----- Original Message -----
Sent: Tuesday, September 26, 2000 3:52
AM
Subject: [PSUBS-MAILIST] Velocity and
Acceleration
"My two cents on the speed and power of personal submersibles:
A submerged body has the inertia of its displaced volume of water. Wet,
dry or anywhere in between, the mass of a submerged boat is equal to
the mass of the water it displaces. This mass has a large effect on the
acceleration (how quickly it starts or stops), but has no effect on the
velocity. The velocity is determined by thrust and drag. Velocity is hard to
calculate with certainty because of the many and mysterious elements of drag
(profile, skin, wave, parasitic), and the complex interactions of hull, prop
and water. Authors of cookbooks tend to present their formula without
showing their homework, which is unsettling for those of us without the gift
of faith. If I have enough cookbooks to compare, I'll
eventually develop some confidence in a common thread. But we have the
additional disadvantage of dealing in an obscure cuisine (small, low
powered displacement hulls.) I'm always on the lookout for applicable
formulae, but I haven't found anything short and sweet. - Joe"
Thanks Joe, that was well said.
We'd have to compare thrust to drag in determining the velocity at
which acceleration would cease and the vessel would achieve equilibrium in
motion. Calculating the drag coefficient of a multi-faceted hull over
a range of velocities is the hard part. Like you, I haven't found any
"short and sweet" way of doing that, either.
So far, with the small E-motor sub designs I've worked on,
thrust/velocity calculations haven't been too important: no matter what I
do, thrust is low and speed is slow. So, I just get the biggest DC
motor I can find, and expect performance to be underwhelming.
But I've got a Hi-Perf sub on paper; I've built a model of it that
moves pretty well through the water; and I wonder how fast a real one
might go with a large amount of thrust. I've got the basics of a hull
and drivetrain downstairs, but finishing the boat will be expensive; and I
don't want to do all that if she's not going to set a speed record. So
now I wish I could accurately determine how fast a certain hull would go
given a known amount of thrust. And right now, my math just aint that
good.
I'm hoping the calculations page Mike is sending me might give me at
least a ballpark guesstimation of what the sub might do. This is where
it really starts to get tough, though.
And another thing concerns me, too. Accurately estimating the
crush depth of a homebuilt hull is hard enough, but what about the
effect of dynamic pressures on the hull at high speed in shallow
water? The hydrodynamic impact-pressure is going to build up against
the leading surfaces of the hull as she accelerates. When we're
talking about a propulsion source similar to those used in drag boats, the
possibility of extremely high speeds for short periods of times exists, and
that means a LOT of pressure on the hull. I would sure feel more
secure if I were able to compute not only the sub's potential speed; but
also to know that the hull's not going to disintegrate if I ever get her
going that fast.
Isn't there some kind of computer program that does this sort
of calculation? I sure hope so, because it's way beyond me
and my little calculator; and the lack of this information is keeping me
from proceeding with a project I have most of the components for
downstairs in the shop at this time. Considering the risks
involved, maybe that's just as well. But still, I'd like to develop
the math skills necessary to figure these sorts of things out.
Pat