Catamaran Hull Design

Hello Nero...

Actually everything you said is true to me. The design is as you said. It's multi-chine and it is bulbous as you said. I dont have a lot of fore foot and the stem doesn't have a lot of overhang, but I flared the bow for reserve bouyancy.

The stern doesn't drag and yes it is swung up as you said. I don't have a lot of rocker because it seems like it contibutes to pitching as does long bow and stern overhangs.

I wish I knew how to put the design on here for you guys to look at. Maybe I'll take some time to learn how to do that. I wonder if my Cad will allow me to do that...although it is not entirely complete.

Well thank's once again for the info...you and Tom are keeping me in brain pain. But hey, ask a question...get ready for target practice.

John

 

Here's the exact passage from Shuttleworth:

I suspect the impact on pitching comes from coupling pitch and roll. The dynamic motion has two parts. One is the excitation by the waves. The other is the boat's response after having been disturbed. Say you jumped off the bow with the boat sitting in flat water. It's going to rock back and forth in pitch for a while until it damps out.

But hulls designed the way Shuttleworth describes won't act that way when the sail rig is loaded so the hulls carry unequal displacement. The lee hull will have its center of flotation ahead of that for the windward hull. So as the boat pitches forward, the lee hull will tend to roll it to windward. As it pitches up, it will tend to roll to leeward. If you looked up, you'd see the mast describing an ellipse, probably aligned at an angle to the boat's axis. With pure pitching motion, the mast tip would describe a fore-aft line.

Now the foils and sail rig have tremendous damping in the roll axis, and contribute very little to damping in the pitch axis. By coupling pitch and roll together, the roll damping quiets down the pitching motion, too. Anything that bleeds energy from a dynamic mode will damp the whole mode, and I think that's what Shuttleworth is achieving.

 

Hello Tom...

I said I would try this and I came up with some numbers, a real pain in the cahoona too, but here it is if it is correct. I'm going to have to do the sail calculation thing in reverse if I figure it out, but at least I have the numbers for the CG in different heel and trim angles.

Because it is a Catamaran, I was not sure on how much heel to use, but I figured that at 35 Degrees I will probably be screaming "LOOSE THE MAINSHEET AND HEAD HER UP" like my grandfather used to scream at me while he did 4 G's to reach the cockpit from the bow to slap me across the nuggin. I did however pitch it to the point where the deck is submerged at 35 degrees and 10.01 degrees which looks like I would probably have had a heart attack by then.

Here we go... 1 hull/ 3000#'s/ CB=CG=225.7" aft.

 

Hello Tom...

Okay, I worked on some of this and I figured I'd share this info. I hope I got it right.

Well, there it is...close approximations, but close. Care to share your analysis on the hull thus far and make some recommendations? What's next for me to do at this point?

Thank's

John

 

"What's next for me to do at this point?"
It wouldn't hurt to read up on boat & hull design a little.

Ted Brewer Presents A Primer on Yacht Design
http://www.boatdesign.net/forums/showthread.php?p=33960#post33960
http://www.boatdesign.net/forums/showthread.php?t=6721&highlight=boatbuilding+book

A catamaran typically won't heel more than about five degrees unless you're flying the weather hull, and if anything, I think the pitch should be the same or less. You should be able to close reach without heeling too much, and broad reach without pitching too much. For that, you need to know how much the rig will be trying to roll & pitch the boat, so you can make the hulls & rig match.

 

Hello Skippy...

I read quite a bit...tried to study everyone as much as possible and sort of settled in the middle for those things that were in disagreement.

I only went that far on the heel and pitch to try to get a comparable for a tri like Tom suggested, but if you look closely, I did a 5deg. heel just for the Cat. I think that even 5deg. might be a little high, but in order for me to fly the leeward, I need about 8.9deg with the wind dead on beam and by that time I will be at panic stations. I have no intention of flying anything except the jolly roger.

As for matching Rig and hull...thats why I'm here...to belabor the different issues and learn. I would appreciate any suggestions on the design as I've described it so far. Critique is always welcome.

Thank's

John