Asymmetric hulls

[Jimbo1490]

The Prindle 19 as well as the Tornado, on which the P19 design is based, have pivoting centerboards rather that traditional daggerboards. They beach quite well and after 17 years of ownership I have never broken a board after literally thousands of beachings. The difference in ease of turning is really amazing. I think everyone learning to sail in a beach catamaran as I did first learns to tack by backwinding the jib to carry you around. Later you learn to tack more properly so as not to lose so much speed in the tack. But in a rough sea (20-25 kts seas 5-8', IIRC) the quartering sea would not even allow my P16 to go safely into the irons. And trying to travel backward the heavy seas almost pitchpoled me backwards twice! Of course you should never go out solo on a P16 in 5-8' seas but for the record, it was only 10-15kts and 3-5 ' when I set out. These things happen with the ocean A small lake like Lake Murray (50,000 acres) is MUCH safer.

Since owning the P19, we actually look forward to the rough stuff because the boat handles it so well! I have sailed along for miles in a deep reach with the tip of the lee hull buried with no apparent affrect. The boat does not even seem to slow down. I have never even come CLOSE to a pitchpole, even with severe weight/balance abuse, downwind with spinnaker flying, etc. The other great rough sea beach cat is the Nacra/Inter 20 and of course any of the Supercat/ARC ships. The Tornado would be a great heavy air boat but for the lack of buoyancy forward of the front crossbeam. Not that it's bad, mind you; it's just not outstanding.

Jimbo

[Stephen Ditmore]

I've discussed this issue with Rudy Choi and with a fella in Gloucester Massachusetts (USA) who built a proa that was dead flat on the leeward sides.

The proa builder said it was too extreme. The boat would actually climb to windward, he claimed, but he felt the vessel was not as fast as it could have been.

Rudy Choi would multiply the inside offsets of his catamarans by a constant factor to get the outside, and claimed that some of his designs, when flying a hull, had effective waterlines closer to symmetrical than they would have been had he built symmetrical hulls.

That raises another possibility.... achieving effective asymmetry by canting symmetrical hulls.

Today there are analytic tools like Splash CFD that I think should be employed if possible to answer these questions. As with modern foil design, the shaping should be an outgrowth of the flow analysis, where possible.

I'd love to hear from Tom Speer on this topic.

[Leo Lazauskas]

Originally Posted by Stephen Ditmore

I've discussed this issue with Rudy Choi and with a fella in Gloucester Massachusetts (USA) who built a proa that was dead flat on the leeward sides.

The proa builder said it was too extreme. The boat would actually climb to windward, he claimed, but he felt the vessel was not as fast as it could have been.

Rudy Choi would multiply the inside offsets of his catamarans by a constant factor to get the outside, and claimed that some of his designs, when flying a hull, had effective waterlines closer to symmetrical than they would have been had he built symmetrical hulls.

That raises another possibility.... achieving effective asymmetry by canting symmetrical hulls.

Today there are analytic tools like Splash CFD that I think should be employed if possible to answer these questions. As with modern foil design, the shaping should be an outgrowth of the flow analysis, where possible.

I'd love to hear from Tom Speer on this topic.

Here's a link to a paper that might help the discussion along a bit...

TUCK, E.O. " Can lateral asymmetry of the
hulls reduce catamaran wave resistance? ",
20th International Workshop on Water Waves and
Floating Bodies, Spitzbergen, Norway, 29 May-1 June 2005.
Proc. ed. J. Grue, University of Oslo.
http://internal.maths.adelaide.edu.a...s/vortex04.pdf

I have verified Ernie Tuck's calculations and agree that asymmetry is likely to have some minor advantages but only when the hulls are not optimally-spaced. If they are at their optimal positions (for a particular speed) then asymmetry is unlikely to lead to any significant advantages.

All the best,
Leo.

[yipster]

Leo, thanks for the " Can lateral asymmetry of the hulls reduce catamaran wave resistance? " input
great study and math on wave resistance yet other factors like stability, windage, seaway,
construction and amaacces may be more in favor of cambered hulls

[Leo Lazauskas]

Originally Posted by yipster

Leo, thanks for the " Can lateral asymmetry of the hulls reduce catamaran wave resistance? " input
great study and math on wave resistance yet other factors like stability, windage, seaway,
construction and amaacces may be more in favor of cambered hulls

No argument there. It would be nice to see some actual computations or (repeatable) experiments to verify any performance claims. Advantages in construction and access are, of course, not really comparable to performance.

