ballast

[David Buchner <Buchner@wcta.net>]

At 19:06 -0700 13<8<1998, Martin Sanderse wrote (about the Submanaut):
>Ballast/Bouyancy:  A water ballast tank located within the pressure hull
>provides +- 100
>lb bouancy and may be manually pumped or blown with air.  External ballast
>tanks
>presently under construction.  (Martin's note:book published in 1976 - gee
>I've been
>interested in this a long time)

This is something I've been wondering about. How much buoyancy does one
need to change? Like, how big should I be thinking about making those
tanks? Say I've built something which weighs the same as the water it
displaces -- with batteries, occupant, air, etc., altogether it's neutral
so it just sits there just below the surface. Add weight, or stand on the
dock and push down on the deck with your foot, and it goes down. I think
the ballast tanks then are basically just how big they need to be to make
it float as far out of the water as it needs to. They become the only thing
holding it at the surface - flood them and you take away all the buoyancy.
So the hull and gear as a unit are a strictly underwater affair and these
add-on pontoons keep it surfaced. The smaller I can get away with the
better, though, to require less compressed gas to blow them out.

No?

Another thing I'm pretty fuzzy on, because I haven't thought it out yet, is
this. Displacement is determined by how much hull is actually below the
surface of the water, right? So a 31 cubic foot bubble of air enclosed in a
canopy needs a ton to sink it. But when it's at the surface, say only half
of that is beneath the waves and the other half is poking out of the water.
I'm having a hard time wrapping my mind around this right now (I'm
currently building my house, and my head is full of fractions of inches,
and general making-things-out-of-wood type problems), but isn't there some
kind of thing here where it takes a lot more weight to initially get this
thing underwater, and then less so once the whole thing is underwater? Or
is it the other way around - it takes less extra (besides the hull volume)
to float it, when part is sticking out of the water? Or, no, wait - it
takes *more* because any part of the hull that's not displacing water is
just dead weight, hanging up there in the air. Or am I completely screwy
here?

Obviously, it's a good thing I'm nowhere near building anything and I'm
mostly just engaging in idle speculation. And when I do build something,
it's likely to be a lot less ambitious that the projects many of you are
talking about -- basically a bubble I stick my head in, with a frame to
hold on a trolling motor, and make shallow dives for a couple of minutes at
a time in small lakes. But it's great fun to read about all these exciting
ideas.

Now I'll go back out and deal with the hornets' nest on the shed... pray
for me...


---------
David
buchner@wcta.net
my website, mostly about building my house: http://customer.wcta.net/buchner
Osage MN USA