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Another note here as it relates to MBT's. (Correct me if I'm
confused).......A psub with it's internal air bubble, will weigh just enough to
be "neutrally buoyant " with the ballast tanks free of air. The size of the
ballast tanks relates to how much of the sub you want above the water
line when you blow the tanks. This tank volume depends on the "dry weight " of
the portion above water when the sub is surfaced. There really is no limit to
how large the ballast tanks can be, other than how much air you're
carrying, and how many times you want to blow the tanks. A little ballast tank
will get you back to the surface, but may not get the hatch out of the
water.
On some of the K boats I've seen, the tower and just a bit of the hull
sticks out above water, which is why the tower should be 30 inches tall to give
you the recommended free board from ABS.
This determines the volume of the ballast tanks clamped onto the ends of
the pressure hull.
If larger tanks are installed, more of the sub sticks out. The displacement
of the bubble below the water line equals how much steel can be lifted above the
surface.
Here's how I designed my tanks......
I'm not sure exactly how much volume is in my ballast tanks yet as I'm
still welding them together. When I'm done welding them, and before I cut
the holes in the bottom, I'll pressure test them like I'd do with a plumbing
system to make sure there's no leaks. Add a gage, pump them up, and wait 24
hours to see if they hold the pressure. It should only take about 3 or 4
psi.
Next step is to fill them up with water and measure how much
water they hold.
This will tell me how accurate my calculations were and how much of the sub
will be exposed above water when the tanks are blown dry.
With no tower, raising the hull and hatch above water will take a lot of
bubble.
Once I'm sure there's no pin holes, I'll cut in the bottom holes and add
the water diverters. These are designed to keep water from flowing into the
tanks while being towed behind a bigger boat.
On the weight distribution, the front section and back section need to be
equal in weight so the sub stays level when submerged.
Because my rear ballast tank is larger than my front ballast tank, the sub
would sit " nose down " when surfaced. To compensate for this,
a portion of the internal ballast weight ( lead blocks ) will be on
a sliding mechanism moved forward and back with an electric actuator.
When submerged, the weight will be more or less centered. When I get to the
surface, the weight will be shifted back slightly to drive the back half down,
making the sub level at the surface. The sliding weight can also be used to
change dive angle when submerged, or re-balance when the occupants shift weight
by moving within the sub.
The reason for the larger rear ballast tank and shifting weight, is because
I will be mounting a small outboard motor on the back end for surface transport,
then removing it when diving. The extra ballast tank volume should be just
enough to keep the outboard out of the water, plus a little more for wake
action.
On the "nuke" subs, the ballast tanks must be huge to get that much of the
sub up above water.
My concern is how much air I need to carry, and how best to utilize the
limited volume. My plan is to have two 80 cu. ft. scuba tanks inside, with an
extra tank strapped below the rear fairing as a back-up.
It's possible to add two more tanks inside. I kept a space open for that if
needed.
These are, of course, in addition to the two 40 cu. ft. tanks I'll be
carrying for escape procedures, and the small oxygen tank tied into the scrubber
system.
I have enough room below the rear fairing for a large tank. Maybe will use
a welding type tank that holds 140 cu. ft. but for now will just mount another
80 cu. ft. scuba tank and plumb it into the HP air manifold, valved off, and
ready to use if the two main tanks get low.
I don't expect to need to blow the tanks dry while submerged at any great
depth, but I want enough capacity so that I CAN if I need to.
I believe the right way to surface is to motor up near the surface and then
blow the tanks, but small adjustments while at depth will be needed for the
VBT's and maybe I'll be "riding the bubble" sometimes too.
I can see a scenario where I may want to be "heavy" so I can settle on the
bottom, and blowing air into the VBT while deep will be required to regain
"neutral."
Now, having never driven a sub, I may be all screwed up here. If anybody
sees a flaw in this thinking, let me know. I may have to go back to school
!
Frank D.
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