In the following it is attempted to define the mechanical propulsive efficiency η of ferry crossings, not the transport efficiency of the vessel itself nor its motor, controller or battery. That is, the efficiency is defined as 100% if the water (+ air) resistance times the speed of crossing (= propulsive power Pp) is equal to the mechanical power produced. Meant is the best efficiency at one operating point, not the practical efficiency average.
η is defined as the ratio between energy output and input. Energy is force times distance or power times time. η can also be calculated as the ratio of power or force output and inputs.
η = Pp / Pm (motor drive shaft power).
If the (small) friction from bearings and seals is neglected, η = ηpropeller, the propeller efficiency. This has a maximum at one speed that falls off toward low or high speeds. The maximum ηpropeller of an excellent propeller can be taken as ~85%, of an ultimate (but highly impractical) propeller as ~90%.
η = Pp / (Pp + frictional power loss Pf)
Pf = crossing speed (Vcr) * total frictional force (e.g. from chain links and pulleys). The virtual slip from chain slack excess is neglected.
(Rivers or tidal streams with current speed (Vcu) at right angles to Vcr)
The apparent speed (Va) meeting the ferry equals the square root of Vcu^2 + Vcr^2.
η = Pp at speed Vcr / Pp at speed Va
As Pp is about proportional to speed^3, η ≈ (Vcr / Va)^3 = (Vcr / SQRT(Vcr^2 + Vcu^2))^3