Airboat

Overview

An airboat is a flat-bottomed boat pushed not by an underwater propeller but by a large aircraft-style propeller mounted in the air above the stern. Because nothing hangs below the hull, the boat can run over water only a few centimetres deep, across mudflats, floating vegetation, sandbars and even wet grass. This is what makes it the standard craft for the Florida Everglades, Louisiana swamps and similar marshes, and for search-and-rescue and wildlife work in shallows where any conventional boat would run aground or foul its propeller.

The boat is organised around moving air. The Hull is a flat scow that skims the surface, the Engine turns the Propeller Assembly to push a column of air astern, and the Propeller Cage guards that spinning propeller and carries the engine. The Rudder Assembly steers by deflecting the propeller's slipstream, the Helm Station puts the driver up high with the controls, and the Fuel System and Electrical System systems run the engine.

Hull

The Hull is a flat-bottomed scow with no keel, no skeg and nothing protruding below the waterline. The Hull Pan is a welded aluminum or moulded composite pan, stiffened by transverse Hull Rib ribs, with a slick UHMW Skin sheet of UHMW polymer bonded to the bottom so the boat slides over grass, mud and oyster bars without grabbing. Spray Rail rails along the sides throw spray down and help the flat hull bite when it pivots in a turn.

Because the boat operates far from help in remote wetlands, buoyancy and a safe deck matter. Foam Flotation foam packed into the hull keeps it floating even if swamped, and a raised non-skid Deck Sole keeps the crew above any water in the bilge. The flat bottom planes at very low speed and draws only a few centimetres, so the boat can transition from open water onto a mud bank or saw-grass prairie without stopping, sliding across the wet vegetation on a film of water.

Engine and propeller

The Engine is usually an automotive or aircraft V8 run as a stationary engine mounted high at the stern. The Engine Block carries Piston pistons driving a Crankshaft that turns the propeller, with Cylinder Head heads on each bank and a Carburetor metering the mixture. Sitting in still marsh air with no forward draught at low speed, the engine cannot rely on ram air, so a dedicated Cooling Fan forces air over it, and an Exhaust System system vents the gases. A Spark Plug per cylinder fires the charge.

The Propeller Assembly turns engine power into thrust. Engines spin far faster than a large propeller can run efficiently, so an Reduction Gearbox, usually a Drive Belt drive over a Helical Gear Pair in a Gearbox Housing on Ball Bearing bearings, slows the Propeller Shaft down. The shaft carries an Propeller Hub clamping three Propeller Blade blades. The slow-turning, large-diameter propeller moves a big mass of air rearward, and the reaction drives the boat forward. Thrust comes entirely from this air column, which is why the boat works the same whether floating, sliding on grass, or crossing a mud bank.

Cage and steering

Spinning a large propeller at head height a metre behind the crew is dangerous, so the Propeller Cage surrounds it completely. Cage Ring hoops frame the propeller disc and carry a Cage Mesh guard screen that keeps people, branches and debris out of the blades. The cage is built onto an Engine Stand that raises the whole engine and propeller above the deck, and a Rudder Mount hangs the rudders in the slipstream behind the prop.

The boat is steered entirely by air. With no underwater rudder, the Rudder Assembly uses a pair of Air Rudder vertical vanes set in the propeller's slipstream behind the cage. Pushing the Steering Stick works a Rudder Linkage through Rudder Cable cables to swing both rudders together, deflecting the column of air to one side and yawing the boat. Steering only works while the propeller is turning, so the driver keeps power on through turns, and the flat hull lets the boat pivot quickly almost in its own length.

Helm, fuel and electrics

The Helm Station sits the driver up high. An elevated Helm Seat gives a clear sightline over the tall saw grass, where the only way to see open channels is to look down on them. From there the driver works the Steering Stick for the rudders, an Throttle for engine speed, and an Control Panel with ignition and gauges, while a Headset protects against the engine noise, which is loud enough that hearing protection is mandatory.

The Fuel System feeds the engine from an Fuel Tank mounted low for stability, through a Fuel Filter and Fuel Line drawn by a Fuel Pump. The Electrical System system runs off a Battery charged by an Alternator, cranked by a Starter Motor, with an Ignition System system firing the plugs and a Light Bar of spotlights for running the marsh at night. Airboats are used for wetland tours, alligator and waterfowl management, swamp and flood rescue, and access to shallows that no other powered boat can reach.