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  #1  
Old 7th March 2012, 05:42 PM
tevake tevake is offline
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Default centerboard: nacelle mounted(c.n.m.)

It seems to me that catamaran design would profit greatly from use of the c.n.m. location, but most reports say it doesn't work... Surely this state of affairs can't be satisfactory to someone versed in fluid dynamics or marine engineering... Afterall, if a) outboards function using cavitation plates, and b) hull-mounted boards give adequate windward performance, then, surely, there should be a solution for c.n.m.? For instance, if a NACA blade profile appropriate for , say, 8 knots, were fitted with a permanent cavitation plate (thus ruling out a daggerboard nacelle mount), then would not that plate prevent the migration of bubbles down the blade? I assume that the latter is why c.n.m. doesn't perform?
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  #2  
Old 7th March 2012, 06:13 PM
dmmbruce dmmbruce is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Presumably you are thinking of having the centreboard on the centre line of the nacelle. Then having it at approximately the centre of rotation of the hull as it pitches.

If that is so, it will have to be a long board. At least say 1.5 metres. That will mean some impressive pivot mechanism to support the side loads when it is down, since you won't have the luxury of having a large part of it supported by the hull in the nacelle. Conventional dagger boards are supported by the depth of the side hulls.

On my 26ft Heavenly Twin I have a central Outboard engine with a long leg. It is mounted in the aft part of the central nacelle which is just aft of the middle of the boat and is at the point of rotation of the boat. It is immersed up to say 10cm from the base of the engine. Nevertheless, in choppy conditions, I can get the prop over revving very near the surface. I can't see it to know if it breaks the surface. This would not do for a centre board if it is to be effective going to windward in a chop. So, as I said above, it would have to be big/long and the forces on the hinge would be considerable. My boat's nacelle is permanently just in the water. If you are considering this for a boat with a bridgedeck clearance, then this will make matters worse.

Overall, it seems to me like an interesting idea that may be too hard to engineer. But I don't know.

Mike
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  #3  
Old 7th March 2012, 07:14 PM
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Default Re: centerboard: nacelle mounted(c.n.m.)

To overcome the problem of getting the centreboard low enough into the water but still providing sufficient support for it, would it be feasible to fit the main centreboard within another board that itself can be partially lowered? So the outer board would be hollow and more substantially built than the inner board, with a telescopic method of operation. The outer board could be built to be lowered to just above the water. The inner board would then slide through it, and into the water. The outer board, especially, would need to be strong enough to support the substantial side loads but maybe this could be overcome by design + use of carbon fibre?

Cost and complexity are two reasons I can think of why you should not do this, and there are probably sound engineering reasons against the idea too, but if you really want to try it, it would be interesting to watch progress.
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  #4  
Old 7th March 2012, 09:36 PM
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Tropic Cat Tropic Cat is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Everything you guys said.... and the fact that the water beneath the bridge deck is constantly disturbed by the bow waves of the two hulls. So disturbed that any center board would have to go deep to compensate. I'm sure it's done all the time but the sheer size of the board would make this less than an ideal solution when scaled to cruising catamaran size.
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  #5  
Old 7th March 2012, 09:50 PM
smj smj is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

The first few Cherokee35's had this setup. After the first few they went to mini keels. I think the Tomcat also uses a centerboard located In the center of the bridgedeck.
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  #6  
Old 8th March 2012, 04:05 AM
Sand crab Sand crab is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

There's a 38' cat for sale on CL in the Gulf Coast area with a center daggerboard similar to what you describe. I posted about it earlier but not sure which thread. It's a great price if, if, if, if, if,,,,,,
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  #7  
Old 9th March 2012, 12:42 AM
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Default Re: centerboard: nacelle mounted(c.n.m.)

Didn't some of the Stilettos have a centrally mounted board? Don't think it would work for cruisers though for the previously mentioned reasons.
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  #8  
Old 9th March 2012, 03:29 AM
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searenitysail searenitysail is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

FSMike is correct--Stiletto 27s did have a centerboard, which tended to break under load-especially when racing. There is a VERY active OPEN Stiletto 27 class in Sarasota, Florida, where a company buys old Stilettos and refurbishes them. All of the OPEN class boats have been retro-fitted with dagger boards.

I closely looked at the Tomcat 9.7 at the Annapolis sailboat show in 2006. I posted this on CF some time ago in a thread comparing the Tomcat and the Gemini 105:

One area that hasn't been discussed is the single centerboard on the TomCat vs the twin boards on the Gemini. It is possible that many forum members are not familiar with the setup on the TomCat--I wasn't when I got on it in Annapolis and I was surprised at the design choice.


I was not impressed, in fact I was concerned. And though there were some things I liked about the Tomcat, the centerboard arrangement was the main reason I would not have picked it over the Gemini. I suggested someone who might have a Tomcat discuss this. This is what was posted by chichester, a Tomcat 9.7 owner in 2010:

http://www.multihulls4us.com/forums/...t-37944-2.html

OK, from my experience in owning the TomCat for 4 years...The centerboard mechanism does not work...it will break or seize. Although the builders of my Tomcat know about this problem, they have done nothing to help me as an owner or to change the set up. There are also problems with the mechanism to raise and lower the rudders because the slot they fit in is cut too small. Another problem was that the rudders kicked up anytime a little bit of seaweed was caught on them....As far as performance goes, the TomCat is not very good. It is a fractional rig so the headsail area is way less than the Gemini's masthead rig. Also, because it has no back stay, the shrouds start way aft. The problem with that is the downwind angle of the sail is limited because it hits the shrouds and the boom can't go far enough forward.

So, I guess there is some argument against a centerboard setup in the case of the Stiletto and Tomcat 9.7, anyway.

