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Old 03-24-2023, 09:00 PM   #11
mlangsdorf
 
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Default Re: Gurps vehicles to design boats?

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Originally Posted by Polydamas View Post
Isn't freeboard determined by the volume of hull which the weight of the ship forces underwater? On still water, if the loaded weight is the density of water times the volume of the hull then the deck will be just above water.
Visualize two boats. 1 has 6" thick wooden boards above the waterline and a roughly 7' freeboard. The other boat, with the exact same hullform beneath the water but 1' thick steel plating instead of the boards. If both boats have the same draft, the steel plated boat should have around 6" of freeboard. So freeboard is determined by the volume and density of the volume above the waterline.

Vehicles 2e doesn't really give you the tools to determine to determine that. I personally think that the sleekness and ratio of weight to flotation rating have something to do with it, but the interaction is fairly complex.
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Old 03-24-2023, 10:23 PM   #12
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Default Re: Gurps vehicles to design boats?

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Visualize two boats. 1 has 6" thick wooden boards above the waterline and a roughly 7' freeboard. The other boat, with the exact same hullform beneath the water but 1' thick steel plating instead of the boards. If both boats have the same draft, the steel plated boat should have around 6" of freeboard. So freeboard is determined by the volume and density of the volume above the waterline.
If you put the same additional mass in the bilge, wouldn't the freeboard change the same way? Either way, the extra mass of the ship pulls it lower in the water until it either displaces its mass in water or hits bottom.

Estimating volume under the waterline at different loads for different hull shapes is really complicated, its probably 15 years since I nerded out about GURPS Vehicles so I can't comment on the realism of its ship drafts. I think the designer's notes for GURPS Classic Low Tech had some early Bill Stoddard tweaks.
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Old 03-24-2023, 11:25 PM   #13
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Default Re: Gurps vehicles to design boats?

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The thing that has long perplexed me in GURPS Vehicles is figuring the volume of boats and ships.

GURPS Vehicles envisions a ship as a rectangular prism, which has length, width (beam), and height (draft). Total volume is the product of these three, and a ship's ability to float is based on its volume. But in reality, a ship's "height" is the sum of its draft (the part of its height that is below the waterline) and freeboard (the part that's above the waterline). I could never figure out how to represent the part of the ship's volume that's above the waterline. Of course a ship can have superstructures, such as its fore and aft castles; but the water never comes up to the top of the mail hull, unless it's a submarine (or the original Monitor, perhaps). And it just seems too arcane to say that a ship's main hull includes both a body and a superstructure that may not even be separated by a deck.
Superstructure vs body is indeed a problem. It's reasonably clear in a modern bulk freighter, oil tanker, or flush-deck warship, but it's not nearly so clear in a modern ship with a forecastle or an age-of-sail ship with extensive forecastle, poop deck, etc.

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Secondary to this, how do you figure a ship's draft? It's simple for an actual rectangular prism. But what if the ship approximates an ellipsoid, or a pair of triangular prisms extending forward and backward, or something more complex?
The shape of the hull's cross section is the bigger consideration, and Vehicles doesn't consider this, except as part of the volume cost of various lines, at all. Thus it's folded into the calculation for draught as part of the variable given for the ship's lines.

Freeboard, as far as I can tell, is best calculated by taking the block the ship fits within, getting its length and width, and thus total height and subtracting draught from that height. This assumes that the body represents all of the ship's useful flotation, and that it's uppermost deck is reasonably flat and level, and thus some sailing ships don't have superstructures, because the gunwhales are high enough that they effectively make the forecastle and poop the upper level of the hull with the deck between as an open part of the ship's body.
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Old 03-24-2023, 11:31 PM   #14
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Default Re: Gurps vehicles to design boats?

