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Re: [PSUBS-MAILIST] Re: Velocity and Acceleration
My boat will be making surface transits to the dive site, so I have to
consider wave drag. But then, I'm able to use an air breathing engine on the
surface. Have you tried that NACA link that I posted some time back in your
search for dirigible papers? - Joe
----- Original Message -----
From: Michael B. Holt <mholt@richmond.edu>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, September 26, 2000 7:05 AM
Subject: [PSUBS-MAILIST] Re: Velocity and Acceleration
> > 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.
>