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Old 07-23-2011, 04:37 AM   #1
Christopher R. Rice
 
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Default Water Move for Spaceships

I'm attempting to work out a proper method for giving a spaceship design (from GURPS Spaceships by the most excellent Mr. Pulver) a water-based movement. Other than the maddeningly unhelpful line from the Star Galleon, there seems to be no easy way to do it.

Quote:
Originally Posted by Spaceships 7
A star galleon is a wooden-hulled sailing ship, but it is
equipped with enchanted astral sails enabling it to fly in the air
and voyage through space. It uses a 300-ton (SM +7) streamlined
hull 90 feet long with nautical lines. It also functions as a
conventional sailing ship.
There seems to be the hint of having regular sailing vessels with the capacity to move through water...but no actual rules for it. I know I could just rule it to perform as a vessel of a similar nature but I'd rather have some sort of solid guidelines I could use. I've got a few odds and ends I've toyed with but I'll be honest, they seem sloppy and contrived to me. So I've come to pick the ginormous collective brain of the fora. :)

So thanks much for anyone who replies!

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Old 07-23-2011, 05:31 AM   #2
vicky_molokh
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Default Re: Water Move for Spaceships

Here's a previous attempt to tackle the issue.
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Old 07-30-2011, 03:45 AM   #3
Christopher R. Rice
 
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Default Re: Water Move for Spaceships

Quote:
Originally Posted by vicky_molokh View Post
Well that shows that I am not the only one looking for a solution. Anybody out there with mad Vehicles skillz got any ideas? I really am at a loss here and it's sort of important for my campaign.

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Old 07-30-2011, 05:54 AM   #4
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Default Re: Water Move for Spaceships

Water speed (surface speed in mph) - no contragrav...
6 * (LWt * 2000)^(1/9) * (Acceleration)^(1/3) * (Modifier)^(1/3)

LWt is the ships rated mass, Acceleration is the space acceleration in Gs (though some notes state that rockets and rams give only 1/2 their rated thrust in water and should cost a bit more to retain full thrust in space and air), ^(1/9) is ninth root and ^(1/3) third or cube root. Modifier is the hydrodynamic lines modifier, something from 1 to 20 (avg lines are at 10). Same works for underwater performance just as well, except the modifiers are different (avg nautical at 3). To make it into 4th ed stat line i suppose you need to further divide the result (in mph) by two.

Acceleration rate is same as air acceleration.


Crush depth (yards) is different... but i suppose... dDR being the lowest armor rating on the ship.
10 * (dDR + 1) * Frame * Shape / SM

Frame probably should default to 1 and Shape to 3 (maximums being 4 & 24 respectively)
(30 * (dDR + 1)) / SM

No guarantees for the math nor for any of them notes.
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Old 07-30-2011, 10:51 AM   #5
chandley
 
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Default Re: Water Move for Spaceships

If you happen to have Gurps Vehicles, p.26 of that book has a very familiar looking table. You can work backwards from that, submerged weight, and your own judgement on how good your ships hydrodynamic lines are to get a surface water speed and a submerged speed.

I didnt concern myself with my ram-rockets differing performance in the much denser water than air, since I had already made it a reconfigurable system (to switch between high and low thrust options) and figured the cost and tech needed to adjust nozzles for space, air, and water all fell under that. Given how much a reconfigurable system costs, using that to cover your ability to get full rocket thrust underwater should do, Id think.

I do wonder if we could figure out a vortex system to get underwater speed up. It seems to be a TL 9 design feature if the real world is any indication, and spaceships like the Vorpal Blade feature it prominently. But I digress.
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Old 07-30-2011, 11:45 AM   #6
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Default Re: Water Move for Spaceships

Well... I was referring to Vehicles expansion 1 which has some content about underwater propulsion, including vortex combustion, underwater & adaptive rockets and rams, as well as supercavitation.
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Old 07-30-2011, 11:57 AM   #7
chandley
 
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Default Re: Water Move for Spaceships

Quote:
Originally Posted by Wanderer View Post
Well... I was referring to Vehicles expansion 1 which has some content about underwater propulsion, including vortex combustion, underwater & adaptive rockets and rams, as well as supercavitation.
Ah my apologies Wanderer, I dont have VEX1 so I didnt make the connection. Thanks for letting me know though, maybe I should pick it up.
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