The Governing Mechanism
A fixed-pitch propeller's power demand varies as approximately the cube of shaft RPM. At bollard (zero vessel speed), the propeller absorbs maximum torque for a given RPM because there is no advance speed angle of attack. The engine must supply exactly the power the propeller demands at whatever RPM the system reaches equilibrium.
The maximum static thrust occurs at the RPM where the engine's power output curve intersects the propeller's power demand curve. This intersection point depends critically on the shape of the engine power curve at sub-rated RPM not just the maximum rated power.
Why Two Engines with the Same Rating Can Produce Different Bollard Thrust
Two engines rated at identical maximum power can have very different power curves between idle and rated RPM. When the propeller's power demand curve intersects engine curve, produc higher thrust at the intersection point.
Using NavCad to Compare Engine Options
NavCad allows direct entry of manufacturer power curves for any engine model. To compare bollard thrust from different engine options:
- Create an IC Engine Object for each candidate engine, entering the RPM-power pairs from the manufacturer's power curve. Be sure to include points across the full RPM range, not just the rated point.
- For each engine, run a towing analysis at zero speed with the same propeller, gear ratio, and hull data.
- Compare the static thrust results, noting the equilibrium RPM for each engine.
- Also run free-running analysis to confirm that the change in engine has not affected design-speed performance.
Propeller Effects
The propeller's power demand curve shape at low RPM also influences bollard thrust. Propellers with lower torque absorption at bollard conditions (i.e., a flatter torque curve advance) allow the engine to reach a higher RPM at equilibrium, producing more thrust. This is a propeller design parameter that can be optimized independently of the engine selection. See the related article on bollard analysis setup.
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