[PSUBS-MAILIST] Re: Velocity and Acceleration

["Michael B. Holt" <mholt@richmond.edu>]

> Marsee Skidmore wrote:
> 
> 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.

There's a lot of math to determine speed versus horsepower, but all
of it I've found so far has limitations that are recognized.

> 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. 

Submerged, wave drag isn't a factor.  That reduces the horsepower
requirements by an order of magnitude.  The rest of it is quite real.

> Authors of cookbooks
> tend to present their formula without showing their homework, which is
> unsettling for those of us without the gift of faith.

Look in older books; that is, books written before 1950.  They
have all the math in them, and the derivations of the formulae.
Very comforting.

(What you're seeing -- the "cookbook" approach -- is an attempt to
remove from the designer the need to do all the math.)  

> If I have enough
> cookbooks to compare, I'll eventually develop some confidence in a
> common thread. 

I'm trying to do that now.  I have two programs that test math: I have
Busby's and I have one from a book published in 1917.  There will be
more as I plow through my library.

I also have a spreadsheet that has 255 designs in it.  I'm testing 
it by charting the characteristics, and thereby to uncover any 
relationships.  This is an ongoing thing, too.  NOTE: if anyone 
wants the spreadsheet to play with it, it's available.

> 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.

I suspect nothing will be as easy as we hope it might be.  I'll
report as I develop programs and uncover new math.

I keep thinking we're all overlooking something obviously useful
from airhsip design.  It'd have been written in the 20s or the 30s.