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Cat Length question

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Old 11-17-2006, 10:36 AM
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Default Cat Length question

Theoretically, shouldn't a 38' Cat be able to pack more air underneath than a 37' 9" Cat assuming everything thing else is equal? I realize we're talking about a minuscule difference. But is there any reason the boat that's 3" shorter would be faster if everything else was equal?

Looking for technical responses please. No BS, No polotics.

Please.
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Old 11-17-2006, 10:57 AM
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Default Re: Cat Length question

Okay, I'll play. Theoretically a 37'9 1/2" boat could have less wetted surface in the water and be faster-overcoming the decrease in possible lift. Remember that most boats only have a fraction of the hull wetted-esp cats. Assuming a 2 foot wetted surface at speed the 2.5 less inches would have a much greater effect than 2.5 inches of a lift area that is 30 plus feet long. So there ya are...thanks for playing.
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Old 11-17-2006, 10:58 AM
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Default Re: Cat Length question

The width, height and funneling of the tunnel will also effect the amount/velocity of the air underneath the cat.
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Old 11-17-2006, 11:17 AM
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Default Re: Cat Length question

Quote:
Originally Posted by insptech
Okay, I'll play. Theoretically a 37'9 1/2" boat could have less wetted surface in the water and be faster-overcoming the decrease in possible lift. Remember that most boats only have a fraction of the hull wetted-esp cats. Assuming a 2 foot wetted surface at speed the 2.5 less inches would have a much greater effect than 2.5 inches of a lift area that is 30 plus feet long. So there ya are...thanks for playing.
the extra 3" inches wouldn't be necessarily wetted surface on a cat, the sponson could be 3 inches longer and would have negligible effect on the running attitude of the boat, although 3 more inches of tunnel could effect the running attitude greatly....

but to answer the question, tunnel length and boat length aren't mutually exclusive...a longer cat won't pack more air unless it has a longer tunnel....and if a longer cat has a longer tunnel, it also has more weight to lift now... kind of a viscous circle...
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Old 11-17-2006, 11:25 AM
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Default Re: Cat Length question

I don't know where the 2.5" is coming from, whether it's the tips of the sponsons or the transom which would affect the tunnel and airfoil, but either way we're talking about a difference of 0.5%. Say it is in the tunnel area, lift and drag behave linearly with surface area as defined by L (or D)=1/2 * rho * V^2 * S * Cl (or Cd), where rho=density, V=velocity in fps, S=planer area in sqft, and Cl=coefficient of lift (or drag). Again, 0.5% differences...maybe.

After running a quick CFD comparison, I see no appreciable difference.
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Old 11-17-2006, 12:01 PM
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Default Re: Cat Length question

Quote:
Originally Posted by CigDaze
I don't know where the 2.5" is coming from, whether it's the tips of the sponsons or the transom which would affect the tunnel and airfoil, but either way we're talking about a difference of 0.5%. Say it is in the tunnel area, lift and drag behave linearly with surface area as defined by L (or D)=1/2 * rho * V^2 * S * Cl (or Cd), where rho=density, V=velocity in fps, S=planer area in sqft, and Cl=coefficient of lift (or drag). Again, 0.5% differences...maybe.

After running a quick CFD comparison, I see no appreciable difference.
the 2.5 inches in question is overall boat length from tip of sponson to the top of the transom... there is no set length for tunnels...
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Old 11-17-2006, 12:01 PM
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Default Re: Cat Length question

Is being 3" short going to give ANY advantage? Does it have ANY disadvantage?

PS - Hello to my old friends Uncle Teddy and Nick
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Old 11-17-2006, 12:07 PM
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Default Re: Cat Length question

Quote:
Originally Posted by Sean H
the 2.5 inches in question is overall boat length from tip of sponson to the top of the transom... there is no set length for tunnels...
Gotcha! So, given two boats exactly 38' in overall length, their tunnels can have literally an infinite combination of characteristics (within their width limitations) pretty much indepentant of OAL. Therefore, differences in lift and drag are a moot point.
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Old 11-17-2006, 12:08 PM
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Default Re: Cat Length question

Quote:
Originally Posted by MarkMathews
Is being 3" short going to give ANY advantage? Does it have ANY disadvantage?

PS - Hello to my old friends Uncle Teddy and Nick
Back at ya! How's it going? Hope all's well.
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Old 11-17-2006, 12:12 PM
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Default Re: Cat Length question

Life is good.
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