Originally Posted by mikereed100
Interesting. Having Kiwiprops I can
but hesitate as one often hears that you don't want to overload a diesel, specifically, that one should be able to reach max rated rpms at WOT.
Forever the prevailing wisdom has been the engine should reach redline RPM at full throttle.
This ensure three things.
1. Maximum power will always be available if you need it.
2. under normal conditions, it just won't be possible to overload or "lug" your engine.
3. No training or understanding or instrumentation required to operate the engine, any one can do it.
For most of us, the ideal situation is moderately overpropped, meaning most of us want enough load on the engine so that at half RPM she will hit full operating temps., but that also means you can't operate at full throttle for long as the engine will be overloaded, what that means is excessive fuel
See Diesels of course have no throttle valve, they pull in a full cylinder full of air at each revolution whether at idle or full bore (atmospheric engines).
What you do is add fuel, not air and fuel like a spark ignition engine, just fuel. Too much fuel and you quickly get to what is known as the smoke point, the issue with being there is exhaust
temperatures are excessive, aluminum
pistons will melt right at about 1400 F, but other damage is done as well before then from over temperature operation. Engine oil
and or water
may not overheat, but the temps in the combustion chamber are dangerously high
So if your moderately overpropped, you can motor
along happily at half RPM or less forever and life is good, you just have to accept that your 40 HP engine is now say a 25 HP engine, and never let yourself get into a position where 40 is needed, cause it's not there anymore.
Overpropped is especially useful when motorsailing as if your not overpropped on engine alone, your underpropped under engine and sail.
So, you want your cake and eat it too? That is a controllable pitch prop, like a car in overdrive as your cruising along on a long flat road, need power? Downshift for the acceleration, then back to overdrive to cruise
. Think of motorsailing as going down a long hill, a really high gear
would be useful then.
I believe though for most of us a true controllable pitch prop doesn't exist, the closest I know of is the Autoprop that will increase pitch until an engine is loaded even at low RPM and motorsailing, but you don't control it and like any automatic thing, it works, but if you could control it and knew what you were doing and had the instrumentation, you could do better manually.