thumb|Two U.S. Marines of the [[Maritime Special Purpose Force operating a Diver Propulsion Device (DPD)]]A diver propulsion vehicle (DPV), also known as an underwater propulsion vehicle, sea scooter, underwater scooter, scuba sled or swimmer delivery vehicle (SDV) by armed forces, is a compact underwater propulsion device, usually used as a diving equipment by scuba 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, and the battery power of the DPV. Time limits imposed on the diver by decompression requirements may also limit safe range in practice. DPVs have recreational, scientific and military applications.
DPVs include a range of configurations from small, easily portable scooter units with a small range and low speed, to faired or enclosed units capable of carrying several divers longer distances at higher speeds.
The earliest recorded DPVs were used for military purposes during World War II and were based on torpedo technology and components.
Structure
left|thumb|Hobbyist-built DPV with T500 thrusters
A DPV usually consists of a pressure-resistant watertight casing containing an underwater thruster, or a battery-powered electric motor, which drives a propeller. The design must ensure that the propeller cannot harm the diver, diving equipment or marine life, the vehicle cannot be accidentally started or run away from the diver, and it remains approximately neutrally buoyant while in use underwater.
Application
DPVs are useful for extending the range of an autonomous diver that is otherwise restricted by the amount of breathing gas that can be carried, the rate at which that breathing gas is consumed, which is increased by exertion and diver fatigue, and the time limits imposed by decompression obligation, which depend on the dive profile. There are accessories that can be mounted to a DPV to make it more useful, such as lights, compasses, and video cameras. Use of a DPV on deep dives can reduce the risk of hypercapnia from overexertion and high breathing rate.
Limitations
thumb|Two divers scootering with heavy duty DPVsDPV operation requires greater situational awareness than simply swimming, as some changes can happen much faster. Operating a DPV requires simultaneous depth control, buoyancy adjustment, monitoring of breathing gas, and navigation. Buoyancy control is vital for diver safety: The DPV has the capacity to dynamically compensate for poor buoyancy control by thrust vectoring while moving, but on stopping the diver may turn out to be dangerously positively or negatively buoyant if adjustments were not made to suit the changes in depth while moving. If the diver does not control the DPV properly, a rapid ascent or descent under power can result in barotrauma or decompression sickness. A DPV can increase the risk of a silt-out if the thrust is allowed to wash over the bottom.
History
thumb|An Italian maiale type manned torpedo, at the [[Royal Navy Submarine Museum.]]
thumb|Israeli manned torpedo. 1967.Human torpedoes or crewed torpedoes are a type of diver propulsion vehicle used as secret naval weapons in World War II. The name was commonly used to refer to the weapons that Italy, and later Britain, deployed in the Mediterranean and used to attack ships in enemy harbours.
The first human torpedo was the Italian Maiale ("Pig"). In operation, it was carried by another vessel (usually a normal submarine), and launched near the target. It was electrically propelled, with two crewmen in diving suits and rebreathers riding astride. They steered the torpedo at slow speed to the target, used the detachable warhead as a limpet mine and then rode the torpedo away. The nose of the torpedo was filled with pounds of TNT and would be hung under a ship's keel.
The idea was successfully applied by the Italian navy (Regia Marina) early in World War II and then copied by the British when they discovered how effective this weapon could be after three Italian units successfully penetrated the harbour of Alexandria and damaged the British battleships and , and the tanker "Sagona." The official Italian name for their craft was "Siluro a Lenta Corsa" (SLC or "Slow-running torpedo"), but the Italian operators nicknamed it "Maiale" after their inventor Teseo Tesei said that it was making the noise of a pig while moored on the beach. The British versions were named "chariots".
thumb|Motorised submersible canoe
The Motorised Submersible Canoe (MSC), nicknamed Sleeping Beauty, was built by British Special Operations Executive (SOE) during World War II as an underwater vehicle for a single frogman to perform clandestine reconnaissance or attacks against enemy vessels.
For long-range missions, SDVs can carry their own onboard breathing gas supply to extend the range of the swimmer's scuba equipment. For extended missions on land, a team can be re-supplied by contact with other SDVs. In the latter usage, SDVs can stealthily plant mines and other bombs on ships or port infrastructure and then retreat to a safe distance before detonating the explosives. In addition to destroying targets, the SDV can mislead enemies as to where they are being attacked from.
