What does "thermal efficiency" refer to in marine engines?

Thermal efficiency in marine engines refers to how effectively an engine converts the energy stored in fuel into useful mechanical work. Specifically, it is the ratio of the useful work output of the engine to the energy input from the fuel being consumed. Higher thermal efficiency indicates that a greater portion of the energy from the fuel is being converted into work, which is particularly important in marine applications for both performance and fuel economy.

The other options focus on unrelated aspects. The total weight of fuel doesn't reflect how efficiently an engine uses that fuel. Similarly, the efficiency of the ship's heating system pertains to thermal management rather than the engine's energy conversion capabilities. Lastly, a vessel's speed through water is a performance metric, not a measure of energy efficiency. Thus, the definition pertaining to the ratio of useful work to energy input is the most accurate and relevant description of thermal efficiency in this context.