Re: batteries in pressurized compartment...?

[shawl@torchlake.com (Jonathan Shawl)]

Hi all,
asmyth wrote:

> Michael Holt said:
>
> >   The difficulty with
> > it would be that the entire design has to be "fixed" before any one
> > system
> > is started.
>
> Actually, its the opposite! The reason I am taking this approach is that I
> want to know the actual weight and displacement of little itsy bitsy
> hard-to-calculate things like actuators or props BEFORE I can dimension the
> pressure hull.
>

I have found that all those little things don't add up to enough to even worry
about. The reason is that you usually come up short on overall weight and have
to add something like 1000 to 2000 lbs of lead just to get it to dive.
My sub is made from a 500 gallon LP tank, The entire sub ready for a dive is
about 4500 lbs, I had to add about 2000 lbs total, keel 600 lbs, lead weights
1000, and a 400 lb drop weight. I have enough extra ballast weight that I could
have even added heavy ribs and still have been under weight. It's pretty hard
to go over weight unless you are building a Alvin, or other very deep diving
type sub.

> My design is very simplistic. All the wiggly wet bits are in two external
> pods, which attach to a cylindrical pressure hull. As you can gather from
> the spreadsheet I sent in a few days ago I am thinking about hull
> calculations, however the exact dimensions will not be determined until I
> finish the pods, at which point I'll simply weigh them submerged and
> determine their center of gravity submerged. That will tell me how much
> positive buoyancy the pressure hull needs (with its contents of course),
> which is a much simpler calculation. I feel this approach will give me a
> lower margin of error than if I tried to calculate the whole lot up front.

It don't sound too simple to me. As far as the margin of error on the "little
wigglies", I just don't see that the displacement of the small parts has to be
calculator all that close. Heavy and or large displacement parts should be
calculated as close as you can and then is you were to just estimate the "small
wigglies" I think you would come up with a workable figures. Just adding a
layer of paint to the entire sub adds up because it covers the entire sub but
it is still just a few pounds overall, about 10 lbs displacement on my sub. I
just didn't bother calculating the motors and paint and rub rails and stuff
like that because what does it matter if you still have to add 1000 lbs or more
of lead when you get all done?
Just my thoughts.

> The basic assumption permitting this approach is that my diving depth
> objective is simply "as deep as possible". If I hard a hard figure in mind,
> I could not make depth a function of everything else.

Well that is one way to look at it. You must be trying real hard to squeeze out
the last inch of depth from your hull design.
I hope you don't spend 20 years just designing it. Good luck
Jon Shawl

http://www.yel-o-sub.com