Quote:
Originally Posted by lancelot9898
From one of my Perkins manuals it states that the max rating of 4000 rpm should not be run for longer than 45 minutes of every hour with the rpm reduced by 20% during the 15 minutes before returning to max rating. However it goes on to state that if the wide open throttle setting is less than the max rating then it too should be backed off by 20% every 15 minutes on the hour. In other words the WOT setting should be treated as the max rating.
I've always heard the term "over proped" and understand that to get the hp rating of the engine that certain rpms should be obtained. However I do not understand if I'm operating the engine at a lesser rpm that I'm over loading it even when I'm not able to achieve the desired rpm rating. Is there some physical sign(temp rise, smoke from exhaust, strange sounds) to show that the engine is being overloaded??
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If you can not achieve the max rated RPM of the engine the engine is over propped. I don't know how to be any clearer.
I will use my own Westerbeke a an example as Westerbeke is very clear in their language about prop sizing. I was 350 RPM off this spring with a new prop and was told in no-uncertain terms by both the distributor, Hansen Marine, and Westerbeke them selves in Taunton, MA that I should most definitely have my prop re-pitched with no if's and's or but's about it.
Here's the basic gist. The MAX rated RPM on my four cylinder 44B Four is 3000. Here is exactly what Westerbeke says, in writing.
"Westerbeke recommends a propellor that will allow the engine to turn 3000 RPM under way at full throttle."
Pretty darn clear!!!
My MAX RPM is 3000 and my engine should be able to hit 3000 at WOT. This does not mean that this is where you run the engine but if it can turn max RPM then everything else falls in line behind it and you won't be loading the engine. I cruise at 75-80% of WOT and the 350 RPM difference made a large difference in the tone of the engine. Because I was 350 RPM off, Westerbeke recommended that I also temporarily back my normal cruise RPM off by 350 RPM to "try" and compensate for the over propping and to NOT overload my engine.
By backing my RPM off by 350 it put me at an RPM outside of the recommended cruise RPM range for the engine and one too low for long term use. This was according to the Westerbeke engineers in Taunton that I spoke with. Fortunately the prop maker had me a new prop in about two weeks time and I only had to put about three or four hours on the engine this way..
Many good surveyors also check to make sure an engine can hit MAX rated RPM during sea trials.