Nomad Willy
Guru
Tom,
"overpropped means you achieve more than full RPM."
No. Under proping will allow higher than max rated rpm. And I think a tiny bit of it is ideal. then you can run almost at will right up to max rated rpm. My engine man says one can run WOT for one hour at a time. If I was under proped a bit I could run even longer without overloading the engine and conversley if I was over proped I could'nt run anywhere near max without being (or being in danger of) overloading. Like a truck going up hill in too high a gear, lugging the engine. The thing that wears out engines is not rpm, noise or the speed of the piston but the high temperatures and most important the forces generated. At mid-stroke the piston is pushing down w the force of an explosion and pushing the piston sideways (lots of force) because the crank pin is off to one side. An engine makes more noise at high rpm but it's much easier for the engine. Under proped and sing'in along at 80% of rpm is easy for the engine if proped right or a tad under proped. That's like downshifting a truck when you CAN make it over the hill in 9th gear but it's much easier on the engine to downshift to 8th gear. If you ALWAYS have a very light load you can over prop like a truck going up a slight grade. If you have a go fast go slow trawler you can over prop but the option of go fast will be lost.
"You cannot go wrong by propping by the book, especially with a new TierII JD. In general, the book says to select a prop size*to allow full RPM to be achieved. "
Absolutely right Tom.
"overpropped means you achieve more than full RPM."
No. Under proping will allow higher than max rated rpm. And I think a tiny bit of it is ideal. then you can run almost at will right up to max rated rpm. My engine man says one can run WOT for one hour at a time. If I was under proped a bit I could run even longer without overloading the engine and conversley if I was over proped I could'nt run anywhere near max without being (or being in danger of) overloading. Like a truck going up hill in too high a gear, lugging the engine. The thing that wears out engines is not rpm, noise or the speed of the piston but the high temperatures and most important the forces generated. At mid-stroke the piston is pushing down w the force of an explosion and pushing the piston sideways (lots of force) because the crank pin is off to one side. An engine makes more noise at high rpm but it's much easier for the engine. Under proped and sing'in along at 80% of rpm is easy for the engine if proped right or a tad under proped. That's like downshifting a truck when you CAN make it over the hill in 9th gear but it's much easier on the engine to downshift to 8th gear. If you ALWAYS have a very light load you can over prop like a truck going up a slight grade. If you have a go fast go slow trawler you can over prop but the option of go fast will be lost.
"You cannot go wrong by propping by the book, especially with a new TierII JD. In general, the book says to select a prop size*to allow full RPM to be achieved. "
Absolutely right Tom.