There is a small case to be made for over pitching older normally aspirated engines like the Perkins or Lehmans to gain a bit (and it is a small bit- 10%) of fuel economy and reduce engine noise due to lower rpms while cruising at or below displacement speeds.
But I know your engine- the Yanmar 370 I suspect, as I have had two- one on a MS 34T and the other on my current MS P34. If you are cruising at 13 kts then you are running at about 3,000 rpm and hopefully you are currently pitched a little over rated rpm to about 3,400 rpm.
If you were to over pitch even as little as 200 rpm, ie your wot would now be 3,200 rpm, you would be over loading your engine while cruising at 13 kts and maybe 2,900 rpm. And if you expected to get another half knot or so by running at 3,000 rpm you would really be overloading it.
But if all you ever want to do is to run at 8 kts, then like the NA engines above, you could over pitch by several hundred rpms and gain a bit fuel economy and less noise. But don't ever think about running at 13 kts again.
You may have the Cummins 370 engine, in which case the numbers change by several hundred but the conclusion is the same.
David
David,
Just to clarify the definitions: (see the diagram)
I believe "over pitched" takes more of a bite of water for every revolution
And an "under pitched" takes a smaller bite
(correct me if I'm wrong)
[/IMG]
The over pitched prop will run at a lower RPM, give more speed for a given RPM but have less torque.
The under pitched prop will run at a higher RPH, with less speed, and will have more torque.
I know there are applications where it pays to favor one or the other. Like a tug boat, or a tow boat that really needs the torque to push would favor an under pitched prop.
And a boat that's going a long distance and a fairly constant cruise speed might favor a over pitched prop. (like our Trawlers).
There are also variable pitched props, that can be changed as needed. Not sure if there are cost effective for our trawlers, however.
Now, I understand that we still have to operate within the limits of the engine, and don't want to overload the engine.
Overloading might occur if one has a low pitched prop (over pitched) and tried to accelerate with full power to get away from something quick. Similar to putting your car in 3rd gear and trying to rapidly accelerate away from the stop light. It can be done, but at less power and takes longer to get up to speed.
Underloading might occur if one has an under pitched prop and operating at a low but constant speed at really low power. I don't believe it's that harmful as overloading and can work out fine if we occasionally "race" the engine as prescribed in the engine operations manual.
So, if my goal with to increase speed at a lower fuel consumption, I'd increase the pitch of the prop (overloading) and instead of operating at 3000 RPM for a certain speed, I'd be operating at 2800 RPM.
Does this make sense?
So, I think we can agree that there could be some variables out there, perhaps not enough to matter. But I'm curious if folks have done this, and to what extent and the results.