Propeller Selection & Engine Matching

A Panamax bulker leaves the yard delivering 6,320 kW to her propeller at 84.0 rpm. A year later her records average 6,900 kW at 82.6 rpm: more power and fewer revolutions at the same time. That is not two faults but one. At the revolutions she is turning, her reference propeller curve asks for 891 kW less than she is burning, and if we assume 250 sea days a year at 175 g/kWh and 550 US dollars a tonne that gap alone is worth about 515,000 dollars a year. It is the floor of the bill, not the bill: at those revolutions a clean ship would also be going faster, and compared at the same speed through the water the same condition costs her more, several times as much on the rule of thumb in the propulsion guide of MAN Energy Solutions (now Everllence). Nothing was installed wrongly. The propeller and the engine were matched on paper before the ship existed, and what the records are reporting is that match ageing.

Matching is where the hydrodynamics stops being an academic exercise and turns into an order for a casting and an order for an engine. A common error in a propulsion contract is to treat the gap between the power the propeller wants and the rating the engine is bought at as a single lump of safety factor and then argue about its size. On the ship above that gap is 1,756 kW, 27.8 per cent, and it is three separate decisions taken for three different reasons and expressed in two different units. Set them too tight and the engine works against its own torque limit in the first head sea of the winter; set them too loose and the ship carries an engine, a shaft and a fuel bill she never needed.

The course carries one vessel, a Panamax bulk carrier of about 75,000 tonnes deadweight, from an empty design space through to an engine order and then out into service. Inside it: why the design problem has a family of answers rather than one, and what has to be frozen before it has a single answer; the choice of diameter and shaft speed, which is where most of the achievable efficiency is decided; the blade area a propeller needs before cavitation decides for it, and the classical screening check together with an honest account of what that check cannot see; the engine's layout diagram and what derating actually buys; the load diagram drawn to real constants, with each limit line explained by what physically fails if you cross it; the sea, engine and light running margins, applied one at a time in their own units; and the interventions that move an operating point back where you want it. Two Excel workbooks ship with the course: engine-matching.xlsx, which takes a propeller design point and three margins to a specified MCR, computes the corner points of the load diagram around it and tests an operating point against them, and light-and-heavy-running.xlsx, which turns a measured power and speed pair into a light running percentage and the extra fuel at the revolutions measured.

The opening lesson, The Two Propulsion Problems: Direct and Inverse, is free to read as a sample. By the end you should be able to review a propeller proposal and say whether its diameter, its revolutions and its area are defensible, place a specified MCR inside a layout diagram and justify the margin stack that produced it, plot a measured operating point on a load diagram and state its margin to the torque limit, and read a year of a ship's own power and rpm records as a statement about how her match is ageing.