[fhrussell]

Originally Posted by Stephen Ditmore

That raises another possibility.... achieving effective asymmetry by canting symmetrical hulls.

I'd love to hear from Tom Speer on this topic.

But if you cant symmetrical hulls, then when heeling, the hull is in a symmetric attitude...correct? Lock Crowther did this early on. The Catana's are also designed this way...if I am following you on this.

Yeah,...where is Tom?!?!

[Stephen Ditmore]

Originally Posted by fhrussell

But if you cant symmetrical hulls, then when heeling, the hull is in a symmetric attitude...correct?

Affirmative. Conversely, if you don't cant a symmetrical hull it will become asymmetrical as it heels (esp if rocker and waterline half breadths are very different).

Yipster, buddy.... by "cambered" do you mean rockered, or something else? Perhaps you're saying one could create an asymmetric hull by taking a symmetric hull and plotting the transverse offsets from a cambered axis (the way some asymmetric foils are created from symmetric foils). Do you know of any designer who has done it that way?

[yipster]

Stephen D, good to check my rough language but after a check it is the outstanding camber as in cars frontwheels i mean.
sitting here with two envellops bottoms out wider, thinking bout how computers determine the uniform car design nowaday's.
tv meanwhile tells planes structurals are computerised but still need a windtunnel, by the time its all virtual,
materials, pricing and what else computerised it probably becomes a reversed process where the sum be the input?
now these envellopes on the desk can also open each "hull" asymetric, sb wider than ps and mirrored making them asymetric right?
hmm hairspray and color inkt testing next?

[dutch1955]

Hi my name is Curtis
I'm looking for a trailerable tri with schoal draft, large cockpit for day cruising with friends. The Corsaire Sprint comes the closest, would asymetrical amas work on a tri? Barnegat bay is my homewaters and sandbars and mud flats surround me.the R22 mono is 2nd choice but I want more choices of where to sail (speed) like going around the island instead of up and downthe bay all day.
Does such a boat exist? thanks

[fhrussell]

Asymetrical amas are not unusual. I know Jim Brown had designed them many years ago and the general concensus seems to be positive. Also, Newick has an interesting asymetric ama design that starts as an ellyptical shape. You should look at a Tremolino, too...if that is the size that suits you.

[mattotoole]

Originally Posted by fhrussell

another thought on asymmetric hulls. Talking with Roy Seaman, (son of Warren Seaman (CSK), designer of several Nacra Cats, multiple winner of the Worrell 1000, batten maker, and helmsman on Aikane X5 when they broke the TransPac record) he is not a big fan of asymmetric hulls. His comments were that up to a certain speed they make sense. Aikane X5 was capable of speeds above 30 knots. Roy claims that at about 25 knots the boat would 'shudder'. The Randy Smyth designed rig was very powerful and wanted to push the boat faster than 30 knots, but the hulls, being asymmetric, wouldn't go through the water as efficiently as the rig needed.

I raced a Hobie 16 for a few years. I can vouch for their having an upper speed limit beyond which they don't go easily, no matter how well sailed or how strong the wind. My experience was before the days of handheld GPS, but it seems to be around 15-18kt. I'm sure a modern H16 sailor could corroborate this. The H18, OTOH, would easily go faster and faster. This may not be all due to hull symmetery though -- the H16 has bouyancy and trim issues which may cause at least as much drag.

It was a planing issue...he said the hulls wanted to 'break out', but couldn't because they didn't plane at all, like a D-section hull would ala Tornado, Nacra, etc.

None of these cats plane. Skinny hulls are just easy to push beyond conventional hull speed.

[mattotoole]

Originally Posted by dutch1955

Hi my name is Curtis
I'm looking for a trailerable tri with schoal draft, large cockpit for day cruising with friends. The Corsaire Sprint comes the closest, would asymetrical amas work on a tri? Barnegat bay is my homewaters and sandbars and mud flats surround me.the R22 mono is 2nd choice but I want more choices of where to sail (speed) like going around the island instead of up and downthe bay all day.
Does such a boat exist? thanks

You might consider the Windrider 17. The actual cockpit is small, but there's plenty of lounging room on the tramps, and plenty of bouyancy to carry a few folks. Of course it's more exposed than a Corsair/Farrier tri would be.