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Originally Posted by mlangsdorf View Post
Vehicles 2e doesn't really give you the tools to determine to determine that. I personally think that the sleekness and ratio of weight to flotation rating have something to do with it, but the interaction is fairly complex.
It's relatively simple for ships with square or nearly square cross-sections. You can mostly ignore (or rather, fold into some constant) the bow and stern having different cross sections, and treat the ship as a simple block. It's less easy if the ship has a round bottom to the hull, but at least with vertical sides the sinkage with load is constant. Then we have ships with angled or curved hull sides at about the waterline, and a that point it become not 'simple' at all, especially if the angle changes or it's a complex curve, and the odds are that it won't be consistent along the main body of the hull, so can't be abstracted away with little loss like the bow and stern can.

Stick to spaceships, they're simple.
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Old 03-24-2023, 11:36 PM   #15
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Default Re: Gurps vehicles to design boats?

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Freeboard, as far as I can tell, is best calculated by taking the block the ship fits within, getting its length and width, and thus total height and subtracting draught from that height. This assumes that the body represents all of the ship's useful flotation, and that it's uppermost deck is reasonably flat and level, and thus some sailing ships don't have superstructures, because the gunwhales are high enough that they effectively make the forecastle and poop the upper level of the hull with the deck between as an open part of the ship's body.
I don't understand how you would do that. What numbers that you obtain during the vehicle design process would give you a way to figure the total height?
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Old 03-24-2023, 11:50 PM   #16
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Default Re: Gurps vehicles to design boats?

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I don't understand how you would do that. What numbers that you obtain during the vehicle design process would give you a way to figure the total height?
My mistake - I thought VE2 gave body length, but I must've confused it with some other design system.

However, there's another way. Because it does give draught, and it does give flotation, we can get a rough idea by multiplying draught by flotation and dividing that by weight to give total height. Subtract draught from that to give freeboard. That shouldn't be too far off for most modern ships, and large age of sail ships, but will be less and less accurate for smaller and faster ships.

It should also allow a guesstimate of length and beam, in conjunction with the length/beam ratios given for various lines.
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Old 03-25-2023, 11:14 AM   #17
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Default Re: Gurps vehicles to design boats?

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Originally Posted by Polydamas View Post
If you put the same additional mass in the bilge, wouldn't the freeboard change the same way? Either way, the extra mass of the ship pulls it lower in the water until it either displaces its mass in water or hits bottom.
I think so, but that doesn't change the fact that losing 1' of freeboard isn't a big deal to the wooden board ship but turns the iron plate ship into a submarine.
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Old 03-26-2023, 07:47 AM   #18
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Default Re: Gurps vehicles to design boats?

I have to admit, I’m confused as to what Ve2 actually does, but here is the bottom line…whatever it does do does it translate to mechanics in the game?

Also does it take into consideration of the five different points of sail to the wind?

I think I might have to go back to school and get my engineering degree in boat design. LOL
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Old 03-26-2023, 09:18 AM   #19
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Default Re: Gurps vehicles to design boats?

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I have to admit, I’m confused as to what Ve2 actually does, but here is the bottom line…whatever it does do does it translate to mechanics in the game?
That's actually the main thing it does. Some of the 3e mechanics are not directly compatible with 4e mechanics, though. In particular, 4e has changed the way HP and explosive damage are calculated, simplified vehicle maneuverability rules, and reduced weapon Acc by about half (the idea being that Acc now represents "under pressure" accuracy and you apply modifiers for a peaceful day at the firing range, rather than the other way around).
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Also does it take into consideration of the five different points of sail to the wind?
Vehicles includes basic rules for sailing vessels, but if you want much detail, you will probably want to consider adding Vehicles Expansion 1
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Old 03-26-2023, 10:13 AM   #20
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Default Re: Gurps vehicles to design boats?

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Originally Posted by briansommers View Post
I have to admit, I’m confused as to what Ve2 actually does, but here is the bottom line…whatever it does do does it translate to mechanics in the game?

Also does it take into consideration of the five different points of sail to the wind?
Ve2 will give you mass and a simple answer for volume as well as HP and DR. It will handle crew size, cargo capacity, accommodations and armament and other things like that in pretty good detail. You would need Expansions I for more detail about sailing.

You might also want the Gurps Vehicle Builder program to automate all the stuff..

http://www.sjgames.com/gurps/vehiclebuilder/

I'm afraid it's a fairly old program now but I beleive it still works.
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