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Old 02-20-2015, 12:23 PM   #1
PTTG
 
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Default How dense are boats?

I'm working on an expansion for Spaceships that will work with various hull types, shapes, and TLs. To do this, I need to know what a reasonable volume-per-tonnage rating is for most ships.

Before anyone posts "well, obviously, less dense than water," true, but I'd like to know some more details, particularly the ratio of loaded weight to unloaded weight.

As you might have gathered, this is a slightly more granular approach to Spaceships.

I imagine that ships of varying materials have differing ratios of capacity and dry mass. My current system assumes that ships can be made of generic wood (representing any timber fine enough for shipbuilding), Iron sheets, and steel, or can be made of some combination of wood and metal for high strength and lightness.

I'm not too concerned about TL, but given the variety of materials available, it's late TL 5.

EDIT: I got a very helpful PM which answers my basic question adequately.

Quote:
Originally Posted by Agemegos
The density of a ship turns out to be its displacement divided by its gross register tonnage divided by 2.83168. (Because 100 cubic feet of volume will hold 2.83168 tonnes of water).

Last edited by PTTG; 02-25-2015 at 11:44 AM.
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Old 02-20-2015, 12:54 PM   #2
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Default Re: How dense are boats?

If the ship sinks in fresh water, it's 32 cf/ton or less. If it floats, it's 32 / (fraction of ship submerged). 100 cf/ton is a standard assumption. This is unlikely to vary substantially for different materials, since you just add air space until you reach the floatation you want.
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Old 02-20-2015, 02:38 PM   #3
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Default Re: How dense are boats?

Does that ratio extend to other vehicles? How much of a normal TL5 ship's mass is "dry weight"?
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Old 02-20-2015, 08:58 PM   #4
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Default Re: How dense are boats?

Quote:
Originally Posted by PTTG View Post
Does that ratio extend to other vehicles? How much of a normal TL5 ship's mass is "dry weight"?
Wikipedia gives the USS Constitution:
Tonnage: 1,576
Displacement: 2,200 tons
Complement: 450 including 55 Marines and 30 boys

If Tonnage isn't dry weight then I don't know what it could be.
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Old 02-20-2015, 09:14 PM   #5
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Default Re: How dense are boats?

Quote:
Originally Posted by Balor Patch View Post
Wikipedia gives the USS Constitution:
Tonnage: 1,576
Displacement: 2,200 tons
Complement: 450 including 55 Marines and 30 boys

If Tonnage isn't dry weight then I don't know what it could be.
It can also be a measure of volume.

This is one of those things that can be really confusing, words don't always mean what they usually mean when talking about sea vessels.
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Old 02-20-2015, 09:24 PM   #6
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Default Re: How dense are boats?

A "boat" that has a density equal to that of water will be a submarine.

A density of 0.9 (about 56.25 pounds per cubic foot) would give you an extreme design kind of like that of the Monitor, most of whose volume was below the waterline when it was in operation. A more normal ship would probably be about 0.3-0.5, putting from half to two-thirds of its volume above the waterline; lower density ships will tend to have less hydraulic drag, I think. An airship has an effective density very close to 0 relative to that of water (though neutral with respect to that of air).
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Old 02-20-2015, 09:47 PM   #7
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Default Re: How dense are boats?

"Tonnage" is usually calculated, and involves various complicated formulas based on ship length and width, or rules for what part of the ship counts as internal volume. The goal is to calculate cargo volume. For "gross register tonnage", a "ton" is 100 cubic feet by definition. "Deadweight tonnage" (DWT) is a measure of the capacity in units of weight.

"Displacement" is the weight of water actually displaced by the ship, so it has to equal the actual weight of the ship and contents. The ratings you see are usually a maximum loaded displacement, and thus isn't the weight of just the ship structure itself, though as with tonnage there are other variants with various levels of loading.

Subtract DWT from max loaded displacement and you'll get pretty close to the weight of the ship itself.
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Old 02-20-2015, 11:13 PM   #8
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Default Re: How dense are boats?

Quote:
Originally Posted by Anaraxes View Post
The goal is to calculate cargo volume.
For cargo ships, that is. For warships, it isn't (and this means a 10,000 ton warship is in fact a different size from a 10,000 ton cargo ship...).
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Old 02-20-2015, 11:18 PM   #9
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Default Re: How dense are boats?

Quote:
Originally Posted by Anaraxes View Post
"Deadweight tonnage" (DWT) is a measure of the capacity in units of weight.
To add to that, note that deadweight includes everything that isn't the body and equipment of the ship. It includes cargo, passengers, crew, fuels, provisions, and ballast.
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Old 02-21-2015, 03:12 AM   #10
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Default Re: How dense are boats?

Quote:
Originally Posted by Johnny1A.2 View Post
It can also be a measure of volume.

This is one of those things that can be really confusing, words don't always mean what they usually mean when talking about sea vessels.
Or vice versa. The shipping usage is of course original, and started out as a volume measure - how many wine tuns you could fit in the holds, which determined how much the government charged you in taxes. The weight definition is the later add on.

But yeah, it's confusing, that's because it is *still* a set of legal terms that determine things like taxes and tarrifs and usage or transit fees - so it's subject to the usual complexity of any tax code. And of course motivates people to *deliberately* do things to make the value unrelated to any number you might actually be interested in, so they can pay lower fees on the same amount of whatever the useful quantity you care about they actually have.
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