Marshall
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  #9  
Old 9th March 2012, 04:47 AM
ggray ggray is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

I don't think cavitation is the real trouble with surface piercing foils, it's that a surface piercing foil--or any surface piercing object, for that matter--will generate waves, or wake, and that means much more drag.
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  #10  
Old 9th March 2012, 11:53 AM
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Tropic Cat Tropic Cat is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Oh I think you can safely say it's a cavitation issue. If the Tomcat designer had tank tested the design, he would have changed it.

How many of those boats were sold? Are they still building?
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  #11  
Old 9th March 2012, 04:48 PM
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searenitysail searenitysail is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Quote:
Originally Posted by Tropic Cat View Post
Oh I think you can safely say it's a cavitation issue. If the Tomcat designer had tank tested the design, he would have changed it.

How many of those boats were sold? Are they still building?
According to chichester in 2010, 13 boats.

It looks like my link to the CF discussion of Gemini vs. Tomcat which includes chichester's comments about his 4-year ownership of the Tomcat was edited to point to the M4Us login page.

If anyone is interested in chichester's comments about the centerboard and his exchange with me, they appear on page 3 of this thread:

CF dot com /forums/f48/gemini-or-tomcat-37944-3.html

I can't believe I have to resort to this over here!

BTW, as of yesterday a reference to Multihulls4us (just like that) was not deleted in a thread on CF asking for information about Heavenly Twins catamarans! Just checked today--it is still there!

Marshall
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  #12  
Old 11th March 2012, 04:57 AM
tevake tevake is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

thanks guys for your thoughts onc.n.m, I guess I will be conservative and use a daggerboard in one hull, mounted in a centerboard case, so that the daggerboard can kick back up into theslot if it gets hit. I think someone mentioned that the problem there was that the daggerboard gets stuck in the cushioning material... the reason I was considering a c.n.m. was because I have a Deutz 2 cyl diesel that will be located in the nacelle anyway...
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  #13  
Old 14th March 2012, 03:17 AM
tevake tevake is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

to dmmbruce: thanks for your ideas about the outboard: I did not have any problems when I had a 10hp Chrysler Sailor long shaft mounted on the aft beam on a lowerable rail system; the prop was about a ft below the surface. The cat I'm building now will have a nacelle for the Deutz diesel, and the steel frame could be modified to support a c.n.m., but I'm apprised by the volume of replies to my thread to CEASE & DESIST. Better, I suopoose, would be a drop keel, which always keps the slot blocked...
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  #14  
Old 14th March 2012, 03:22 AM
tevake tevake is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Quote:
Originally Posted by ggray View Post
I don't think cavitation is the real trouble with surface piercing foils, it's that a surface piercing foil--or any surface piercing object, for that matter--will generate waves, or wake, and that means much more drag.
You make a good point; that is the difference between a "surface piercing"outboard shaft, and a c.n.m.: the latter produces a wave without compensating by producing drive and having a cavitation plate.
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  #15  
Old 14th March 2012, 03:31 AM
tevake tevake is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Quote:
Originally Posted by searenitysail View Post
FSMike is correct--Stiletto 27s did have a centerboard, which tended to break under load-especially when racing. There is a VERY active OPEN Stiletto 27 class in Sarasota, Florida, where a company buys old Stilettos and refurbishes them. All of the OPEN class boats have been retro-fitted with dagger boards.

I closely looked at the Tomcat 9.7 at the Annapolis sailboat show in 2006. I posted this on CF some time ago in a thread comparing the Tomcat and the Gemini 105:

One area that hasn't been discussed is the single centerboard on the TomCat vs the twin boards on the Gemini. It is possible that many forum members are not familiar with the setup on the TomCat--I wasn't when I got on it in Annapolis and I was surprised at the design choice.


I was not impressed, in fact I was concerned. And though there were some things I liked about the Tomcat, the centerboard arrangement was the main reason I would not have picked it over the Gemini. I suggested someone who might have a Tomcat discuss this. This is what was posted by chichester, a Tomcat 9.7 owner in 2010:

http://www.multihulls4us.com/forums/...t-37944-2.html

OK, from my experience in owning the TomCat for 4 years...The centerboard mechanism does not work...it will break or seize. Although the builders of my Tomcat know about this problem, they have done nothing to help me as an owner or to change the set up. There are also problems with the mechanism to raise and lower the rudders because the slot they fit in is cut too small. Another problem was that the rudders kicked up anytime a little bit of seaweed was caught on them....As far as performance goes, the TomCat is not very good. It is a fractional rig so the headsail area is way less than the Gemini's masthead rig. Also, because it has no back stay, the shrouds start way aft. The problem with that is the downwind angle of the sail is limited because it hits the shrouds and the boom can't go far enough forward.

So, I guess there is some argument against a centerboard setup in the case of the Stiletto and Tomcat 9.7, anyway.

Marshall
Searenity; Hi, it is great to hear from you and about chicester's experience on the tomcat. I have seen the construction of the Saerunner centerboard, and understand that it works reliably, doesn't clunk or bind, and really helps getting off a leeshore. Does anyone have experience with Searunner centerboards??
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  #16  
Old 14th March 2012, 04:20 AM
tevake tevake is offline
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Default Re: centerboard: nacelle mounted(c.n.m.)

Quote:
Originally Posted by Sand crab View Post
There's a 38' cat for sale on CL in the Gulf Coast area with a center daggerboard similar to what you describe. I posted about it earlier but not sure which thread. It's a great price if, if, if, if, if,,,,,,
hi sandcrab! thanks for the note about the 38 ft with c.n.m. i couldn't it...could you give me more info; I would try to find out how it sails, you know....
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