Hydrofoil Boat

Overview

A hydrofoil boat is a fast passenger craft that lifts its hull clear of the water at speed, riding instead on small underwater wings called foils. At rest and at low speed the vessel floats on its Hull like any conventional boat. As the Propulsion System accelerates the craft past roughly 18–22 knots, the Foil System generates enough lift to raise the whole hull out of the water, leaving only the thin foils and struts immersed. With the hull no longer plowing through the surface, drag drops sharply and the boat cruises smoothly at 36–45 knots while using far less power than a planing hull would need at the same speed.

The example described here is a 27 m commuter ferry carrying 60–120 passengers across coastal and inter-island routes. It is built from welded 5083 marine aluminium with stainless steel foils, displaces 60–90 tonnes, and is driven by a single 900–1,200 kW high-speed diesel. The cabin is enclosed and air-conditioned, and the vessel is conned from a forward Navigation Bridge much like a small aircraft cockpit.

How foiling works

A foil is a wing that works in water instead of air. Because water is about 800 times denser than air, a hydrofoil only needs a small area to carry the whole boat. The Forward Foil sits below the bow on the Forward Strut and carries roughly 60% of the load. The Aft Foil sits below the transom on the Aft Strut and carries the rest while also setting the boat's pitch trim. Lift rises with the square of speed, so below take-off speed the foils cannot support the boat and it stays hullborne. Above take-off speed the lift exceeds the weight, the hull rises, and the boat settles at a height where lift and weight balance.

This design uses fully submerged foils, which stay completely underwater during flight. Fully submerged foils give a smooth ride in waves because they are not disturbed by the surface, but they are not naturally stable: left alone they would either climb out of the water or sink back into it. The Foil Control System solves this. It is an active autopilot that continuously adjusts the foils to hold a steady ride height.

Foil control

The ride-control loop runs about 50 times a second. A forward and an aft Height Sensor measure how deep the foils are below the surface, using ultrasonic ranging and submersion pressure read by a pressure-sensor. A Gyro Unit reports the boat's pitch, roll and vertical motion. The Autopilot Controller, a sealed unit built around an mcu on a marine-grade board, takes these readings and computes how much each foil should change its lift. It commands four Flap Actuator units, each driving a trailing-edge Foil Flap through a servo-motor. Deflecting a flap a few degrees changes the foil's lift almost instantly, so the controller can hold the hull at a steady cruising height of about 1.5 m above the water even as waves pass underneath. The same loop damps roll by deflecting the port and starboard flaps differently, keeping the cabin level in a beam sea.

Hull and structure

Although the boat flies at speed, the Hull still has to carry it at rest, during take-off, and on landing. It is a welded aluminium planing hull, 5–8 mm Hull Plating over a grid of transverse Frame Set frames and a central Keel Beam box girder that also anchors the forward strut. The hull is divided by watertight Watertight Bulkhead walls into buoyant compartments so flooding one section will not sink the boat. A Spray Rail runs along each chine to throw spray downward during the planing take-off run, and the reinforced Transom mounts the aft strut and the drive line.

Propulsion and steering

Power comes from a high-speed common-rail Marine Diesel Engine of 900–1,200 kW. Because the propeller has to sit at the bottom of the aft strut, well below the hull, the drive turns through a down-angle Reduction Gearbox built around a gearbox-housing and a gear-pair, then runs down the strut on the Propeller Shaft to a Supercavitating Propeller. At foilborne speed the propeller works in a regime where the water around the blades vaporises, so a supercavitating profile is used that is shaped to run efficiently inside its own vapour cavity. The engine is cooled by a seawater Cooling Circuit and isolated from the hull by rubber Engine Mount blocks.

Steering is hydraulic. The Helm Wheel drives a Steering Pump that pressurises a Steering Cylinder, which swings the Tiller Arm to set the angle of a strut-mounted Rudder. The rudder stays immersed at all speeds, so the boat steers the same way hullborne and foilborne, though turns at 40 knots are gentle to keep the foils from broaching the surface.

Cabin, electrical and fuel

The Passenger Cabin is enclosed and laid out like an aircraft, with rows of reclining Passenger Seat units, large bonded Cabin Window panes, and a 12 kW HVAC Unit for climate control. Passengers wear a lap belt during take-off and landing transitions. The Electrical System runs at 24 V DC: an engine Alternator charges a sealed AGM Battery Bank, and a Distribution Panel feeds the control, navigation and cabin circuits through individual breakers. Diesel is held in two baffled Fuel Tank units feeding the engine through water-separating Fuel Filter elements in the Fuel System.

Variants and use cases

Hydrofoil designs split into two families. Surface-piercing foils form a V or ladder shape that pokes through the surface; they are self-stabilising because lift falls automatically as the foil rises out of the water, so they need no active control, but they ride more roughly in waves. Fully submerged foils, like the ones on this craft, stay underwater and give a far smoother ride at the cost of needing the active Foil Control System. Sizes range from small 5 m sport foilers up to the large passenger hydrofoils that have run on rivers and coastal routes for decades.

The main use case is fast passenger transport on protected and coastal waters: commuter ferries, island hops, and harbour shuttles where a 40-knot crossing beats both a slower displacement ferry and a road detour. The smooth foilborne ride reduces seasickness compared with a planing craft slamming through chop, which matters on routes that run many times a day in moderate seas.