ship dimensions (TL1)

[whswhs]

My current GURPS Fantasy campaign has the PCs sailing about in a merchant ship. I came up with its stats by scaling up one of the ships in GURPS LTC3. But I happened to be looking at GURPS Vehicles, and I started out to build the ship using those rules. That led me to reanalyze the problem using some simple geometry, and think about what it said about things I found perplexing in GURPS Vehicles.

This is a ship with 15 tons load and 2.5 tons vessel. I figure that it's a "round ship," since it's a cargo hauler, so I analyzed it as a half ellipsoid.

A cargo of 15 tons is 53,600 pounds, which displaces 537.6 cf of water. I figured that it usually rode low in the water, about 2/3 of its volume being below the waterline, which gives it 806.4 cf. After fiddling around, I decided that it looked like average hydrodynamic lines, and per GURPS Vehicles that gives it an equivalent volume of 967.68 cf. This is equivalent to a cube just under 10' on a side, and surface comes to 587 sf (there isn't a neat simple formula for the surface area of an ellipsoid, so I went with the approximation).

Taking the entire 2.5 tons or 5600 pounds as hull and top deck, and taking the density of wood as half that of water, I got 179.2 cf of wood. That gives an average thickness of 0.32 feet or 3.88 inches. I tentatively rounded it down to 3 inches, which looks to imply DR 3.

Now, since this is a half ellipsoid, the full ellipsoid's volume should be 1075.2 cf. That can be computed as (4/3)πabc. Length of 45 feet gives semimajor axis a of 22.5 feet, so bc is 20.535 sf. Setting deck to keel distance as 6 feet gives midline to side 3.42 feet, so beam is just under 7 feet. That's a ratio of 6.43 between beam and length, which does fall into average hydrodynamic lines.

The GURPS Vehicles formula for draft gives draft of about 2.75 feet, which gives freeboard of 3.25 feet. That seems wrong, since I worked out the volume as 1.5x the displacement; it ought to be at least 4 feet, and presumably more than that, as the submerged part of the ship is narrower than the part above the waterline.

All this is just a crude first approximation, of course. I'm only writing it up to illustrate the kind of thing I'd like to know about a craft, and to be able to relate somehow to a vehicle design system.

[mlangsdorf]

The Vehicles 2e formula for draft always seemed off to me: Take a 15,000 ton heavy cruiser loaded with armor plate to the safe limit of its flotation rating. It has the same draft as a 15,000 cruise ship that is much larger but is only loaded to 25% of its flotation rating, as long as both ships have the same hydrodynamic lines.

I'm not exactly sure how, but I'm fairly sure flotation rating should have some effect on draft.

[Rupert]

Not really. If a ship displaces 15,000 tons it doesn't matter what overall volume it has, with the same shape (indicated by the lines) and length, it'll have the same draft. For the cruise ship to have less draft than the cruiser it needs to be longer (and with the same lines, wider).

[hal]

If it helps any (which I know it won't sadly) - none of the attempts to build historical Napoleonic era warships using the idea of 60% of the displacement of a ship is at the water line or below, and 40% above, ever matched the given historical draft values with GURPS VEHICLES 2nd edition.

Then again, as I've alluded to in the past, none of the mast heights ever matched reality either. I had to look up actual rigging formulas for historical masting of ships - before I could have some semblance of realism for design.

Try doing a search on "Age of Sail" threads to see if any of those threads/conversations might contain data that you wanted to get your hands on.

[whswhs]

Originally Posted by hal

Try doing a search on "Age of Sail" threads to see if any of those threads/conversations might contain data that you wanted to get your hands on.

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

[hal]

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.

When you say Dhows - I think of Lanteen rigged hulls, essentially smaller craft kind of things. So, what time period? 1400's? 1500's? 1600's? I'm guessing NOT 1700's plus ;)

[hal]

By the by? Went looking for a picture of Dhows, and came across this one that you might like. It's for a 1/85th wood model...

[(E)]

The assumed density used for the wood is about right for a medium density soft wood like radiata pine. Presumably the boats would have been made out of a hard wood to prevent rot. Depending on the area/ boat building tradition up the density of the wood by 50% for oak or acacia.
There is a lot of guess work involved in estimating the density of wood though.
It may also increase the DR per inch.
See here for more detail on that

[whswhs]

Originally Posted by hal

When you say Dhows - I think of Lanteen rigged hulls, essentially smaller craft kind of things. So, what time period? 1400's? 1500's? 1600's? I'm guessing NOT 1700's plus ;)

There were some larger ships with similar designs. But I referred to dhows not for the sail configuration, but for the hull construction, which is edge-joined wooden planks sewn together. My understanding is that this goes far back in history, and probably even into prehistory. I'm envisioning this setting as early to middle Bronze Age, but with some of its cultures advanced in one or another science.

For example, this ship has two square-rigged sails, one on a main mast near the stern and a smaller one at the prow that gives a little extra thrust and can be used to help with steering by angling it to the wind.

[whswhs]

Originally Posted by (E)

The assumed density used for the wood is about right for a medium density soft wood like radiata pine. Presumably the boats would have been made out of a hard wood to prevent rot. Depending on the area/ boat building tradition up the density of the wood by 50% for oak or acacia.

Not implausible. But I was trying to work mainly with the material in GURPS Vehicles, which doesn't talk about the varying density of wood.