Assymetric Cat Hulls

Attention, the boat you can see on this link is a heavy changed version of my original design. The changes where made without my consent !
The original is on the multihull gallery page 3.

Best regards

Bernd Kohler
K-designs
France

 

If posted already, I apologize:

The hulls in an asymmetrical hull catamaran have their zero lift lines at a toe-in angle. Think Clark Y vs symmetrical airfoil. This means that when going upwind with the hulls at a slight angle to the boat water velocity vector, the upwind hull produces more and maybe all of the lift. This is the reason why the forces generated by both hulls do not cancel each other. To be more precise, they only do so at 0 degrees water flow [directly ahead or astern]. Once you have an angle the forces become asymmetrical. The increased drag at higher angle of attack also allows the boat to resist the rotation associated with broaching. I remember Bernd Kohler commenting about this.

 

When you mean with upwind hull the lee hull you are correct. It is a very good explanation otherwise. A-symmetrical cats have a lot to go for, they are only no great load carriers. When you overload them the getting sluggish.

Bernd

 

You are perfectly correct. What I should have said was upstream hull, meaning the hull closest to destination.

 

Patty Cat by Rudy Choy

This video reference was just recently posted on another multihull form:

http://youtu.be/tjTAptscfQ4

While it doesn't really deal with the vessel characteristics so much, it does depicit the actual sailing involvement of a TV movie star that had a lot of influence with some of us older generation that sought to go out exploring the ocean/islands.

I commented on this over here:
http://www.boatdesign.net/forums/boat-design/motor-sailers-philip-rhodes-john-alden-16721.html"In this same era, 59-61, there appeared on TV a wonderful new series called “Adventures in Paradise” written by noted author James Mitchener and staring Garner McKay as the ex-Koran vet Adam Troy who bought an old sailing schooner Tiki and set up a trading business among the South Sea Islands.
http://capitainetroy.free.fr/eng/home.html
http://www.fiftiesweb.com/tv/adventures-in-paradise.htm

This was truly adventure inspiration, and certainly a big spark to my interest in cruising the world upon the sea.

 

The question in post#1 of this thread is "Are there any significant gains to be had by having assymetric hulls?"

The answer is emphatically ----NO.

This is proven by over four decades of multihull sailing experience.

The stunning victory of Derick Kelsals "Toria", with it's three symmetrical round bilge hulls laid that old chestnut to rest in 1966.

The hulls job is to support the displacement of the boat with the least possible drag.

Leeway prevention is the job of the various arrangements of the water foils.

 

History has proven (?) you right. Saying anything more would just create another meaningless argument like the resin infusion vs. everything else guys are having.

 

Prindle catamarans had assymetric hulls and faired very well in competition

 

So did Hobie 16. For several decades, Rudy Choy's assymetrical hull racing and cruising cats ruled the Pacific.

 

"Had" (past tense).

(past tense)

Present day. All significant Mullties have symmetrical hulls. QED.

 

But the Hobie cat assymetrical hull was a far inferior design to that of Prindle...its not just a question of one flat side and one curved one.

And it evolved from their 16 to their 18 assymetrical designs.


...and Rudy's hulls were not much more than Hobie's in basic concept


If you ever get a chance lay a Prindle hull and a Hobie hull next to one another upside down

 

In a Tri with symmetrical amas and a vertical centerline, once it heels and the ama starts to submerge, it starts to become assymetric.
This is usually addressed by canting the amas out at the keel.
Lock did it on the B24 by introducing di-hedral on the crossarm tubes.
Many early designers didn't bother, with the result that the induced assymetry was the wrong way and thus detrimental.

 

Asymmetrical Proa Hulls

I have read the above and wish to toss out a similar inquiry, regarding asymmetrical hulls for a proa type boat. I do not wish to instigate but seek info. I am clueless.


Below is a very basic outline of the very old but nifty Malibu Outrigger which I am too slowly redesigning and plan to build, in a turbo configuration, I hope.

It occurred to me that if it was a shunter, it might benefit from asymmetric main hull.

If it was a tacker, would a similar plan be advantageous overall, considering the different tacks?

I do not think so, but submit for knowledgeable confirmation or not.

 

Asymetrical Proa Hulls

I watched Malibu Outriggers sail at Malibu in the '50s and '60s at, would you believe, Malibu beach in CA. They were NOT proas, they were outriggers. They had a bow and a stern, came about like any other non-proa sailboat. So on one tack, they were "Pacific" and on the other "Atlantic" in proa terms. Most of all, they were splendid beach and surf boats.

Not sure what you mean by "turbo" rig. The rig they carried appeared to be efficient and certainly ample, especially for the usual Malibu off-shore breeze.

 

Malibu Outrigger

Yes, must have been fun to watch them sail...I have been a fan since the 60s.

I know the application of the term proa is loose when referring to the MO, etc. Still, as a tacker, mutual principles might apply and both are 'riggers.

The turbo-ing refers to open center and toe rails, bow/stern floatation and storage area, using pool noodles, lifting foils (maybe) or even asymmetrical hull and long skeg, 12' long/wide akas to very high and wide foam outrigger/ama with rudder and storage perhaps, trampoline, rotating mast, and maybe Bermudan cat rig...half vertical bow, spray rails and carbon spars.

Basically, not primarily a surf boat but would do, maybe occasional beaching.

No class org really, so will build according to plans I have for MO 1640 and incorporate logical changes as above.

Might catch on, with prices of boats and current economy. Wood 1200, glass/epoxy 500, trailer ?, sail ?. Carbon spars already own.

Also will use lighter wood/epoxy/glass...target is 70ish Portsmouth, but comfy beer cruises...what is not to like? Scale model hull pic below...now trying to find wood supplier in Twin Cities area.