I recently contended with a few of my colleagues here at Power & Motoryacht that most of the propellers we deal with in boat-test reports are of the fixed-pitch variety and therefore, to mention fixed pitch as a qualifier in a list of specifications constitutes an exercise in basest redundancy.
Shoot—I was wrong! While my take on fixed-pitch props may be ever-so-slightly the case, I was reminded of another, dissembling type of prop by reading Dick Snyder’s Everything You Need To Know About Propellers, a large and excellent paperback book that, according to both the folks at Brunswick Corporation (which published the book originally) and Amazon (www.amazon.com) is now deplorably out of print. Anyway, according to Snyder, despite its resemblance to a fixed-pitch prop, the dissembling type has what’s called progressive pitch (sometimes called “camber”), meaning the pitch starts low at the leading edge of a given blade and subtly increases to the trailing edge. Pitch numbers for such props are assigned using an average between low and high readings.
The type of pitch we sometimes deal with here at the magazine—and indeed the one I was thinking of when I started my harangue the other day—is called constant, true, or flat pitch, and I think you might also reasonably call it fixed pitch. Props of this sort have the same pitch wherever a measurement is made on the blade. While this type is quite common (especially in comparatively slow-speed applications), it is, to my undying chagrin, far from universal.
Controllable- or variable-pitch propellers are a different animal altogether, by the way. Airplanes and ships have them, tractor tugs typically have them as well, and some large yachts have them too.
This article originally appeared in the February 2013 issue of Power & Motoryacht magazine.