How amphibious vehicles work

Floating: buoyancy and displacement

Anything placed in water is pushed upward by a force equal to the weight of the water it displaces, a rule known as Archimedes' principle ^[1]. A floating vehicle therefore settles until it has pushed aside its own weight in water, so its sealed hull must enclose enough volume below the waterline to do so ^[1].

A safe amphibian also needs freeboard, the height of the hull above the water, and reserve buoyancy, extra sealed volume that keeps it afloat if waves break over it or water leaks in ^[8]. Investigating the 2018 sinking of the tour vessel Stretch Duck 7 on Table Rock Lake, Missouri, the US National Transportation Safety Board noted that the converted World War II DUKW had low freeboard, an open hull and no internal subdivision or flotation, and it named the lack of a Coast Guard requirement for sufficient reserve buoyancy as a contributing factor ^[8].

Hull shapes: displacement and planing

Most amphibious cars and trucks behave as displacement craft in the water: the hull pushes water aside, and its top speed is limited by its waterline length ^[2]. A common rule of thumb puts the maximum speed of a displacement hull in knots at about 1.34 times the square root of the waterline length in feet, so a 16 ft (4.9 m) waterline gives roughly 5.4 knots (6 mph, 10 km/h) ^[2].

A planing hull has flatter bottom surfaces and hard edges called chines. As speed rises, water pressure on those surfaces lifts the hull until it rides on top of the water rather than through it ^[2]. Fast amphibians use planing hulls; Gibbs, for example, describes its hulls as one-piece composite moldings that give a stable planing surface ^[3]. To plane, the vehicle also has to lift its wheels clear of the water, because wheels hanging below the hull add drag ^[4].

Propulsion in water

The simplest method is to let the wheels or tracks do the work. Small amphibious all-terrain vehicles such as the ARGO use a one-piece polyethylene hull and low-pressure tires with a chevron tread that paddles the vehicle through water ^[12]. The World War II Landing Vehicle Tracked was driven on both land and water by its tracks, which carried specially shaped shoes ^[13].

Many amphibious cars and trucks drive a propeller from the gearbox. The DUKW of World War II sent power through a transfer case to a propeller, and the driver could select the propeller or the front axle as needed ^[11]. The Amphicar 770 of the 1960s had twin propellers mounted under the rear bumper ^[10].

Some amphibians are boats first. The Sealegs, a boat fitted with retractable wheels, uses an ordinary outboard motor in the water and a separate small engine on land to drive hydraulic wheel motors ^[7]. Water jets, used on fast amphibians, draw water in through an intake in the bottom of the hull and force it out through a nozzle at the stern; because nothing projects below the hull, they can run in shallow water ^[6]. Gibbs pairs its jets with a BMW motorcycle engine on the Quadski and says they bring it to planing speed in seconds ^[3].

Retracting wheels and suspension

On a slow amphibian the wheels can stay down in the water, but a fast one must lift them. A Gibbs patent granted in 2021 describes a hydraulic actuator and linkage that swing the suspension arms and wheel upward by at least 75 degrees, and preferably 90, so the wheel clears the waterline even with a deep V-shaped hull ^[4]. On land the linkage locks in an over-center position so the suspension works normally ^[4]. Gibbs says the complete change on its Quadski takes four seconds at the push of a button ^[3].

Keeping water out

Every opening at or below the waterline, and every hatch or air intake that waves can reach, has to be sealed or protected, and any water that does get in must be pumped out. The DUKW carried a high-capacity bilge pump system that could keep it afloat even with holes up to 2 in (50 mm) across in its thin steel hull ^[11]. Openings remain a weak point: NTSB investigators found that waves flooding through a bow air intake hatch that was not weathertight caused Stretch Duck 7 to sink ^[8].

Cooling the engine

Small boats usually cool their engines with the water they float in, but an amphibian must also run on land, and a car-style radiator is hard to place and to feed with air ^[5]. A Gibbs patent filed in 2003 describes one answer: an air-cooled radiator behind a front intake that a hinged flap closes in the water, plus a second heat exchanger cooled by outside water drawn in under the rear of the hull, where the supply continues as the hull rises onto the plane ^[5].

Steering, entering and leaving the water

Steering methods vary with the drive. The Amphicar used its front wheels as rudders ^[10]. A water jet steers by turning its nozzle to direct the outflow to one side, and a reversing bucket gives astern thrust ^[6]. On the Sealegs, hydraulic steering turns the front wheels on land and the outboard in the water ^[7].

An amphibian enters the water by driving down a ramp or shore until it floats. On the Sealegs, the wheels are retracted hydraulically once the boat is afloat, which lets it reach full water speed ^[7]. Leaving the water often means crossing soft sand or mud. The DUKW was the first vehicle that let the driver change tire pressure from the cab, deflating the tires for sand and inflating them for roads ^[11].

Road rules and marine rules

Legally, an amphibious vehicle is usually both a motor vehicle and a boat. In California, for example, a vehicle-vessel combination must be registered for road or off-road use and separately registered as a vessel, and the two registrations have separate expiration dates ^[9]. Rules differ by state and country. Vehicles that carry paying passengers face added oversight; after the Stretch Duck 7 sinking, the NTSB made recommendations to the US Coast Guard on reserve buoyancy and passenger survivability ^[8].

Related entries

• GMC DUKW "Duck"Military amphibians · 1942 to 1945
• Landing Vehicle Tracked (LVT, amtrac)Military amphibians · 1935 (Alligator); 1941 to 1945 (wartime LVTs)
• Amphicar 770Amphibious cars · 1961 to 1968
• Gibbs AquadaAmphibious cars · 2003 to 2004
• Gibbs QuadskiATVs and work machines · 2012 to 2016
• SealegsAmphibious boats · 2004 to present
• HovercraftHovercraft and aircraft · 1959 to present