On every warship with a SQS-26 or -53 sonar, there were sailors who had to be medically qualified to be divers. They weren't divers, they had no training as divers, but they had to be medically qualified as though they were divers.
In an earlier post, there were pictures of one of those large sonars. The sonar domes had rubber windows on them, for tests had shown that a rubber window was a lot better at conducting acoustical signals than a steel dome was. "Better" means that you might track a target that you couldn't or detect a target that you might have missed. "Better" means that you kill a submarine, and that, Gentle Reader, is what it was all about.
What you had, therefore, was part of a very large rubber tire on the front and sides of the sonar dome. A cruiser or destroyer could push that very large sonar dome through the water at over thirty knots. To prevent the rubber window from being pushed in from that much force, the sonar dome was pressurized with water. Fresh water was used in order to prevent the corrosion that would have occurred from sea water. There were high and low alarm points on the pressurization system, the alarms rang in both Sonar Control and on the Bridge. The importance of keeping the sonar dome pressurized was such that the alarm box was placed right next to the Captain's chair on the Bridge.
In really heavy seas, the bow of the ship would come out of the water, sonar dome and all. When the bow came back down, that large sonar dome would slam into the sea; the entire ship would quiver from the force of the impact and then the bow would continue down until the ship started up the front of the next wave. Warships did best in heavy seas by sailing into the waves. (By 'heavy seas," I mean seas with wave heights of thirty feet or higher.) If you were on the Bridge while that was going on, you would hear the loud beeping of the alarm as the dome was slammed down into the sea, as that would briefly overpressurize the sonar dome.
It was sort of an informal gauge of how bad the seas were by the number of beeps you heard from the dome alarm each time the bow came back into the sea. It was not unheard-of for a ship to suffer enough damage to its sonar dome in really heavy seas that the ship would have to be drydocked for repairs. (I'll blog about that process another day.)
Back to the point: There were periodic checks that had to be done to the interior of the sonar dome. If a bad sonar transducer element was detected during a source level check, it might need to be replaced. The sonar was designed so that you could replace elements of the transducer without having to drydock the ship. But keep in mind that you have a large dome that is, at its base, over twenty feet under water. That is a lot of pressure on the rubber window. What was done was that the fresh water in the dome was pumped out and replaced with compressed air. There was an air lock in the access trunk to the sonar dome. (A "trunk" on a ship is a narrow vertical shaft that goes through one or more decks.) The sailors who would do the work inside the sonar dome had to have diver's physicals, since they would have to work in a pressurized area.
And that is why you had sailors who had to be medically qualified for diving duty on warships.
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