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Old 09-05-2008, 20:08   #91
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Therapy,

I think I recall seeing the link you are talking about. From what I recall the point the poster was making was that while a semicircle presents the lowest wetted surface for a given displacement, a square shape will handle an increasing load better. In other words, the wetted surface increases less for a square shape than a circular shape as weight is added. It was an interesting link, I wish I could find it.

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Old 09-05-2008, 20:09   #92
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"A cube has the highest volume with the least surface area" If you have to pick from among shapes made out of planes (flat surfaces,) only. Without this restriction, a sphere has the least surface for a given volume
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Old 10-05-2008, 00:14   #93
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sail area displacement ratio.

Tried googling up a few dimensions for both "magnitude" and "afterburner".
From what I can determine the 80' mono "magnitude" weights approximately 3 x as much as "afterburner" has twice the wetted surface and only 50% more windward sail area.

Are we still sure the problem of "no go" in light winds is a wetted surface problem?
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Old 10-05-2008, 00:41   #94
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Catty, I don't know of any way to find wetted surface without a lines drawing. I'm as sure as I can be without actually seeing numbers for the specific boats that wetted surface is the issue. It is basically an instance of the surface to volume ratio scaling phenomenon.
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Old 10-05-2008, 04:02   #95
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There is also the height of the mast . A taller mast can sometimes find that extra bit of breeze.
Robert
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Old 10-05-2008, 04:15   #96
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Quote:
Originally Posted by BigCat View Post
David is quite right, and Gideon is wrong if he means to imply that monos are slower than multis in light airs. A light multi has more wetted surface than a monohull, and it has even more when you add more weight to it. What David left out in his explanation for why multihulls are faster when the wind picks up is the "K" factor, which reflects the lower resistance of a much narrower hull. Cal is right about the effect of waterline length, but he is also leaving out the fact that the "K" factor gets higher if you stretch a hull to make it longer.

It is easier to design a narrow hull in a big boat, than in a small one, because the human body isn't scaling up as the boat gets bigger, and interiors are designed around the dimensions of the human body-if you are to have reasonable accommodations.
Big Cat

you have an imaginative mind to say the least , nowhere did I say or imply that Mono,s are slower than Cats in light airs. It must be what you have had to drink, please let me know what it was.

Greetings

Gideon
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Old 10-05-2008, 14:28   #97
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From another site:

...Jason's boat is less than half our displacement. In 2 knots of breeze we sat there watching him pull away. When we did have wind, we would make lots of ground on him but never enough to catch him. I would estimate that the average wind speed for the first 8 hours was only 4-5 knots...our gennaker needs 6 knots to stay full.

In light air, he can sail circles around us without a doubt and I'll have to live with the friendly bashing until we can get a club race on a windy day.
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Old 12-05-2008, 19:04   #98
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Originally Posted by Therapy View Post
I can't find what I saw the other day.

It was a link from here somewhere that showed several hull shapes.

I found this.

"2. Surface Area to Volume Ratio

A cube has the highest volume with the least surface area"

Much of this I don't understand well.
Not meaning to get argumentative, but a sphere does. BigCat already mentioned this. That's why bubbles and planets are spheres and not boxes or torpedo shaped or whatever.
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Old 12-05-2008, 19:14   #99
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BigCat,
Here is the best explanation I can think that will hopefully explain why cats have more wetted surface area per amount of displacement than a monohull:

Given...a sphere has the greatest volume per amount of surface area. Given volume equates to displacement (weight) for a vessel for everything below the waterline. Now given those two things, which shape, below the waterline, is closer to the shape of a sphere? Are two toothpick shaped hulls closer to the shape of a sphere or is a monohull closer to the shape of a sphere? All it takes is a little imagination to visualize this.
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Old 12-05-2008, 19:18   #100
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Strong drink and wetted surface

"imply that Mono,s are slower than Cats in light airs. It must be what you have had to drink, please let me know what it was." Hi, Gideon - I said if, because I found your remarks ambiguous. Alas, I drink nothing stronger than tea, as I have gout (no alcohol allowed) and am allergic to coffee. I would be happy to drink large quantities of both if my health permitted-but my wetted surface for both is zero for the foreseeable future! (Bandaged mouth logo=no strong drink allowed, in this case.)
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Old 12-05-2008, 19:36   #101
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Not meaning to get argumentative, but a sphere does. BigCat already mentioned this. That's why bubbles and planets are spheres and not boxes or torpedo shaped or whatever.
I get that when I think about it but I wish I could find that link I went to the other day. It was from someones post and showed several different hull designs and was inriguing. Partly because I have seen what I said more than once and don't understand it.

I know a sphere holds the most volume. Still not sure about surface area for mass displacement.

Only becaue I read it here - it has to be true!

Maybe someone will find the link again.

Just trying to get my head on it is all.

Not that I will design anything past my cat with twin masts (foam, alumimum and paper - 14in ) done 30 years ago!!
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Old 12-05-2008, 19:37   #102
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Hi, David - It is because there are two of them (hulls) that a catamaran has more wetted surface, not because one is more like a circle than the other one is. Two small shapes will have more surface area than one large one, for a given volume, if the shapes are the same.

Biologists say this is why mammals can't get larger than a certain size, because they will overheat due to a lack of surface area to radiate heat away. I imagine that this is why elephants have big ears, to radiate heat. Meanwhile, little mammals have a hard time staying warm for the same reason. Visualize a poor little shivering Chihuahua.
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Old 12-05-2008, 19:58   #103
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Quote:
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Biologists say this is why mammals can't get larger than a certain size, because they will overheat due to a lack of surface area to radiate heat away. I imagine that this is why elephants have big ears, to radiate heat. Meanwhile, little mammals have a hard time staying warm for the same reason. Visualize a poor little shivering Chihuahua.
Visualizing things in my head does not always prove them correct.
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Old 13-05-2008, 06:36   #104
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Hi, David - It is because there are two of them (hulls) that a catamaran has more wetted surface, not because one is more like a circle than the other one is. Two small shapes will have more surface area than one large one, for a given volume, if the shapes are the same.

Biologists say this is why mammals can't get larger than a certain size, because they will overheat due to a lack of surface area to radiate heat away. I imagine that this is why elephants have big ears, to radiate heat. Meanwhile, little mammals have a hard time staying warm for the same reason. Visualize a poor little shivering Chihuahua.
These are examples of application of the square-cube law.

See > Square-cube law - Wikipedia, the free encyclopedia

This also explains why really big monos (freighters) are the best designs for sea transport. Taken one step further, as Chris White discusses in his book, European's and North American's traditional choice of monos for sea vessels probably has its roots in commerce > these are the best choices for transport owing to the square-cube law - whether they knew that's what they were doing or not. Compare to the Pacific Islanders who adopted multis. They had no need to transport large amounts of cargo for commerce.

Dave
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Old 13-05-2008, 06:46   #105
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You can do all the fancy math and calculations you want, but as someone said earlier, you can't believe any manufacturers published weight; so this argument establishes and proves nothing.
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