Nomad Willy
Guru
Mark is there a 50hp engine in that box astern?
...Then when you see that lots of trawlers w two engines were offered w one it gets confusing. You can bet that most trawlers w twins are overpowered and most other trawlers are overpowered as well.
Mark is there a 50hp engine in that box astern?
"They do provide a prop demand fuel
curve but I haven't figured how to convert that to an efficiency curve."
That is because you cant.
ONLY a BMEP or fuel map will allow one to chose and match efficient engine speeds with desired rational cruise speeds.
"With a hull speed versus rpm chart (owner generated) one can then get a pretty good idea of what's going on in terms of efficiency for that hull/engine combination.
Not really , it is fine (if its accurate ) to know what the burn might be at some chosen loading, but weather the the egine is at high or low efficiency (fewest GPH/HP is unknowable. ON PURPOSE.
Those aren't "trawlers" (capable of exceeding hull speed) in the Chapman sense. They're fast cruisers.
Mark,
What does Chapman say about trawlers and cruisers?
Ski,
I don't have that on paper but it's probably on the Mitsubishi web site that I have bookmarked on my crashed i-mac. Miss that thing.
OK so this sweet spot is a junction where power and torque curves meet. Kind of a best of both worlds thing? Looks like Mark's running his Deere at a little lower load than me. It would be interesting to see all our boats cruise rpm as a percentage of max power rpm. It would not however indicate a percentage of engine load. I often have viewed it as X # of rpm down from max rated. But that's not apples and apples unless all engines considered have the same max power rpm.
Have to say it again . The toque peak is at max load, far different from the prop load. Boats never use peak torque except perhaps when accelerating.
The makers say nothing about how torque varies along the prop curve.
Ski,
"Cruise at max torque" ...
Is that for max efficiency or is there more to it?
I've heard that many times so there must be something to it. However I would think loading would be more important than small differences in efficiency. Most of the engines we're running were designed for duties other than powering boats. As I recall someone said a long time ago that most gen sets run at 1800 and that's why so many have max torque at 1800. What do you know about that? I think my engine is in that catergory but I run it at 2300. My 2300 is a result of choosing the right amount of power, propping the engine for rated rpm (3000) and then driving the hull at about one knot below hull speed. Fuel burn wasn't considered important enough to put on the chart. But my fuel burn isn't important enough to consider 10% this way or that. To me that seems more like a Passagemaker issue.