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Old 19-07-2008, 12:13   #1
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illusion, paradox?

The "facts":

1997 Gemini 105M 34' Catamaran
32'6" LWL
Draft 5' 6"
Displacement 4.25 Tons

2 Hulls Inc



1985-6 Catalac 8M 27' Catamaran
27? LWL
Draft 2' 4"
Displacement 5 Tons


2 Hulls Inc


Anyone??
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Old 19-07-2008, 13:50   #2
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Two different boats, two different spec's.
What's your point?
Here's a third boat (different, again):
The Albin Vega 27 yacht / day boat - information about sailing, cruising and racing the boat
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Old 20-07-2008, 11:15   #3
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shouldn't the larger boat displace more water... not less?
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Old 20-07-2008, 11:45   #4
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The larger boat should displace more water, all else being equal. However, there are a lot of things that are not equal between the two examples.

In this case, you can step aboard both and see why the Catalac displaces more water.

In general, if you have an oak cube, 1cuft in volume floating next to a balsa
cube of the same volume, which one displaces more?

The answer to that question is the same as the example between the Catalac and Gemini. One is made out of heavier stuff and one is made out of lighter stuff.
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Old 20-07-2008, 13:23   #5
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Very good.
At first glance, it would seem that the Gemini should displace considerably more water than the Catalac. In fact it is less... so this solves the conundrum... or does it? Does the Gemini have deep centerboards? Obviously. If it doesn't and the draft really is 5'6" then something is amiss here? Obviously draft / hull shape play a bigger role than materials(at least in this case...?... it's not oak v/s balsa.

gord, nice looking boat ...what's Your point?


Not trying to be obtuse... just perplexed by the initial brainfart I had when I noticed this.
cheers,
mm
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Old 20-07-2008, 13:45   #6
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I am not sure I am clear on your assumptions, but it could just be my english translation. Displacement is not about how deep the hull is in the water. Displacement is weight. And a hull will settle to a "point" in the water once the same weight of water as the boat is "displaced". So a centreboard 3ft down compared to a centerboard up does not mean the hull displaces more. Sorry if I misunderstand your comment.
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Old 22-07-2008, 10:59   #7
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This is Archimedes' principle. The buoyant force acting on a body submerged is equal to the weight of the fluid displaced by the body. And, since the body isn't accelerating up or down, that displacement force is exactly equal to the weight of the body. That is why the word "displacement" is interchangeable with the word "weight."
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Old 22-07-2008, 11:19   #8
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There's considerable difference in the build techniques between these two examples. Unfortunately, reading about them on the internet doesn't truly explain the discrepancy. Come on down to my marina and have a look at both boats. You'll understand why the smaller boat is in fact heavier.

Quote:
Originally Posted by mangomuffins View Post
The "facts":

1997 Gemini 105M 34' Catamaran
32'6" LWL
Draft 5' 6"
Displacement 4.25 Tons

2 Hulls Inc



1985-6 Catalac 8M 27' Catamaran
27? LWL
Draft 2' 4"
Displacement 5 Tons


2 Hulls Inc


Anyone??
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Old 22-07-2008, 13:40   #9
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The Gemini has centerboards in each hull. When raised, the boat will float in about 18-20 inches of water. Having chartered one for a week, one does get a lot of boat for the money but it is not a boat I would use for other than coastal cruising.

s/v HyLyte
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Old 22-07-2008, 15:08   #10
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HyLyte has the correct answer.
When just reading the draft measurements, one might assume that the Gemini is displacing much more water that the Catalac. As it is, the hulls displace almost the same amount with the smaller boat displacing a tad more...because it weighs more. It's the draft measurement that makes the figures look odd at first glance.

thanks,
mm
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