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Re: [PSUBS-MAILIST] air pressure in ballast tank while surfacing

It wasn't just the US WWII subs that used a low-pressure bilge pump to finish emptying the main ballast tanks; the story of the merchant submarine Deutschland's crossing to the USA in WWI, breaking through the British blockade while the US was still neutral, mentions the clatter of the "Lenzpumpe" or bilge pump immediately after surfacing.
One hazard worth thinking about is that, when pumping out the liquid, the air pressure inside the tank cannot be allowed to drop too far below atmosphere, as the saddle tank plating would be thin and might easily implode. No doubt some sort of equalizing valve was provided.
As for calculating the pressure needed for an air pump to expel the remaining water - a less efficient, but also less hazardous solution - the procedure used for calculating hovercraft cushion pressure seems applicable. Simply divide the buoyant force provided by that tank by the area of the air/water interface. Or maybe not so simple, as that area - and therefore the required pressure - would change as the tank emptied. 

Best,
Marc

On 5/19/2010 9:36 AM, Hugh Fulton wrote:

Jens, I am doing similar but with closed buoyancy and using a pool pump to
pump out the water rather than an Air pump to pump in the air.  Regards,
Hugh


-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jens Laland
Sent: Wednesday, 19 May 2010 1:03 p.m.
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] air pressure in ballast tank while surfacing

Hugh

I'm playing around with a design of a 30 ft LOA diesel-electric submarine
(with hp-air compressor), and one of the things I'm considering is the
option to "economize" the use of bottled air when surfacing.

This will be similar to the practice on board the WWII fleet type
submarines, they used low-pressure, positive displacement blowers (10 psi)
to do the same thing.

The maximum height from the bottom water outlets to the top of the tanks
will be approximately 1300 mm [4.25 Ft].

So this means the said pump will never have to work against any head
pressure larger than 0.14 bar [2 psi], that is: ignoring any pressure loss
in piping system.

I was afraid it would be more, but with 2 psi maximum... I actually think
this could work. Thanks a lot for the enlightenment.

Best regards,
Jens Laland




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