A normal hovercraft typically has one means of accelerating forwards or rearwards (a large propeller at
the rear of the vehicle), and one means of steering
(louvers behind the prop).
The Harrier Hovercraft still uses a propeller to move
forwards, but instead of louvers to steer, it uses
attitude jets, similar to those used for VTOL
operation on Harrier class airplanes.
A plethora of sensors (accelerometers, gyroscopes,
doppler radar aimed at the ground below the vehicle)
are used to determine the vehicle's velocity and
rotational velocity relative to the ground.
When the rear of the vehicle starts to drift left or right,
one of the two jets at the rear is used to precisely
counteract that drift.
When the vehicle as a whole is turning at a rate that's
either more than or less than that dictated by the pilot's
steering wheel and the vehicle's forward speed, one of
the front two jets is used to add to or counteract the
Under ideal circumstances, these two effects together
should allow the hovercraft to steer in a way that feels
very much like a wheeled vehicle.
There would be four jet nozzles, one on each corner of
the vehicle, aimed skywards. If the vehicle starts to
lift off the ground and flip, these are used to counteract
that rotation and bring the vehicle's attitude back to
being parallel to the ground. This would only be
necessary at very high speeds, or in very high winds.
To produce small amounts of deceleration, a clutch
disengages the vehicle's propeller from the engine, and
a brake is applied to the prop.
To produce a larger deceleration, hydraulic actuators
deploy four retractable wheels, with brakes of course.
The front two wheels would be swivel casters; this
should not detract from the vehicle's handling, and
avoid the need for a redundant steering mechanism.
Alternatively (and especially useful off-road)
deceleration can be produced by spinning the propeller
in reverse. This consumes more energy, but you can't
expect wheels to slow you down when your hovercraft
is on water.