Propeller Geometry & Blade Definition

A drawing gives the pitch ratio of a 7.00 m propeller as 0.715 at seven tenths of the radius, which makes the pitch 5.01 m. In dock, a superintendent lays a pitchometer on the flat of the blade face at the same radius and reads 4.85 m. Nothing is bent, nothing has been repaired, and neither party has made an arithmetic mistake. The two figures can sit three per cent apart for no other reason than that they are two different conventions applied to the same piece of bronze, and three per cent of pitch is worth a great deal more than three per cent once a slip baseline, a light-running check or a repitching decision has been built on top of the wrong one.

Almost every quantity on a propeller plan behaves like that. Blade area comes in three versions and they differ by around ten per cent on an ordinary merchant blade. Rake as measured on the casting is routinely larger than the rake anybody specified. A pitch, a chord and a thickness are not single numbers at all but radial distributions, quoted at one reference station by convention and misread as a whole propeller by anyone who does not know that. None of this is obscure and none of it is hard, but it is nowhere written down in one place, and the cost of not knowing it lands on the people who inspect, repair, repitch and re-baseline propellers rather than on the people who draw them.

This course is the missing page. It covers what the three views of a plan are and which quantity each one alone can carry; pitch as a surface, an angle and a ratio, and how to establish which of the competing conventions a quoted number belongs to; the blade section as a hydrofoil, with the mean line and thickness form chosen for the suction peak rather than for lift; rake and skew as the two projections of one blade reference line, including the rake that skew induces whether or not it was ever specified; the expanded, developed and projected areas and the cavitation-against-efficiency conflict that the quoted area ratio settles; blade number and hub ratio read as an excitation-frequency decision; and the geometries that give up part of a fixed blade's definition to buy something else, from controllable pitch and ducts through tip treatments to contra-rotation. One ship carries nearly every worked example, a Panamax bulker of about 75,000 tonnes deadweight with a four-bladed 7.00 m propeller, and the same ship runs on through the rest of the propeller and propulsion sequence.

The opening lesson, The Propeller Drawing: Three Views of One Surface, is free to read as a sample. By the end you should be able to pick up an unfamiliar propeller plan and say which view owns which quantity, pin a quoted pitch to its convention before using it, separate specified rake from skew-induced rake, convert between the three blade areas, and read a blade area ratio, a blade number and a hub ratio as decisions with named costs rather than as data. What the course deliberately does not do is size a propeller for you: choosing the diameter, the area ratio and the blade number for a given ship is the work of the courses that follow, and this one exists so that the numbers they produce mean something definite when they arrive.