Re: 1 ATM Sub Question

[hylands@ibm.net (Jon Hylands)]

On Tue, 23 Feb 1999 14:41:39 -0500 (EST), you wrote:

> I have a question regarding water displacement and the
> weight needed to effectively submerge a submersible.
> Does the total vehicle weight needed to become
> neutrally bouyant equal the weight of water displaced
> by the vehicle?  (I understand that a sub can penetrate
> the water when somewhat positively bouyant)

Yes, that is correct for a sub that has neutral buoyancy.

For a positively buoyant sub, you need enough thrust and "wing" foils
to force the sub under water, and must keep moving forwards to keep
from rising (the down-force from the wings must equal the up-force
from the buoyancy).

Most people who built normal submarines use compressed air to pump
water out of ballast tanks in order to change the "weight" of the sub.
For deep diving subs, that is not possible, since the water pressure
outside exceeds the air pressure in your air tank. So instead, for a
deep diving sub, you must either have a positively buoyant sub, or you
must use drop weights. The subs that dive on Titanic all use drop
weights.

Some deep-diving AUVs use drop weights, and some are positively
buoyant with either wings or vertical thrusters. Deep-diving ROVs tend
to be neutral or slightly positive, and use vertical thrusters to move
up & down through the water.

> So if I calculate that my sub displaces X cubic feet
> of water and then multiply X by the weight of 1 cubic
> foot of water, is that how much my sub needs to weigh
> to submerge?

Yes. For large psubs, this quickly becomes excessive.

Later,
Jon