Wet sub

A wet sub is a type of underwater vehicle (submarine) that does not provide a dry environment for its occupants. Usually, wetsuited scuba divers will ride upon the device (as one would ride a motorcycle or with an opening as in an Olympic bobsled), although it can be designed to fully enclose its occupant(s) to provide lower drag. An enclosed vehicle may provide a dry viewing chamber for the occupant(s). The motive force is an electric motor and usually the power source is an electric battery. The depth and duration at depth is typically limited by the requirements of a scuba diver.

This device may be configured as a mine-laying system (particularly for attaching limpet mines directly to a ship) or as a torpedo, or may be used for intelligence gathering in harbours. They have also been used to plant listening and recording devices on undersea cables (see Operation Ivy Bells). For such military use, the operator will use a rebreathing device so as not to leave a trail of bubbles. Such devices are deployed from a service ship or a specially outfitted conventional submarine. An example is the Multi-Role Combatant Craft (MRCC) from STIDD Systems in the US STIDD.

The Motorised Submersible Canoe, developed by the British Special Operations Executive during World War II, is another example.

A wet sub is attractive to the amateur or small commercial builder since many of the problems associated with a dry sub are avoided — only the propulsion and power systems need to be waterproof and life safety is much less of an issue than it is with a sealed occupant chamber. Complete submarines of this type are also available commercially.

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Wikipedia