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Read a propulsion specification as a complete installation, not a single power number. Engine ratings, gearing, propellers and cooling describe different parts of the system.
Engine brands & buying guides →MarineBrands editorial definitions · Reviewed 5 October 2026 · Examples explain context, not operating procedures.
12 of 12 terms
Formation and collapse of vapour cavities where local liquid pressure falls sufficiently low.
Air entering a propeller is ventilation, a different mechanism that may produce similar symptoms.
Also find under: reduction ratio
The specified relationship between input and output rotational speeds in a transmission.
Check the maker’s ratio convention when matching an engine, gear and propeller.
Also find under: inboard engine
An engine installed within the vessel, with power transmitted to a propulsion arrangement.
The word identifies engine location, not whether the boat has a shaft, saildrive or another drive.
Also find under: outboard motor
A propulsion unit mounted externally that commonly combines engine, transmission and lower drive assembly.
Check the boat’s approved installation and the complete model specification, not only advertised power.
Also find under: kW horsepower hp
The rate of doing work; engine output is commonly stated in kilowatts or a specified horsepower unit.
Confirm the unit and rating standard before comparing two advertised engine numbers.
Also find under: pitch
The theoretical forward distance a propeller would advance in one revolution without slip.
Pitch is a geometric specification, not a promise of the boat’s distance travelled per revolution.
Also find under: slip
The difference between theoretical pitch-based advance and actual advance, expressed under a chosen calculation.
Slip is not the same phenomenon as cavitation; avoid diagnosing one solely from the other’s name.
Also find under: revolutions per minute
Revolutions per minute, a measure of rotational speed.
Engine rpm and propeller rpm differ where a reduction gear is used.
A propulsion arrangement with a drive leg passing through the hull, commonly coupled to an inboard engine.
Its seals, installation and servicing are distinct from a conventional shaft arrangement.
An arrangement transmitting engine power through a shaft to a propeller outside the hull.
A repower assessment includes alignment, supports and the existing drive arrangement, not just the engine footprint.
Also find under: Nm
Turning effect of a force, commonly expressed in newton metres for engine output.
Read torque together with engine speed and the rating basis rather than comparing peak figures alone.
Also find under: propeller ventilation
In propulsion, air or exhaust gas entering the water flow at the propeller.
Describe when the symptom occurs and have the installation assessed instead of assuming cavitation.
Original concise explanations for equipment research. Use these specialist resources for the wider technical context and the relevant current documentation for a particular product or activity.
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