Motorboat Theory
Motorboat theory describes the forces acting on a powered boat and how they affect speed, fuel consumption, and manoeuvring behaviour. A fundamental distinction is made between displacement and planing.
Displacement Mode
In displacement mode the boat floats at its full draft and pushes aside a quantity of water equal to its weight. The bow and stern waves it produces determine the maximum speed it can achieve.
Hull Speed
The hull speed is the theoretical maximum speed of a displacement hull. It depends on the waterline length and can be approximated as:
v [kn] ≈ 2.43 · √Lwl [m]If this speed is exceeded, the boat would have to climb its own bow wave. Fuel consumption rises sharply without any noticeable gain in speed.
Planing Mode
In planing mode only a small part of the stern remains in the water. Hydrodynamic lift raises the hull out of displacement. Water resistance drops sharply, allowing significantly higher speeds.
The transition between displacement and planing is called transitional mode. In this phase resistance is greatest and fuel consumption highest.
Trim
The trim describes the longitudinal inclination of the boat in the water. A too bow-heavy trim makes the boat dig into the waves; a too stern-heavy trim prevents clean planing. On many boats the trim can be adjusted by trim tabs or by the angle of the sterndrive.
Paddle-Wheel Effect
The paddle-wheel effect arises because the propeller pushes the stern sideways, as if it were rolling on solid ground. A right-handed propeller pushes the stern to starboard when going forward, and to port when going astern. This effect must be taken into account when manoeuvring in a harbour.
Steering Effect
The steering effect on a motorboat is created by the rudder blade behind the propeller. The water flow generated by the propeller strikes the rudder and turns the boat. It follows that without sufficient propeller flow (e.g. at idle), steering is much less effective.
With outboard engines and sterndrives, the entire drive is swung when steering, so that the thrust acts directly in the desired direction.
Turning Circle
The turning circle is the smallest circle the boat can sail with full helm. It depends on boat length, speed, helm angle, and wind or current influences. When entering a harbour and manoeuvring in narrow waters, the turning circle must always be planned in.