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Hi James. Pretty much every submarine has some weights inside. A
sub's total weight needs to be enough to overcome the air bubble created by
the pressure hull. My pressure hull has over 800 gallons of capacity but doesn't
weigh that much being only 3/8 inch thick.
It takes a lot of extra weight to get that thing down.
My sub doesn't have a normal tower so the top part of the sub that gets
lifted out of the water is just the top hull and hatch. Those two
components weigh about 600 lbs. so by adding lead weights down low, the sub's
center of gravity stays real low. All the heavy stuff like ballast weights,
batteries, and the rest are attached down low on the sub so even when the drop
weight is released, it still can't flip over. The total weight is about 6800
pounds.
The trick is to get the ballast tanks to raise enough of the sub out of the
water so a wave doesn't wash in there when the hatch is open. That portion above
water is called the "freeboard" and ABS has a recommended distance of 32 inches
I think.
I'll not know exactly how much freeboard I'll have until I finish welding
the ballast tanks and pour water in there to see what the capacity really is,
but my "calculated" freeboard is around 24 inches.
I don't put a lot of credence in my calculations so "real time" testing is
my preferred method. If It's critical I'm more likely to build a mock-up to
test and measure rather than relying solely on my dubious math skills.
Frank D.
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