Submarine Depth Measurement

[Mortartube]

Hi, Can anybody tell me what point of a submarine is used to determine it's depth. For instance, if a submarine dived to a depth of 10 fathoms, is that measured from the bottom of the keel (as in ordinary boats), to the surface or is some other point on the hull used as the depth reference?

[Hippy]

The submarine measures its general depth by detecting the pressure of the water outside relative to the atmospheric pressure at the surface. So the deeper the sub. goes the greater the weight of water above it, and a simple pressure gauge is all that is required.

On the surface the pressure of water is nominally zero so the water line around the ship is the datum for measurement, i.e. the point of zero depth.

When submerged and nearing underwater obstacles, or indeed the bottom, sonar (SOund Navigation And Ranging) is a much more accurate means of measuring proximity, and distance is measured from the point on the hull that the sonar transponder is fitted. So the bottom facing transponder on the underside of the boat detects distance below the keel. Forward, rearward and upward facing transponders detect distance from the bow, stern and topside respectively. The clever transponders are able to swivel to detect direction and range simultaneously. Planar Flank Array is a strip of transponders along the sides of the boat with phenomenal passive detection power to alert the crew of nearby ships that can be switched to active mode to fix distance and range from just a couple of pings.

[archbishop]

I don't think that is what mortartube is asking.Generally it is from the the lowest point of the keel.So if the boat dived to 10mtrs it might still be breaching the surface.Also a secondary measurement is taken of depth under the keel,which is the distance to the seabed.

[Mortartube]

Thanks to you both, Archbishop's answer was nearer to my requirements but they were both useful. The site wouldn't allow me to rate your answers for some reason. That's technology for you. Thanks again.

[gmc2002]

There are different measurements taken at different points using different technologies. Pressure differential is used as is Sonar. Active sonar (going ping) is noisy and will get you detected, so is used less often. Whenever depth is quoted, it is usually Keel Depth. . This lets you know from charts or my measurement if you may run aground. Similarly you don't want to surface and be seen when you are trying to be silent and invisible, but you may want to raise a periscope. Another issue under ice is how far to the underneath of the ice, this is another measurement.