ship dimensions (TL1)

[jason taylor]

Originally Posted by Celjabba

Modern adventurers have shown how oceans could have been crossed with
primitive boats.

To be honest modern adventurers "cheat". Not only do they cross oceans policed by a single navy, they know there is something on the other side worth getting to. What they did prove was that Ancients were in fact not absurdly clumsy at shipbuilding craftsmanship. They do not have to worry about ignorance or piracy.

It is for instance romantic guesswork whether China discovered the Pacific Coast and if they did it was a place to repair their nets, and likely by accident. Not because Chinese seafarers were fools but because they did not know America existed.

What propelled Europeans was simply knowing that there was a such thing as India.

[jason taylor]

Originally Posted by whswhs

It's a bit late and not the right culture. I'm going for something more like Indian Ocean dhows and other sewn ships.

In "Sons of Sinbad", Alan Villiers describes having a ride-along on a Kuwaiti tramp sailor.

[Anaraxes]

Originally Posted by whswhs

The real joker there seems to be "submerged length."

Yes. That's why I had to do the algebra to get back to solving for draft. Length should be fairly easy given a profile view, though. The bow usually has a constant rake, and the stern also, when it's raked at all. So the submerged length is a little less than the top deck length, the amount depending on the waterline. But it's just similar triangles, so a straight proportion should keep you close enough for the error bounds we've already got from guessing at Cp.

(Also, I thought it was interesting how similar the coefficients of form sound to what I would have written off as just a gamey approximation had I seen them only in a vehicle design book for some RPG. No doubt these days the CAD programs just spit out a number based on the 3D model. But even in real life, modelling the submerged volume as a box times a coefficient wasn't so error-prone that actual designers didn't use it. They weren't doing 3D volume integrals over curves describing the bulges in the hull or anything like that.)

[whswhs]

Originally Posted by Anaraxes

Yes. That's why I had to do the algebra to get back to solving for draft. Length should be fairly easy given a profile view, though. The bow usually has a constant rake, and the stern also, when it's raked at all. So the submerged length is a little less than the top deck length, the amount depending on the waterline. But it's just similar triangles, so a straight proportion should keep you close enough for the error bounds we've already got from guessing at Cp.

I'm not sure I'm understanding what you're saying. Are you proposing simply to use top deck length and accept the resulting margin of error?

[Rupert]

Originally Posted by mlangsdorf

I didn't say they were the same length, I said they had the same lines and displacement.

A 15 kton heavy cruiser at 90% of it's float rating has an effective hull volume of ~530,000 cf. A 15 kton cruise liner at 25% of its floating rating has an effective hull volume of 1,9200,000 cf. The cruise liner would be twice as long and wide as the cruiser at the same draft, but it is substantially less dense, and depending on its exact size, you would expect it to sit higher or lower in the water than the cruiser.

No, it wouldn't. At about four times the same volume it might simply be four times as high from keel to top of its superstructure (having seen the way modern cruise ships are built this seems likely - I'd hate to try and pilot one in heavy winds). Even if you simply scale it up it'll be more like 1.54 times as big in each dimension, and while that would give it a shallower draft, that shallower draft in proportion to length and width arguably changes its lines (shallow ships, for the same displacement, tend to take more power to reach a given speed, assuming displacement hulls of similar design).

[Rupert]

Originally Posted by whswhs

That's going to require the cargo, stores, and ballast to be much denser than the standard 20 lb/cf.

Ballast can be stone, and thus much denser than that - marble was popular if available, as it had trade value as well as being good ballast. Marble's density is about 170 lbs/ft^3.

However, general cargo has been estimated at 100 cubic feet per ton (so 20 lbs/ft^3 assuming short tons) for centuries. While it obviously varies depending on exactly what is being carried, for a ship carrying random variable cargoes it's an estimate that's withstood the test of time.

Thus a lightly made ship (and dhows are fairly lightly built, in my understanding) is unlikely to have a density much higher than 20 pounds/ft^3. As sea water averages about 64 lbs/ft^3, it's likely your ship will actually ride fairly high.

[whswhs]

Originally Posted by Rupert

Ballast can be stone, and thus much denser than that - marble was popular if available, as it had trade value as well as being good ballast. Marble's density is about 170 lbs/ft^3.

However, general cargo has been estimated at 100 cubic feet per ton (so 20 lbs/ft^3 assuming short tons) for centuries. While it obviously varies depending on exactly what is being carried, for a ship carrying random variable cargoes it's an estimate that's withstood the test of time.

Thus a lightly made ship (and dhows are fairly lightly built, in my understanding) is unlikely to have a density much higher than 20 pounds/ft^3. As sea water averages about 64 lbs/ft^3, it's likely your ship will actually ride fairly high.

Oh, okay. That's useful information. I'll have to do some calculation and see what I come up with. I had been assuming a density of around 40 pounds per cubic foot or a shade more for "round ships" carrying cargo.

[Polydamas]

Trying to be more useful ...

Modelling a roundship hull as an elipsoid does not seem crazy. It would probably be fair to substitute that formula for the generic formulas in 3e Vehicles which try to cover a wide range of body shapes (surface area, draft).

The strakes in the Kyrenia ship, which is about your size, varied from 3.1 to 4.3 cm thick. I think it was mostly Aleppo pine, an article on ResearchGate gives the dry density of Aleppo pine from Macedonia as about 0.55 g/cm^3. A sewn boat usually has a few ribs inside for strength, to keep the cargo out of the bilge water and any sharp pointy cargo off the cords, etc. The mast and anchors and steering oar/oars will add some weight, but an average thickness on the order of 10 cm feels high.

The problem is that I want to check my intuitions against say the Uluburun ship, the Kyrenia ship, and some early dhows, and that is work I usually get paid to do!

[Polydamas]

Originally Posted by whswhs

* Vehicles distinguishes between structure and armor. However, a sewn ship is often a monocoque design, where the outer hull is both the armor and the load bearing structure, with no framing. I'm going to assume that for this ship. My guess is that the fairest way to handle this is to figure the hull out as armor and then determine its equivalent value as structure.

I think that version 2 of Pyramid talked about this in a designer's notes. I don't have time to sort through the old archive, but ... ahha!, your old designer's notes for GURPS (3e) Low Tech!

[Agemegos]

The general formula for the volume of an ellipsoidal cap is doubtless brutal, but in the special case in which the intersecting plane is perpendicular to one of the principle axes of the ellipsoid it seems to me that the volume of the cap must be in proportion to the volume of the ellipsoid as the volume of a spherical cap the same height is to the volume of a sphere, where the radius of the sphere is equal to the semi-axis that the plane cuts. It's a well-behaved linear transformation. The formula for the volume of a spherical cap is tractable.