The Propeller Behind the Hull

At 14.0 knots our reference Panamax bulker, of about 75,000 tonnes deadweight, is towed against 577 kN of resistance. (She is a teaching hull with round numbers, and that figure is light: a real Panamax at that speed needs nearer 680 kN.) Fit her propeller and it has to produce 704 kN. The extra 127 kN is not a measurement error, and it is not lost in the shaft: every newton of it reaches the thrust block. Misread that one number by a hundredth and the delivered power you predict moves by more than a per cent, which is a real sum over a year of steaming. It is one of three interaction coefficients that nobody on board will ever measure, and that every in-service power figure quietly assumes.

This course is about those coefficients and the machinery of belief behind them. It works through the wake field over the propeller disc and the three flows that build it; the difference between the wake a towed model measures and the wake the working propeller obeys, and the two conventions for writing either down; thrust deduction, and why a gain in it is so often written up as a gain in the wake; hull efficiency, which routinely exceeds 1 and is routinely misread as free energy, together with relative rotative efficiency, which absorbs whatever the books will not balance; the full chain from brake power to towrope power, with the honest boundary between what a ship measures and what it infers; the self-propulsion test as it is actually run, including the towing force the carriage keeps applying while the model's own propeller turns; the ITTC 1978 method as a sequence of named, arguable decisions; and what fouling, draught, trim and a seaway do to the tank's answer once the ship is trading.

One ship carries every worked example, so the numbers accumulate rather than restart: the same wake fraction, thrust and torque follow you from the wake survey through the model test to the power prediction, and you can check each step against the last. The opening lesson, "The Wake Field: What the Propeller Actually Swims In", is free to read as a sample.

The boundaries are deliberate. Blade maintenance, hub oil and weardown belong to our course Shaftline, Stern Tube and Propulsor (available shortly). The engine as a commercial choice belongs to Marine Engineering & Main Engines. Turning these coefficients into a propeller and an engine that can live together is the business of the course that follows this one, Propeller Selection & Engine Matching.

By the end you should be able to open a model-basin report, say which identity method produced the wake fraction and whether the figure printed is the model value or the scaled one, rebuild the ITTC 1978 wake scaling yourself and see what it cost, reproduce the propulsive chain from the coefficients quoted, and tell a vendor's real hydrodynamic gain from a number that has simply moved between two columns.