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Old 08-06-2010, 17:12   #1
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Displacement Calculation

Hoping someone here can advise on how the displacement of a vessel is calculated... I realise its not the net weight of the boat and it is the weight of the water that is displaced by the boat. Just dont know how that is calculated and would like to understand it.
Thanks
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Old 08-06-2010, 17:33   #2
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The designer calculates the boat's displacement by determining its underwater volume at a given waterline. The total submerged volume enclosed by the hull surface is the displacement, and it's usually stated as the mass of this volume of water (at some standard density, usually 1000 kg/m3 for a freshwater boat and 1025 kg/m3 or so for an ocean-going boat).

The displacement in any given operating condition is exactly the true mass of the boat and everything on board.

The displacement used by the designer is given at some arbitrary waterline that he/she thinks will represent a normal operating condition for the boat.

The displacement given in a marketing catalogue often doesn't mean very much. Sometimes it's the true design displacement (as intended by the designer), but sometimes it's the weight of the boat when it leaves the factory, without anything in its tanks and without any crew or gear on board.
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Old 08-06-2010, 18:12   #3
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Displacemt Calculation

Thanks Matt, appreciate your input.
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Old 09-06-2010, 06:21   #4
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“The displacement of a boat is its fully loaded weight. This is also equal to the weight of the volume of water the hull displaces or moves aside when it is lowered into the water or launched.”

From The Propeller Handbook ~ by Dave Gerr

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Old 09-06-2010, 17:32   #5
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Jon, for a very very rough number, you can think of a typical displacement hull as a rectangular block that has been sliced diagonally and then put back together again in a "V" shape, so that the (very rough) displacement volume will be in the same range as a rectangular block the length x 1/2 width x depth of the boat.

Very rough place to start, but possibly better than nothing.
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Old 10-06-2010, 14:32   #6
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Hoping someone here can advise on how the displacement of a vessel is calculated... I realise its not the net weight of the boat and it is the weight of the water that is displaced by the boat. Just dont know how that is calculated and would like to understand it.
Thanks
You can weight the boat, calculate it by finding the weight of everything on the boat (64 page of calculation for my 37 foot boat) or use the "Tchebycheff or Simpson" rules. Due to the accuracy of simple crane weight read out ( can be out by 10% and do not forget to deduct the weight of the lifting equipment) the other methods are far more accurate.
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Old 10-06-2010, 18:38   #7
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"You can weight the boat, "
But the weight of the boat has nothing to do with the displacement! Displacement corresponds to the volume of the boat. A boat weighing a thousand tons, built of solid lead with no spaces below decks, and a boat weighing a thousand pounds, built of nothing more than an empty fibergalss shell, might displace the same weight of water, while there was nearly a thousand ton difference in their crane weights.
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Old 10-06-2010, 18:47   #8
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By a very basic principle of physics, the weight of the water that a vessel displaces IS the exact weight of the vessel...period.

In my naval architecture class, the displacement of a ship was determined by adding the sum of the volume of the cross sections (cross sectional area times length). The greater the number of samples cross sections, the more accurate the determination of displacement.

Naval architecture software determines volume at a given draft. The draft readings do determine displacement and weight if you know the water density.
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Old 10-06-2010, 20:11   #9
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"You can weight the boat, "
But the weight of the boat has nothing to do with the displacement! Displacement corresponds to the volume of the boat. A boat weighing a thousand tons, built of solid lead with no spaces below decks, and a boat weighing a thousand pounds, built of nothing more than an empty fibergalss shell, might displace the same weight of water, while there was nearly a thousand ton difference in their crane weights.
Where did you get that one? Bunnings, you naughty, you really like to confuse people.::fl owers:
Without going back to Archimedes, try to get hold of Douglas Phillips-Birt book Sailing Yacht Design, ISBN 0 229 11563 2, quite a good read.
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Old 10-06-2010, 20:25   #10
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Originally Posted by hellosailor View Post
"You can weight the boat, "
But the weight of the boat has nothing to do with the displacement! Displacement corresponds to the volume of the boat. A boat weighing a thousand tons, built of solid lead with no spaces below decks, and a boat weighing a thousand pounds, built of nothing more than an empty fibergalss shell, might displace the same weight of water, while there was nearly a thousand ton difference in their crane weights.
An object floats when it displaces its own weight of water (i.e., the weight of the water displaced by the object is equal to the weight of the object in air).

Thus, the displacement of a floating boat is equal to its weight.

The displacement of an object that doesn't float (e.g., one made of solid lead) is smaller than its weight.
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Old 10-06-2010, 21:05   #11
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Whaaat?

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Originally Posted by hellosailor View Post
But the weight of the boat has nothing to do with the displacement!
Huh? It has EVERYTHING to do with the displacement. The boat displaces an amount of water exactly equal to its weight--if it is floating. (The Titanic now displaces somewhat less than its weight).
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Old 10-06-2010, 22:17   #12
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Quote:
Originally Posted by hellosailor View Post
"You can weight the boat, "
But the weight of the boat has nothing to do with the displacement! Displacement corresponds to the volume of the boat. A boat weighing a thousand tons, built of solid lead with no spaces below decks, and a boat weighing a thousand pounds, built of nothing more than an empty fibergalss shell, might displace the same weight of water, while there was nearly a thousand ton difference in their crane weights.

Displacement, Vessel weight and tonnage are three different things nautically.

Displacement is the weight of water displaced by the portion of a vessels hull immersed in water.

Vessel weight is the weight of a vessel if you measured it on a scale.

For any vessel or object that floats weight and displacement will be the same. It will be the same for any submarine trimmed for neutral bouyancy, though the water in ballast tanks starts to be counted which is why surfaced and submerged displacements are different. For any object that sinks, the weight and displacement will be different.

Tonnage is a measurement of cargo volume, 100cubic feet by most of the systems I am aware of, and is computed by bureaucrats according to arcane rules established by committees so don't ask me to go into detail.
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Old 10-06-2010, 22:22   #13
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Also when calculating the weight of a boat, consideration must be given to soakage.
Roger Marshall in his book DESIGNED TO WIN (ISBN 0 229 11578 0) allow in weight calculation 5% of A. B. C for soakage and bilge water. A = Hull Structure, B = Superstructure, C = Joiner Work.
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