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Old 01-25-2013, 08:15 PM   #24
Marin
Scraping Paint
 
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Join Date: Oct 2007
Posts: 13,745
My guess is that the fuel savings from the reduced load on an engine due to some pitch being taken out of the prop(s) is negligible, as you say. Depending on the kind of engine the reduced load might be beneficial to its service life. Or not.

The biggest benefit in your case sounds like it would be the reduction in idle speed.

Grand Banks boats were all over-pitched from the factory, at least in the first few decades. This made all kinds of sense given the power of the engines and the relatively low cruising speeds people went with them. Like driving in a bit too high of a gear, over-pitching can give you more speed for a given rpm, so as long as an engine is not overworked in this process, you get a more economical boat at cruise.

On the recommendation of the prop shop we had our four bladed props (not original to the boat) pitched down an inch not to lower our idle speed but because the shop said that as a general rule, for the same diameter prop on a given boat, a four-bladed prop should have one inch less pitch than a three-bladed prop to get the same performance and economy. The four-bladed props on our boat were set--- badly--- to the same two pitches the original three-bladed props had been.
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