Velocity of ships

[TheDS]

Hey all, got a question that's been bugging me.

Given the energy of propulsion (shaft horsepower, kw, whatever) and the mass of the vessel (metric tons, tons displacement, whatever), is it possible to figure out the speed a wet ship can travel at (kph, knots, whatever)?

Was going through Warships1.com and copied some info about the battleships, then went through my GURPS Vehicles book trying to figure out how to make wetship-building rules for TNE, and found out that the formulae given in GV doesn't match the realworld performance of any of the 20-some classes of battleships that I took down info for. About the closest I could come was half the correct value.

So far, I have banged my head against a brick wall trying to come up with the correct formula. So far I have determined, by looking at the realworld stats that:

power to weight ratio is only partly responsible for final velocity.

Taking the cube root of that gets me to within 10 percent, so long as we're not talking more than about 35 knots, which is the speed limit of the Iowas.

I am hesitant to use cube root of power, though, because the numbers clearly show that when you double your speed, your fuel consumption quadruples. This obviously means that cruising at 25% power lets you travel at half the speed of 100% power, so therefore we must take the square root of power instead of the cube root.

I tried next to correlate the amount of ship in the water to degradation of performance, and saw that in many cases, the more ship in the water (multiply draft by beam) the higher the p/w ratio needs to be to get the same speed, but this is hardly a rule. My best guess so far is that there are other factors.

I am starting to feel that instead of dividing by drag, one must subtract drag to get the right final speed, but this will involve lots of experimentation and might not be the right answer.

If anyone has any thoughts on the subject, I would be glad to hear them.

 

[mike wightman]

Here's a thought.
You aren't dealing with power to weight, rather power to inertial mass and then friction (drag) between the hull and water.
Upthrust reduces the effective weight of the ship, but not its mass.
So there is probably some really complicated formula to take account of the water's drag.

 

[far-trader]

Further complicating the issue, at least for conventional hulls, is your wake. The maximum speed of the ship is limited by the wake and length of the hull. Once your wake is longer than your hull your speed suffers since you lose your planing. Something like that. From a foggy memory

You can get around this by using hydroplanes, but I got the idea you were just looking at simple hulls.

But then I'd say it's just a game simulation and close is close enough, sounds like you're closer than some systems

 

[flykiller]

an old subject and well-studied. google naval engineering hull speed and start there, shouldn't take you long.

 

[TheDS]

Aye, that's the REAL killer: not being able to figure out WHAT to search for...

I woulda thought that the cross sectional area of the hull in the water would give a good idea of the drag, but either these ships have a lot of undocumented submerged hull features, or maybe too much wind drag, or who knows?

Thanks for the help so far. I'll check into those search terms and see if I find anything readable. (All too many people like to litter their formulas with lots of symbols and not tell you what any of them mean.)