Bookmark and Share Email this page Email Print this page Print

Prop-loading question

Dec 21, 2007

To the editor: In his recent article on getting better engine efficiency at low speed (“Improving low speed fuel efficiency,” Issue No. 165, October 2007) Nigel Calder says this regarding propeller/engine matching:
“Propellers are generally sized such that this match occurs at maximum engine speed, or something close to it. This is because a match at any lower speed would result in the engine being overloaded at higher speeds, and a match at some projected higher speed would result in the engine never being fully loaded.”
I get half of that — but I don’t get the half that says: “…a match at any lower speed would result in the engine being overloaded at higher speeds…”
It seems to me that a match at a lower speed would result in the engine never being able to run at a higher speed at all. Maybe this is because I don’t understand what it means to “overload” a diesel?
Can Calder clarify this?

- Martin Gardner lives in Venice, Calif., teaches sailing part time in Marina del Rey and is currently between boats, his last being a wooden, 25-foot Wittholz catboat.

Nigel Calder responds: In many circumstances (for example, doing a bollard pull, with the boat tied off at the dock) an oversized prop will simply not let the engine get up to full speed. Underway, the conditions are more dynamic and you will be able to get the engine up to higher speeds. None of the small marine diesels used in recreational boats, and also in a lot of small commercial craft, are rated at full continuous power, so this kind of use will shorten the engine’s life.

Edit Module