Sail Plans...again

[Aussie]

OK so now you've got me started... Ta to John for being concise and putting in the figures, and MMD for umm, providing the big phrases (and putting in the figures too) ;) and a shout out to Dingo! :D

The idea that John and MMD put across about moments of force working about a pivot point, I believe it's called a Metacentre (over here we call them shopping centres hoho!) makes sense. Like a see-saw really.

Now the next tricky bit (and this is for small boats) is how much does hull shape affect the rotation of the hull in the water. And how can I determine (with muh crayons) the point at which the boat would prefer to capsize.

And yeah, you're right, I should buy a book and read it, but this is much more fun. Jeez, can you imagine what this boat is gonna be like?

[Bruce Taylor]

Calculate the righting moment (righting arm X displacement) at each degree of heel and plot a static stability curve. At some point on your curve, the righting moment will plunge precipitously -- that's yr. point of no return.
Here are some articles about stability:

http://www.trailersailor.com/content/features/read.cgi?14

http://www.dockwalk.com/issues/2002/september/stability1.shtml

http://persweb.direct.ca/tbolt/stabilit.htm

Here's a fun website:

http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm

[ 11-26-2002, 11:55 AM: Message edited by: Bruce Taylor ]

[Ian McColgin]

With the crayons - now that's just my style.

For a small boat make the somewhat arbitrary assumption that the center of mass with no crew aboard is about amidships and, if partly decked, maybe about 1/3 from the bottom of the molded depth.

If you have well drawn plans with a projected waterline, assume the designer knew what s/he was doing and project the weight of the boat from the displace volume.

If you're doing the design, get the weights of some similar boats and figure you'll come close.

A super easy way to get the volume in curved shapes if you have access to a sensitive beam balance: Draw the underwater part of each station on graph paper and cut them out. Weigh them in ajoining pairs against not weights but some standard bits of graph paper with known areas. Now you have the areas for the stations and with the distance between stations you have the immersed volume. Convert to pounts at 64#/cu foot. We only do salt water here.

I made up this method back in '66 when I was taking quantitative chemistry at the university and was not a good enough mathematician - nor could I stand going to computer land and doing punch cards at ohdarkhundred when all the more important people were off the old monster - to do those horrid integrations.

Now you have a projected weight, a center for it, and a plan that will show you the center of bouyancy. Don't be surprised if the center of mass is above the center of bouyancy - it's a little centerboarder. If you just play with a cutout of the center station, tip the drawing to port say 10 degrees. The center of mass will of course now be displaced to port but the center of bouyancy will travel even farther to port. Positive stability.

Keep tipping and you'll see some interesting things.

Take two pretty simple and common designs - say a gunning dory and a sunfish.

The gunning dory has a very narrow flat bottom and about 15 degree deadrise in the garboard. Floating light, she hasn't much beam and the initial stability is very little. That increases steadily as the garboard gets immersed and continues to increase dramatically to the point where the rail touches the water. After that, the stability diminishes rapidly, accelerated by the water pouring over the rail but it does not actually vanish. Add the dynamic effect of crew and you have something that will actually surf broadside without tripping. I've done just that in Leeward in about 8' surf and she stayed with the wave face, really about 50 degrees past verticle but in terms of the water heeled about 10 degrees against the wave.

Now try your sunfish. She'll have lots of initial stability, which stays fairly high as you heel through maybe 25 degrees of so. After that the stability diminishes and somewhere just before 90 degrees the weight of the rig (which puts the metacentric pretty high) will carry her on over. Absent crew of course. But being a nimble sailor, you're already standing on the dagger board and planing on the rail!

Dream on.

[Don Maurer]

Ian,

Add the dynamic effect of crew and you have something that will actually surf broadside without tripping. I've done just that in Leeward in about 8' surf and she stayed with the wave face, really about 50 degrees past verticle but in terms of the water heeled about 10 degrees against the wave. Of course you did that on purpose, right? :eek:

[Ian McColgin]

I learned to do it on purpose.

The first time was an accident. I was bored with sedately coming in bow to the surf, backing in the troughs and rowing against each wave as it passed. I thought I could surf in and keep her from boaching.

My ex (Lee, hense the dory's name Leeward) was in the stern with a steering oar and I had another guy rowing aft of me. We were broaching anyway when he caught a crab. The oar flew up and knocked Lee off her seat, amazingly into the boat. At the same time he was flung forward into my lap so we all three lay in a tangle in the bottom of the boat watching this huge wave loom over us and terrified.

By the way, kayakers do this as well but most effectivly by paddling backwards in the trough and a ways up the wave to gain some speed before turning beam on and high bracing into the wave face.

Ah, speaking of dories in surf: We used to have some great races down in the Hamptons. Each skipper would sit in the unrigged boat and all the crews would shoulder one boat at a time, run into the water and toss it over the surf line. Once all the boats were launched, crews would swim out, we'd rig, and race.

One bright day there was a nice on-shore breeze and I tried to sail surf ashore. I pitchpoled in shallow water busting up the bow of the boat and tearing the rig to shreds.

That was in the '60's. A decade later, with Leeward, I still didn't mind risking a little breakage.

Fun with boats.

[Hans Friedel]

Try to get your hands on Frank Bethwaite High Performans Sailing.

You can also try this free design prog from Hullform.

www.hullform.com (http://www.hullform.com)

Thanks Bruce for the Java link

Hans Friedel