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Multihull Design Forum Discuss the design characteristics of multihulls

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  #1  
Old 11th March 2012, 04:24 AM
tevake tevake is offline
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Default centerboard; nacelle mount

the experience that nacelle mounted centerboards dont work should be examined in regard to a large cavitation plate positioned so it is always submerged when the board is lowered. this would eliminate air being sucked down the bladet. does our readership include anyone with access to engineered fluid dynamics?
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Old 12th March 2012, 06:33 PM
Sandy Daugherty Sandy Daugherty is offline
Boat: PDQ 36 catamaran "Page 83" 2001 Atlantic 42 "Siesta"
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Default Re: centerboard; nacelle mount

As a keen observer only, I think that a blade-nounted anti-ventilation plate would have to be quite large, and even larger at higher speeds. As always, the real-world trumps theoretical efficiencies: On a hinged, kickup board the plate would represent a huge amount of drag any time it isn't perfectly parallel to the flow of water, and that changes constantly as the boat pitches and rolls over wakes and waves, or heals a bit in a gust with some degree of crab.
It would represent a considerable amount of additional surface drag, too.
I remember experiments in the late 60's when AV plates were tried on transom-hung rudders. Did not work.

An alternative is a third "hull", much more than a small nacelle, that is always in the water. It could provide enough plate effect, room for an easily serviceable retraction system, and more storage. Or a generator?
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Old 12th March 2012, 09:22 PM
dmmbruce dmmbruce is offline
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Default Re: centerboard; nacelle mount

Quote:
Originally Posted by Sandy Daugherty View Post
................
An alternative is a third "hull", much more than a small nacelle, that is always in the water. It could provide enough plate effect, room for an easily serviceable retraction system, and more storage. Or a generator?
That is, in effect, what I have on the Heavenly Twin. It is noisy, but you get used to it. It slams, but you get used to it. It drags, but you get used to it.

Not recommended overall!

Mike
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Old 14th March 2012, 03:00 AM
tevake tevake is offline
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Default Re: centerboard; nacelle mount

thanks sandy daugherty for your thoughts. What you have said has convinced me that it would be better to use a single "drop keel" like in the Iroquois. BUT, then you mention transom-mounted rudders, and I'm a believer in them: had one on my Wharram Tangaroa, and it worked great, (together with a suitable profiled skeg. SO, if tansom-mounted rudders work, then why shouldn't a c.n.m.? Do you think it is because the t.m.rudder FOLLOWS the skeg and the hull stern is pointed?
tevake
p.s. this is my first time creating a thread, and I couldn't figure out how to reply to those kind enough to respond to my query. If this goes through, I will try to answer everyone who has posted one...
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Old 15th March 2012, 04:38 PM
Sandy Daugherty Sandy Daugherty is offline
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Default Re: centerboard; nacelle mount

I thought the transom mounted rudders on my Stiletto 30 were OK too, until I got my PDQ 32. The 32's rudders seemed woefully small to look at, but I had an Ahaa! moment when I sailed her. Science bears that out. I didn't say transom mounted rudders won't work. They do of course. and so did a board hanging over the starboard side. submerged rudders just work better, for two reasons: They can have a balance area in front of the axis of rotation to reduce the force required to steer, and the plate effect of the hull above the rudder almost completely prevents air being sucked down to screw up the hydrodynamics around the foil. I say almost, because if the cat is driving through wakes or waves, the hull might come up enough to let air into the flow around the rudder, and the cat heads up from the sudden loss of up to half its steering!
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Old 16th March 2012, 04:01 AM
tevake tevake is offline
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Default Re: centerboard; nacelle mount

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Originally Posted by Sandy Daugherty View Post
I thought the transom mounted rudders on my Stiletto 30 were OK too, until I got my PDQ 32. The 32's rudders seemed woefully small to look at, but I had an Ahaa! moment when I sailed her. Science bears that out. I didn't say transom mounted rudders won't work. They do of course. and so did a board hanging over the starboard side. submerged rudders just work better, for two reasons: They can have a balance area in front of the axis of rotation to reduce the force required to steer, and the plate effect of the hull above the rudder almost completely prevents air being sucked down to screw up the hydrodynamics around the foil. I say almost, because if the cat is driving through wakes or waves, the hull might come up enough to let air into the flow around the rudder, and the cat heads up from the sudden loss of up to half its steering!
I see what you mean, but I am designing for Florida SouthWest coast and the Bahamas, and the transom mount allows a "sheathed sword" style for gunkholing in shallow mangrove lagoons. I had no problems tacking on my Tangaroa, as the skegs were faired into the rudder profiles, and 'leak/through' was kept to a minimum. Incidentally, I found the following passage in the 1st edition - 1973 - of Jim Andrews "Cruising Catamarans":to go back ....to the ...centrally mounted drop-keel slung beneath the bridgedeck:...in my Shearwater the" c.n.m " had to be lifted up clear to do away with some entanglement of bladder-wrack and other marine and man-made ...which perpetually gathered against the leading edge..."p.140
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Old 26th March 2012, 05:44 PM
Sandy Daugherty Sandy Daugherty is offline
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Default Re: centerboard; nacelle mount

I think the number of successful examples clearly support the practicality of a single daggerboard in one hull on a catamaran. Tha means there is only one trunk to build, and the other hull is free of the interior obstruction, and the engineering to support the loads!
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