Re: Re: [gurps] [VEHICLE] of the week 632 – Archvillain's Yacht (1920s Version)

[David Scheidt]

On 12/6/06, Onno Meyer <Onno.Meyer at gmx.de> wrote:
> David replied to me:
>
> > > Archvillain's Yacht (1920s Version) v1.0 (TL6)
> > >  Copyright 2006 by Onno Meyer
> > >
> > >  An Evil Overlord with style will not be confined to a clammy mountain
> > > fastness. With a yacht like this, he - or she - can travel with all the
> > > comforts of home.
> > >  The yacht is powered by twin steam turbines for a top speed just over
> > > 30 knots.
> >
> > Not with a length of 120 feet, it's not.  Speed of a displacement boat
> > is limited to about 1.35 * sqrt (LWL).  (LWL is length on water line,
> > typically less than overall length).   To go faster, you need to
> > plane.  To plane 700 tonnes, you'd need quite a bit more power.
> > Here's a modern boat (it ain't a yacht if it doesn't have sails,
> > dammit) of about the same size, and claimed speed.  Note it only
> > displaces ~150 tonnes, and has about the same amount of power.  It
> > also needs life rafts, and a tender.  (I assume the escape boat is
> > super secret).
> >
> > cool idea, though.
>
> There are two external cradles (davits) in addition to the secret
> dock. Regarding the speed, it follows the rules - 90,000 lbs. of
> thrust for 500 stons (rather less than 700 metric tons). But a
> LWL over 400' is out of the question, I fear. Would a cat hull
> help?

Some. I don't know how you figure out hte hull speed of multi-hulls.

What happens is that as a boat reaches what's called the hull speed,
the stern wave moves aft of the boat, which causes the stern to sink a
bit, which changes the hydrodynamics of the boat, so the amount of
power required for the next knot goes way, way up.  There's a neat
measure of speed of a boat, called the speed/length ratio.   It's the
speed in knots divided by the square root of the length of the water
line.  Two boats travelling at the speeds that give them the same
ratio require about the same amount of force to move each pound of
displacement.  That means that a 9 foot canoe going 3 knots requires
the same force per pound as a 400 ft destoryer going 20 knots!
(Roughly, there are other factors, of course.)  That's one reason that
cargo ships keep getting bigger and bigger.  For the same energy
input, you can move cargo faster; or you can go the same speed and
save fuel.

I'm not sure that a 120 foot heavy boot could be made to do 30 knots
using 20's technology.

-- 
David Scheidt
dmscheidt at gmail.com