Open-Water Performance

Slip is the most-read propeller number at sea and the least trustworthy. Take a Panamax bulker at her design point: the apparent slip a noon report would print for her is minus 2.8 per cent, which reads as a propeller advancing further in one revolution than its own pitch allows. The real slip at that same instant is 30.1 per cent. Nothing is broken and nothing has been mismeasured. The two figures are built on two different speeds, and only one of them is a statement about the propeller.

This course is about the chart that settles arguments of that kind: the open-water diagram, which describes a propeller alone, in uniform inflow, with no hull around it. What is inside: the four numbers the whole subject runs on, advance ratio, thrust coefficient, torque coefficient and open-water efficiency, worked end to end on one ship and carried through every lesson; the dimensional argument that licenses a single horizontal axis, together with the conditions under which that licence is withdrawn; how the curve is actually measured in a towing tank, what has to be subtracted before a coefficient exists, and what a basin report must state for its curves to be usable at all; the Reynolds-number gap between a model blade and a full-size one, the standard correction that closes it, and why, for a merchant propeller, the error runs in one direction; the constructions that let you enter the chart when the shaft speed or the diameter is the unknown; and the Wageningen B-series polynomials, what a systematic series freezes in exchange for its speed, and the point beyond which no series can answer the question you are asking.

The boundaries are deliberate. The blade drawing that supplies the geometry, its pitch distribution, its skew and its area ratio, belongs to the course before this one. The wake fraction, the thrust deduction and the propeller working in the sheared inflow behind a real stern belong to the course after it. Holding this middle ground is what makes the rest of the sequence possible, because a propeller has to be characterised on its own before anything defensible can be said about it behind a hull.

The opening lesson, on what the open-water condition strips away and why a propeller must be described alone first, is free to read as a sample. By the end you should be able to take an open-water curve from a tank or a propeller maker and say which scale and which Reynolds number it belongs to; enter it with your own ship's thrust, speed of advance and diameter to recover the rate of rotation, the torque and the efficiency that follow from them; and tell a supplier's efficiency claim that a series calculation can support from one it cannot.