Diving a Submarine

Some people assume that a submarine dives purely by use of its hydroplanes, others seem to think that it fills up with water, while most may not have given it any thought. So, how's it done?

While on the surface, most submarines have their forward hydroplanes ('foreplanes') either retracted or clear of the water. When the time comes to dive, the 'planes are extended in the horizontal position (neither pointing up nor down). To support the submarine's weight, air tanks - ballast tanks - around the submarine are full of air, giving positive buoyancy.

On diving, 'vents' in the tops of the tanks are opened and the ballast tanks become flooded, normally in a sequence forward to aft, by sea-water coming in through the 'free flood' holes at the bottom of the tanks: - you'll remember, from your schooldays, holding a tube of some sort under water, with your thumb acting as a stopper over the top - the tube will remain air-filled until you let go with the thumb, when water rushes up into the tube.

By the time the tanks are full, the submarine will have taken on a negative buoyancy. The boat won't sink like a stone because of the residual air inside the pressure hull so, to go deeper, the hydroplanes - both fore and aft - will have been pointing downward, acting in a similar way to aeroplanes when they point their 'elevators' downwards. The hydroplanes, like aircraft wings, would have no effect if the boat was not moving forward at sufficient speed for water to act on the 'plane surfaces.

For an emergency dive, or when needing to get deeper very quickly, this process can be accelerated by the flooding of extra internal tanks and by increasing speed. Once the submarine is dived, the vents are shut again. When underwater, the submarine can be manoeuvred, for the most part, by use of the hydroplanes alone. Extra balance, or 'trim', will need to be established by moving water around inside the boat's internal tanks. The boat can return to a position just below the surface - 'periscope depth' - by use of hydroplanes, though sometimes extra buoyancy will need to be gained by blowing some water from internal tanks.

When the boat needs to surface, high-pressure air is blown into the main ballast tanks. This pushes the water back out through the free flood holes (remember the vents are shut). This only needs to be done for a few seconds, until positive buoyancy is regained. The surfacing is aided by the hydroplanes being put into a 'rise' position. Full buoyancy is attained with a 'slow blow' with Low Pressure air.