fresh/salt water bouyancy difference

[Ray Keefer <Ray.Keefer@eng.Sun.COM>]

Hi,

There is a story I once heard. A Captain of a nuclear Los Angelas class
attack submarine took his boat out after an overhaul at Pearl Harbor.

He took it out clear of Pearl, out to the safe zone to dive and proceeded
to submerge. Try as he could. Flood all his tanks: ballast, trim, bilge, 
waste ... he couldn't get the boat submerged.

After a couple of hours of futile attempts to submerge he had to take his
boat back to Pearl. Once back in port it was discovered that the overhaul
crews had removed the margin ballast.

Big military submarines are intentionally designed to naturally a bit
positively bouyant. This is done for a few reasons. 

First it's easier to add weight to a submarine then to add bouyancy, just add
lead bars. 

Second, if in the future you get bigger and better weapons the chances are
they are heavier then the current ones. In which case you just add less margin
ballast instead of having to add additional bouyancy in some way.

The submarine in this story couldn't get under because it was too bouyant.

>From the book "Concept in Submrine Design" there are three types of ballast:

Ballast - pgs. 40 -44
	Permanent-brings equity of weight and buoyancy; enables longitudinal 
		positions of center of gravity and center of buoyancy to be 
		brought into coincidence;  brings vertical position of center of
		gravity enough below center of buoyancy to give adequate 
		separation, for hydrostatic stability

	Margin - to make an allowance for weight growth during design and 
		building and future life

	Variable - carried in sea water; used to make continuous corrections 
		to achieve neutral buoyancy

Regards,
Ray