From what point is the depth of a submersible measured?

Question

When they say that a submarine is at a certain depth (like 200 feet below the surface), from what point on the sub are they measuring from? Is it from the midline of the sub, or the top of the conning tower (which, on U.S. Navy subs, is probably 50 feet closer to the surface than the midline)?

So I assume that the measurement is from the lowest point on the keel, then (unless it is from the tower, of course).

Answer 1

Whenever depth is discussed without any other reference, it's ***usually*** "keel depth" as measured from the keel to the surface -- that's done in consideration of a comparison to the depth of the bottom to know what kind of clearance currently exists.

Of course, when running close to the surface, the current height of the sub in the water as determined by the top of the conning tower/gear is critical -- for obvious reasons of visibility above the water, or cracking into another ship or the bottom of some serious ice! Steerable sonar transponders are typically used to keep track of clearances in multiple directions at once.

Which (keel or con) is more important at a given moment will of course depend upon whether you're more worried about running aground, or running into something above or being seen -- not that you need the con to be at or above the surface to be "seen", of course. The bugger is when in shallow water and you're screwed from both ends -- a trap into which few sub Cap'ns like to become involved.

Edit: Reply to Additional Details:

Yes, measurement from the lowest point of the keel is what's important, since that'd be the first thing to hit if the sub were to run aground! These days, they've got multiple transducers, and can keep track of clearances in all directions, including "sideways". I can only imagine what it must be like to be farting around under a shifting polar ice cap where no map would be worth beans the next time through.

Edit: Note for the poster below:

How'd we get off into "rated depth" (another kettle of fish altogether)? Didn't see that anywhere in the question.

Answer 2

the rating has more to do with the pressure assumed in the design

due to dif in temp/salinity, the depth rating is relative....its all relative

pressure is the key

also there is a dif between static and dynamic pressure

look at this an odd way

a virginia class sub is 400 feet long with an operation depth of 800 feet,

now stand it vertical....