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Old 07-23-2013, 09:13 PM   #28
Nomad Willy
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City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 14,160
I said;

"If one props the engine for rated rpm at WOT no exhaust gas gauge will be needed. You won't be able to overload your engine except at specified engine speeds (almost at WOT) and for specified periods of time."

FF replies;
"IF the engine is being operated at max power (the reason you chose to prop it at max rpm) each and every time a displacement boat is stopped or really slowed by a big wave the engine will be in overload."

The reason to prop at wot at the rated rpm has nothing to do w the ability to run the boat at max speed or max engine output. Nothing to do w it at allll. It's a proper adjustment of the load the engine sees that matches the power output to the resistance of the propeller so the engine can be operated throughout it's range of power output and engine speeds so that all is balanced and working together.

If one is going to operate in very large seas at wot for any more than a few moments he's probably an absolute fool. A good skipper will quickly adjust throttle to match conditions. But an over propped boat will be over propped during much of it's upper throttle range and lack considerable power to climb the big seas. Under propping and a speed limiter set only 100rpm over rated rpm would probably be best for that condition.

The reason one props an engine at rated rpm is basically just a test to know the system is operating correctly. Then one can go to the book and see (for example) that one can operate said engine continuously at 150rpm below rated speed and rated speed is 2500 then the engine can be operated at 2350rpm all day long.

If you over prop an engine you may only be able to apply or generate 90hp w a 120hp engine and the engine may still be over loaded at 2000rpm. You may as well through away your 120hp engine and get a 60hp engine if 60hp is all you need.

But most here HAVE the 120hp engine that is putting out 60hp and about 5% more efficiently than it would if it were propped correctly so why do it right? If you NEVER use more than 60hp running it like that MAY not have a significant downside as long as you KNOW your'e not injecting more than the optimal amount of fuel. In other words not running too rich. If you absolutely know that and NEVER run harder than a 60hp load you probably can over prop for a long time but that's just my opinion and you'll need to live w your own opinion.

Motion 30,
Of course. You've opened the door for what the engine has to offer and you'll probably never notice the slight extra fuel you burn and the slightly higher burn rate will only be noticed at very low rpm. At higher speeds you will probably burn less not more. All around a better way to go.
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Eric

North Western Washington State USA
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