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Old 30-06-2007, 06:57   #1
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Figuring displacement

How do you figure displacement? I know our boat is listed as a 12k displacement. Is this the weight of the boat when manufactured. If the boat is weighed while in travel lift, is this the actual displacement?
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Old 30-06-2007, 10:05   #2
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MSC Tonnage Calculations by Simplified Measurement for measuring displacement.
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Old 30-06-2007, 10:17   #3
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Displacement is the weight of the boat.

It's called displacement because it displaces the same amount of water by weight.

A simple experiment is to fill a measuring cup so full of water that it drips the excess out the lip.

Place an object in the cup that will float and catch the water that drips out. By weighting the over flow of water it will tell you the weight of the floating object.

Now, if you want to measure the volume of an object it would have to sink completely. And by measuring the amount of water it will tell you how much volume it displaces..............................._/)
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Old 30-06-2007, 21:20   #4
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Put your boat in the water and measure how much the water rises on the pilings?

Seriously - It might help to think about it this way. Displacement is how much "water" the boat needs to displace in pounds. If it didn't displace the weight it would obviously sink.

What you might be interested in is "cubic" volume.

1 Pound of water = 0.0160187 Cubic feet

So a 5,000 lb boat will displace at least 80 cubic feet of water. This would be Delmarrey's full cup. Add freeboard, cabin space etc. Greater "volume" will equate to greater bouyancy. I light high volume boat will be like a "cork" on the water. A heavy low volume boat may end up with lot's of water on deck as it crashes through the swell.

To get a very rough idea of cabin volume you can measure the top of the cabin to the top of the keel (the hull height) double the measurement, solve for the volume of a cylinder and divide by about half.
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Old 30-06-2007, 21:39   #5
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I do not think anybody answered your question. The displacement is the weight of the water that is equivalent to the volume of the submerged part of the hull. It is my understanding that the displacement specified by boat manufacturers is the weight of the water displaced when a boat is immersed to her theoretical water line. It means - when the boat is fully loaded. However, fuel and water are not included in such load.
Please correct me if I am wrong.
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Old 30-06-2007, 22:16   #6
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Ted Brewer's (yacht designer) definition:


DISPLACEMENT: If you weigh the boat on a scale, that is her actual (not advertised) displacement and it is the weight of sea water that she will displace when she is afloat. Most designers figure displacement when half loaded (the boat, not the designer) with stores, liquids and crew.

From:
Ted Brewer Yacht Design

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Old 01-07-2007, 00:29   #7
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I'm certainly not the sharpest tool in the shed and there are much smarter folks on here.

I'm just trying to understand that definition. If I have 2 boats of the same weight but the hull shape is very different (one at 42ft with a narrow beam, while another at 50ft with a very wide beam and stern ut both weight the same) and I placed both in sea water and measured the displaced water, I think I would get 2 different numbers - at least in my mind.

So how could that definition be right (i know it must be, the guy know a lot more than me, but someone please explain it to me)?
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Old 01-07-2007, 00:42   #8
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A lot of multihull makers have always listed "displacement" as the light ship weight i.e. empty. 12K empty sounds about right (if a bit optimistic) for a Lagoon 37. Travelift scales are notoriously innacurate anyway!
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Old 01-07-2007, 02:01   #9
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Redbull:
The “smaller” hull will sink more deeply, than will the “larger” hull, of the same weight.
They will each displace the same amount of seawater.
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Old 01-07-2007, 06:27   #10
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Quote:
Originally Posted by Evan
A lot of multihull makers have always listed "displacement" as the light ship weight i.e. empty. 12K empty sounds about right (if a bit optimistic) for a Lagoon 37. Travelift scales are notoriously innacurate anyway!
So when we have 800 lbs of water, etc. etc. etc. your displacement would increase in direct correlation to what stuff weighs.
Travelift said we weighed 20k!!!!!!!!!!!!!!!!
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Old 01-07-2007, 09:08   #11
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Quote:
Originally Posted by Redbull addict
If I have 2 boats of the same weight but the hull shape is very different (one at 42ft with a narrow beam, while another at 50ft with a very wide beam and stern ut both weight the same) and I placed both in sea water and measured the displaced water, I think I would get 2 different numbers - at least in my mind.
As Gord says a narrow long boat will sit deeper in the water vis a vis a broad shorter boat at the same weight.

It would be hard to get a longer beamier boat, as in your example, with the same materials. If that were the case you start thinking hull thickness. i.e. where is the weight?

It's like, "What weighs more? A pound of lead or a pound of goose feathers?"
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