Diver propulsion vehicle
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A diver propulsion vehicle (DPV, also known as an underwater propulsion vehicle or underwater scooter) is an item of diving equipment used by scuba and rebreather divers to increase range underwater. Range is restricted by the amount of breathing gas that can be carried, the rate at which that breathing gas is consumed under exertion, and the time limits imposed by the dive tables to avoid decompression sickness. DPVs can have military application; an example is the Diver Propulsion Device (DPD) from STIDD Systems in the US.
A DPV usually consists of a battery-powered electric motor, which drives a propeller. The design must ensure that: the propeller is caged so that it cannot harm the diver, diving equipment or marine life; the vehicle cannot be accidentally started or run away from the diver; and it remains neutrally buoyant under all conditions.
DPVs are useful for long journeys at constant depth where navigation is easy. Typical uses include cave diving and technical diving where the vehicles help move bulky equipment and make better use of the limited underwater time imposed by the decompression requirements of deep diving.
For many recreational divers DPVs are not useful. Buoyancy control is vital for diver safety: The DPV has the potential to make buoyancy control difficult and cause barotrauma if the diver ascends or descends under power. Visibility of less than 5 metres makes navigating a DPV difficult. Also, many forms of smaller marine life are very well camouflaged or hide well and are only seen by divers who move very slowly and are very vigilant.
 Diver-tugs, tow-behind, scooters
The most common type of DPV tows the diver who holds onto handles on the stern or bow. These are efficient because the divers ride in the slipstream as opposed to a "ride-on-top" which increases drag and reduces battery life. Tow-behind scooters are even more efficient by placing the diver horizontal and above the slipstream; the diver is attached with a harness and backplate or a BC with a front crotch-strap D-ring clipped to the scooter with a bolt snap and tow leash.
 Manned torpedoes
These are torpedo or fish-shaped vehicles for one or more divers typically sitting astride them or in hollows inside. The well known human torpedo or chariot was used by commando frogmen in World War II. Similar vehicles have been made for work divers or sport divers but better streamlined as these do not have warheads; the Dolphin made on the Isle of Wight (UK) in the 1970s is an example. Some Farallon and Aquazepp scooters are torpedo-shaped with handles near the bow and a raised seat at the rear to support the diver's crotch against the slipstream. The Russian Protei-5 and Proton carry the diver attached to the top. The New Zealand made Proteus is strapped onto the diver's cylinder.
The Subskimmer is a submersible rigid-hulled inflatable boat (RIB). On the surface it is powered by a petrol engine, when submerged the petrol engine is sealed and it runs on battery-electric thrusters mounted on a steerable cross-arm. It can self inflate and deflate, transformimg itself from a fast, light, surface boat to a submerged DPV. Started in the 1970s by Submarine Products Ltd. of Hexham, Northumberland, England, Subskimmer is now a tradename owned by Marine Specialised Technology.
 Wet subs
As DPVs get bigger they gradually merge into submarines. A wet sub is a small submarine where the pilot's seat is flooded and the diver must wear diving gear. Covert military operations use swimmer delivery vehicles (SDVs) to deliver and retrieve operators into harbors and near-shore undetected. An example is the Multi-Role Combatant Craft (MRCC) produced by STIDD Systems in the US.
These are unpowered boards (usually rectangular) towed by a surface boat with two ropes. The diver holds onto it and keeps it submerged by adjusting the angle like an upside-down aerofoil. It is named after the manta ray fish.
 See also
- "STIDD Military Submersibles". Retrieved 21 September 2010.