Center nacelles and wave pounding in cats.

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Pretty cool stuff ,Raps-thanks!

 

Perhaps you might have placed that posting over on a mast, or sail, or wingmast related discussion?...probably better response?
...maybe here?
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-30.html

http://www.boatdesign.net/forums/sailboats/wing-drive-6698.html

http://www.boatdesign.net/forums/multihulls/homebuilt-wing-mast-30519.html

I just don't see where it relates to wingdeck slamming?

 

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Brian, the raised center nacelle on Coates cat seems relevant to the discussion to me.......Maybe you were thinking of post #60?

 

what if the center nacelle is able to move virticaly, so when its smooth you can ride on the outside sponsons and when its rough you can dip the center nacelle to be in contact with the water. The drag issues are not the primary concern in a high seas scenario and by dipping the center nacelle your eliminating a lot of pounding and lowering the CG. All you'd need is some hydraulics and the connecting arms could be the only real moving part.

just an idear
cheers
B

 

Righto Doug!

The boat reminds me of strings, the new gougeon catamaran.

 

So for a sailing 40´cat...

Hi all.

I'm new to the forum but bare with me.

It seems from what I read that a "square" 40ft cat should ideally be greater than 0.9m clearance, and the bridgedeck should be a short as possible.

But that would mean a total superstructure height to LOA ratio of around 1:4 if you want to have full standing headroom in the bridge. That is a seriously boxy boat, and what about the effect of all that windage in heavy weather scenarios.

Surely the bridgedeck need not be that high across its entire length. Couldn't it step down to 0.7m or even less at the saloon/cockpit bulkhead while retaining a minimum of 0.9-1m clearance elsewhere. Afterall, the most vulnerable areas for slamming are those nearest the ends of the craft. By the time a wave gets to the middle of the bridgedeck wont the boat have risen somewhat to the wave? That would get the superstructure height down to a more reasonable 2.5m on a 12-13m LOA while retaining a high bridgedeck over 90% of the bridgedecks surface. Also wouldn't breaking up the flat surface, or even giving the bridgedeck positive curvature fore and aft, strengthen the deck whilst reducing the area that slams at any particular time. Particularly the latter seems like a sensible way of of reducing slap as no matter how the wave hits the deck, its curvature would deminish the force of the blow.

 

Cross Referenced Discussions

I thought it would be interesting to cross reference these two interesting discussions on this subject on two different forums. Perhaps some info will be brought over to from each respective forum.

Bridgedeck clearance in manufacturers specs
http://www.multihulls4us.com/forums/showthread.php?t=4067

Brian

 

Quote:- Please keep this discussion going as I am most interested in hearing about other experiences. Quote.

My only experience in a Cat in open ocean conditions was when we sailed from Sydney to Lord Howe in 1981.
We were in a southerly buster and the waves were forty ft high , but 400 ft long.
We were sailing in John Hitches Spindrift 45 catamaran and hitting speeds of 17 kts.
The boat would climb fast up the watery slope and smash through the breaking crests in a huge ball of foam. I don't recall any heavy slamming, particularly as we rested in the big saloon rather than the hull double bunks. (Reason why I don't like double bunk cabins in the wings of any multihull other than a dock hugging "Roomeran").
I remember that we could paddle through under the wing deck on our sailboards without banging our heads.
Perhaps this and the wide stance between the hulls made all the difference.

 

Interesting observation just recorded 6/8/2015....

http://www.cruisersforum.com/forums/f109/which-power-catamaran-76926-8.html#post1843120

 

I was going back and reviewing some discussions we had about these central nacelle ideas. I think these two deserve repeating.
Brian

 

There will always be pounding, only character will be different. Anyone here serious enough about technology converting the slamming into forward thrust?

 

You might start with your idea to convert the slamming force to forward motion.

 

I understand this discussion concerns "sailing" cats. My only cat was a '65 PowerCat motored catamaran for river cruises. Never had a pounding problem, heavy rig...the only irritating problem was going airborne and coming back down...when all gear-loose went forward...I figure now speed precluded the pounding...but with the outrigger I use the tramp to dampen the wash, until the speed (VMG) picks up...

Am I missing something?