low energy-consuming propulsion

Propulsion systems, in their relatively recent history, have been energetically inefficient, in the sense that the energy transformations, which characterized them for a long time, were not particularly efficient.

The waste of energy caused by the propulsion systems over time has been significant.

A large part of propulsion systems involve two distinct energy transformations.

A first transformation is carried out by an engine that transforms the chemical energy of a fuel or electrical energy into kinetic energy, for example, on a ship the marine engine transforms the energy of the fuel into kinetic energy of the crankshaft.

A second transformation transforms the kinetic energy, supplied by the engine, into propulsive thrust by means of a propulsive device, in the marine example, the second transformation is carried out by the propeller which produces the thrust that moves the ship.

Leaving aside the energy efficiency of the engines and considering only that concerning the propulsive part, many propulsion devices are not particularly efficient. 

Remaining on the nautical example, the current propellers produce turbulence and noise and this fact involves not a little waste of energy.