Re: [PSUBS-MAILIST] VBT positioning

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Captain Nemo" <vulcania@interpac.net>
To: <personal_submersibles@psubs.org>
Sent: Sunday, July 15, 2001 6:50 PM
Subject: Re: [PSUBS-MAILIST] VBT positioning


OOPS!  I was sitting here running this off the top of my head, and I realize
I goofed in my math.  Let me clarify what I meant to say...

>
> If, say, the gross weight of the example boat (above) is equal to 44 cubic
> feet of sea water (2816 #), and the desired surfaced freeboard is equal to
> that provided by 10% of the pressure hull's displacement;  I'll want to
> mount 4 cubic feet worth of ballast tanks outside and below the waterline
on
> the 40 cubic foot pressure hull if, when the tanks are  flooded, I want
the
> boat is to submerge at neutral buoyancy.

The pressure hull displaces 40 cuft (2560 lbs.)  The external tanks displace
an additional 10% of that: 4 cuft (256 lbs).  The total weight of the boat
is 2560 lbs.  The external tanks add their 256 lb. empty buoyancy to the
boat, providing a total buoyancy of 2816 lbs, and thereby giving a freeboard
that is equal to 10% of the hull displacement.  When the tanks are flooded,
buoyancy is reduced to the gross weight of 2560 lbs., and the top of the
hull is just barely submerged at neutral buoyancy.


> If, however, the design calls for the gross weight of the boat  to equal
the
> displacement of only 40 cubic feet of sea water (2560#), and I still want
a
> 10% freeboard, I can place a 4 cubic foot tank tank inside the hull, where
> it's volume won't add to the overall displacement; and when it's flooded,
> the water-weight it takes on will overcome  the 10% positive buoyancy /
> freeboard, enabling the boat to submerge.
>

Similar mistake to the above.  The displacement of the hull is 2560 lbs.,
but now the gross weight is 10% less than that, or 2304 lbs.  With the empty
4 cubic foot VBT inside the hull, she'll sit with a 10% positive freeboard.
And when we add 4 cubic feet of water to the tank (256 lbs), the combined
weight of the boat and ballast water will equal 2560 lbs, and the boat will
be submerged at neutral buoyancy.

Whew!  Glad I caught that one; and I hope I didn't confuse you too much!

Pat