Catamaran Hull Design

[BluWtrSail]

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

[tspeer]

Here's the exact passage from Shuttleworth:

"2.Pitching and hobbyhorsing.

Many early multihulls were prone to hobbyhorsing, and pitching. This was caused by too much rocker on the hull profile, and fine V sections both fore and aft. As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem, because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull (ref 2 and 3 ). This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind. At the same time windward performance is improved, because the apparent wind direction is more stable across the sails."

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.

[BluWtrSail]

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.

[quote=tspeer]
If there's a difference between the Cb and the cg, then there's a moment (torque) applied to the hull.

THERE IS NO DIFFERENCE.

This can only be the case (assuming steady conditons) if there's some kind of external load (besides gravity) acting on the boat. For a sailboat, the sail rig applies this kind of load.

I WILL HAVE TO REVERSE CALCULATE THIS AND DRAFT THE FOOTPRINT BECAUSE I HAVEN'T FIGURED IT OUT YET.

You can then map out the X-Y location of the Cb as a function of the heel and trim angles.

X=LONGITUDINAL, Y=LATERAL

CB
@ 0 DEG. HEEL, 0 PITCH = 225.7"X, 0"Y / WITH 0" OF ARM
@ 1 DEG. HEEL, -.01 PITCH = 211.9"X, .3"Y / WITH .4" OF ARM
@ 5 DEG. HEEL, -1.01 PITCH = 199.0"X, 1.3"Y / WITH 2" OF ARM
@ 10 DEG HEEL, -2.01 PITCH = 187.5"X, 2.5"Y / WITH 4.2" OF ARM
@ 15 DEG. HEEL, -3.01 PITCH = 177.3"X, 3.5"Y / WITH 5.7" OF ARM
@ 20 DEG. HEEL, -4.01 PITCH = 168.5"X, -135.3"Y / WITH 102" OF ARM ??!!
@ 25 DEG. HEEL, -5.01 PITCH = 161.8"X, 9.5"Y / WITH 5.7" OF ARM
@ 30 DEG. HEEL, -6.01 PITCH = 153.2"X, 11.4"Y / WITH 6.8" OF ARM
@ 35 DEG. HEEL, -7.01 PITCH = 147.0"X, 14"Y / WITH 7" OF ARM
@ 35 DEG. HEEL, -8.01 PITCH = 139.9"X, 15"Y / WITH 6.2" OF ARM
@ 35 DEG. HEEL, -9.01 PITCH = 132.9"X, 16"Y / WITH 5.3" OF ARM
@ 35 DEG. HEEL, _10.01 PITCH = 139.9"X, 15"Y / WITH 6.2" OF ARM
The final degree of pitch required to put the deck in the water is -16.24".

There will be some external load that can be applied that will result in the boat achieving any given heel and trim angle...

GOTTA WORK THIS OUT NEXT IF THE BRAIN DOESN'T SHUT DOWN.


Likewise, you can map out the X-Y location of the virtual cg for different combinations of apparent wind speed and apparent wind angle.

I'M GOING TO WORK ON THIS NEXT, BUT IT WONT HAVE ALL THAT INFO YOURS HAS OR BE AS NICE...NEED SOME TIME.

When you superimpose these two maps, you get the multihull footprint for that design. It allows you to quickly see what the heel and trim angles will be for different wind conditions for that hull design and that rig design.

YUP, JUST GOTTA WORK ALL THIS OUT FIRST...WANNA LEND A HAND?
BUT IF YOU DO IT FOR ME, I REQUEST THAT YOU EXPLAIN IT SO I CAN UNDERSTAND HOW TO WORK THE PROBLEM.

Thank's again for the pain...

Oh sugar...was I suppossed to calculate these at 6K#'s? Ahh man...
John

[BluWtrSail]

Hello Tom...

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

Originally Posted by tspeer

If you haven't arranged things that way in your design, then the boat won't be floating on her lines!

I WILL ARRANGE IT SO THAT EVERYTHING IS WHERE IT SHOULD BE!

In addition to the center of buoyancy, another key parameter is the center of flotation (LCF). This is the centroid of the waterplane. As the buoyancy changes, the change will be added or subtracted at the center of flotation. John Shuttle worth has some excellent advice on this in his article, Multihull Design Considerations for Seaworthiness.

I'M NOT SURE HOW JOHN CREATES THAT EFFECT, BUT EVEN THOUGH I DON'T KNOW HOW TO DO IT OR EVEN KNOW HOW TO CALCULATE MY OWN LCF, I WILL TAKE A VERY EDUCATED GUESSTIMATE AND SAY THAT IT IS JUST AFT OF THE CB JUST BY LOOKING AT THE WATERPLANE AREA.

In "Sailing Yacht Design - Practice" (edited by Claughton, Wellicome & Shenoi) Alexander Simonis..., The LCB for his designs varied from 53.1% to 56.43%, and the center of flotation varied from 53.53% to 58.42% of the waterline length.

MY CALCULATIONS SHOW THAT THE LCB AS A LIGHTSHIP RACER IS (3k#'S X 2) = 58.7% OF LWL AND AS A FULL CRUISER (4K#'S X 2) = 56.8%. ANY IDEA WHAT THESE INTERPRET INTO OR RECOMMENDATIONS ON WHETHER I SHOULD ADJUST THEM?

This is also partly why the bows have a slight angle forward instead of being plumb as seen on earlier cruising designs." For this design (the Moorings 4500), the LCB is at 55.23%, the LCF at 56.36%, the CE at 55.95%, and the center of lateral resistance (CLR) at 56.03%.

THE CE IS AT 47.3% OF THE LWL AND AT 16.08' ABOVE WATERLINE AND THE CLR IS AT 42.8%

Shuttleworth also presents a stability factor,
SF = K * sqrt( (0.5*B*D)/(SA*CE) )
B = beam, measured between hull centerlines (0.5*B is distance from c.g. to lee hull)
D = displacement
SA = sail area
CE = height of the center of effort of the sail rig above the c.g.
K = 9.48 in English units (ft, lb, mph)
K = 0.202 in SI units (m, kg, m/sec)

THESE ARE APPROXIMATE
SA = 600 SQ, GENOA (375) AND MAIN (225)
B = 10'
D = 6000#'S (LIGHT)
CE = 15.23' ABOVE CB
SF = 24.29...IS THIS RIGHT? HOW DO I INTERPRET IT? iS THAT GOOD, BAD, AVERAGE OR NORMAL?

Shuttleworth also recommends that the forward pitchpole stability be 81% of SF and the diagonal stability (combined heeling and pitching) be 115% of SF for a trimaran. I can't find his recommendations right now for a catamaran, but the pitchpole and diagonal stability ratios were a bit different.

HOW DO I SOLVE FOR THESE?


...displacement is 6000 lb. If boat were sailing dead downwind and the sail rig were to apply a moment of 83,400 in-lb, ...this moment could be produced by 417 lb of sail force acting on a center of effort that is 200 inches up.

TOM WOULD I SOLVE FOR FT-LB BY DIVIDING IN-LB BY 144? HOW DID YOU GET THE 417LB AND 200 INCHES?

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

[Skippy]

"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/sho...3960#post33960
Need reading material and assistance for an amateur with a serious interest!

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.

[BluWtrSail]

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