Hi Don. I was gone all day, and I see that Jim K. has pointed you in the
right direction. ABS is the first starting point.
On the thickness question, I would suggest making it as thick as you can
reasonably deal with for a couple of reasons.
First off, stronger is better, and when you run the calcs the difference
between 1/4 and 1/2 inch is quite substantial. The actual work involved is
really almost the same, and the material cost, although higher, is a small
percentage of the total cost. Like I said, if I was starting new, I'd have
used 1/2 inch.
The second thing is, to get the sub to dive, it's gotta be heavy. In my
case, that's going to mean almost 2000 lbs. of lead. It would have been
cheaper to use some of that weight as steel, rather than the lead. The extra
strength is just a bonus.
The stiffener design is more complicated in a saucer, but again, the work
involved is mostly the same. I'll be using 1/2 inch by 3 inch stiffeners, but
without the "T" portion. The "T" part is to prevent buckling of the stiffener.
I'll be using round stock between the ribs like a ladder configuration. This
is easier to fabricate than the "T" shape. Especially when you consider that
the saucer shape ribs can't be just rolled to round. They are ovals, and must
be made to fit individually rather than just making a dozen rings all the same
and welding them in. The ribs are all the same, just not round. I'll make a
pattern, cut the ribs from plate, and fit them in. The round
stock "ladder" bars can be cut to fit and welded in after the ribs are
all done. Although not quite as strong as the "T" flange, the locations
and spacing can be adjusted to nearly approximate the same strength. They will
also serve as attachment points, and handles. The fabrication difficulty
and time spent making the "T" shaped ribs will be a lot less with
the "ladder" method. Painting and paint maintenance will also be easier.
The biggest down side to the saucer shape is the drag. With a straight
cylinder or "cigar " shape, it's pretty efficient. Not so with the saucer
shape. I'm trying to over come this with sleek fairings.
On the plus side though, the draft is less, so the sub can be launched in
shallower water. Because these subs don't go very fast any way, the difference
in drag is small, and offset by the increased area available for forward
windows. Not to mention interior room. I've been inside a few shapes, and find
the "Flattened Sphere" the most comfortable. With the cigar shape,
there's a lot of interior space that you have to crawl to. With the saucer
shape, I can just reach over to everywhere. It's not crowded at all, and for
the same volume, nicer to sit in.
Again, every one's design is driven by what their
sub is used for. My primary use will be video, but I also want to do some
salvage. I plan on mounting a metal detector array under the sub, to search
the sandy bottom areas near my Florida home. With a grabber arm, I may be
able to find some stuff and pick it up, or attach a lift bag from inside the
sub. (Still working on that idea)
I've also made the sub modular, so different shaped
structures can be bolted on to the front and rear. I'm hoping to replace the
rear structure later with a larger section containing a diesel motor,
additional batteries, fuel tanks, and a gen set for charging while motoring on
the surface. Maybe even an on-board compressor.
This will also be a one atmosphere hull, but not connected to the main
pressure hull. Bolted on, with connecting thru-hulls for controls, but access
will be limited to the surface.
The alternative ( actually my original plan) was to have a
motor section that would stay on the surface while the small pressure hull
submerged. I think a pontoon boat could fill that role, and have enough deck
space for the required equipment and crew spaces. Plus it could act as a float
limiting the depth the sub could sink to in case of a system failure. This of
course would require a tether, which has it's own set of problems. It is still
possible to dive without the tether, but a tethered sub could have some
serious power supplied from above via a 220V line and hard wire communications
to the surface, and maybe even an air line. For now, I'll keep it simple, get
it working, and use it for a while to gain experience. Usage will help me
understand what works, and what changes I'll need. That's why the modular
design. The pressure hull itself is straight forward, and everything else can
be changed relatively easy by unbolting and re-configuring.
Try building full size mock-ups with card board and wire.
Sit inside, cut holes for windows, change the shapes, locations of things, and
generally figure out what shape suits you best. I spent a long time playing
around with card board shapes, and it was a lot of fun, and almost free. That
was the beginning of my adventure, and really got me hooked on the idea that I
actually could build one of these. Its fun and really cheap. Frank D.