1
Most people will prefer a newer "motorsailor" cat with lots of room, ugly deck cabin & two big engines. We see them all the time in & around St-Martin/Sint Maarten, with drunken charter "sailors", mainsail up & engines running non stop to keep the ice machine running
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Old 27-10-2016, 10:31   #10
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Re: Any Cats with a retractable centerboard?

The above are very true, but in addition...

A centerboard also needs to be much longer. Assume 3' of projected board, and 3' of bridge deck clearance...

In the daggerboard case you need a boardable to contain 1tonn*1.5'=1.5foot*tons of side force. Why 1.5 not 3? Because the side force is distributed evenly over the entire blade so the average location of the load is 1.5' down the blade.

In the centerboard case it needs to be able to contain 1tonn*4.5'=4.5foot*tonns of side force. Because you need to add together the average projected blade dimension, plus the bridge deck clearance. So the board needs to be three times as stiff.

This is a massive structural issue. It means the center board is probably more than double the cost of a single one, the trunk needs to be far stronger, and the mechanisms also need beefing up. At some point it isn't just easier and cheaper to move to two daggerboards.

Of course this is just one element. Daggerboards can also be towed in and asymetric, meaning they are even more efficient than a central board can be, and of course the end plate effect as mentioned.

Finally there is an aration issue. A surface piercing blade can suck air down the blade and cause it to ventilate. This is even worse when the blade is in the front of the boat since wave action can help to aerate it.
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Old 27-10-2016, 12:37   #11
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Re: Any Cats with a retractable centerboard?

Quote:
Originally Posted by jdazey View Post
It's actually a daggerboard, not a pivoting centerboard, but Cat's Meow is a truly impressive vessel.

Cheers,
Ah ha good catch,, It was early.
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Old 27-10-2016, 14:44   #12
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Re: Any Cats with a retractable centerboard?

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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.
You may not realize lifting foils and daggerboards perform entirely different functions?
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Old 27-10-2016, 14:48   #13
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Re: Any Cats with a retractable centerboard?

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....
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Old 27-10-2016, 15:04   #14
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Re: Any Cats with a retractable centerboard?

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Originally Posted by Dulcesuenos View Post
Probably the coolest Catamaran I have stepped foot on. 78' with retractable centerboard.. Sail Trekker - The Catamaran
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,

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Old 27-10-2016, 15:54   #15
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Re: Any Cats with a retractable centerboard?

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You may not realize lifting foils and daggerboards perform entirely different functions?


Ha thats funny Mr 44c

Back to school for you.

Whats providing the leeway resistance here then if its not the verticle portion of the lifting foil.
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