Manned torpedoes
These are torpedo or fish-shaped vehicles for one or more divers typically sitting astride them or in hollows inside. The human torpedo was used to great effect by commando frogmen in World War II, who were able to sink more than 100,000 tons worth of ships in the Mediterranean alone.
Wet subs
As DPVs get bigger they gradually merge into submarines. A wet sub is a small submarine where the crew spaces are flooded at ambient pressure and the crew must wear diving gear. Covert military operations use wet subs to deliver and retrieve operators into harbors and near-shore undetected.
Towed sleds
thumb|Towed diver sled with independently controlled planes
These are unpowered boards (usually rectangular) towed by a surface boat which function as diving planes. The diver holds onto the sled and may use a quick-release tether to reduce fatigue. Depth control while submerged is by adjusting the angle of attack. Sometimes known as manta-boards, after the manta ray. Towed sleds are useful for surveys and searches in good visibility in waters where there are not too many large obstacles. The route is largely controlled by the towing vessel, but the diver has a limited amount of control over vertical and lateral excursions.
Modern DPVs
DPVs currently in service include:
Italy
;Cosmos CE2F series such as CE2F/X100-T
:Two person DPV
Poland
;Błotniak
:One person DPV
Russia/USSR
;Protei-5 Russian diver propulsion vehicle
:One person DPV clipped on to the diver
;Sirena DPV
;SEAL carrier
white 2015 human crew SDV with 30+ kn on surface and can be parked on the sea floor. There are three different modules for the Carrier providing different applications including: SEAL SDV – Swimmer Delivery Vehicle for SOP missions, SEAL AUV – Autonomous Under-water Vehicle for MCM missions, and SEAL RWSV – Remote Weapon Station Vehicle for Fire Support.
;Smart SEAL
:A downsized SEAL Carrier, with 30+ kn on surface and can be parked on the sea floor.
;Sub SEAL
:Electrically powered SEAL carrier capable of diving 40 m and carrying 6 divers with 600 liter of balanced load, with 30 nmi range at 5 kn. Can be mounted either inside the tube or transported on deck.
;Torpedo SEAL
:SDV is designed to be stored inside a 21 in torpedo tube, and it comes in two versions, the one person version 2.5 m and a two person version 3.4 m.
United Arab Emirates
After purchasing US submersible manufacturer Seahorse Marine, Emirate Marine Technologies of United Arab Emirates has developed four classes DPV/SDV, all of them built of glass reinforced plastic and carbon composite materials:
;Class 4
:Three-ton two-person DPV using silver-zinc external battery packs to power an 8 kW motor. Sensor and control suites include a sonar, echo sounder, GPS, electronic compass and electronic mapping facility and an onboard computer. They have a range of 60 nmi (110 km) at 6 (maximum speed 7) kn and can operate down to 50 m. Maximum payload is 200 kg.
;Class 5
:Eight-ton two-person DPV using nickel-cadmium external battery packs to power an 8 kW motor. All other specs are identical to Class 4 except the maximum payload is increased to 450 kg.
;SDV-X Dolphin
white 2015.75 tonne 6 to 8-person SDV developed by Columbia Research Corporation built of fiberglass. Maximum range is 56.5 nmi (104.5 km) at 5 kn but burst speed of 8 kn can be achieved with its powerful brush-less DC motor and silver-zinc batteries. Maximum depth is 91 m, although it will usually be kept around 45 m. The hull is of molded high-density polyethylene plastic to provide a low magnetic signature while folding bow planes and rotating motors allow the vehicle to be deployed through a 76-cm escape hatch. The Sea Shadow features electronic speed control with a manual back-up mode and an option of electronic drift compensation. Speed, depth, battery duration and voltage details is shown on a liquid crystal display, while there are visual warning signals on low battery power as well as unsafe rates of both ascent and descent. Sea Shadow has a range of 5 nmi (9.25 km) at 2 to 3 kn and at a maximum of depth of 30 m.
;Swimmer Delivery Vehicle
:Deployment of SDV
;Swimmer Transport Device
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