Engines are heat sinks.
The more (cyl wall/combustion chamber)area the more heat is lost and the less efficiency.
Brooksie’s post #6 nails it IMO. But there are variables like electronic control and turbochargers. But if you take two engines where the only variable is the engine size the small engine wins every time from what I can see.
But I think one can come very close to the truth using the specific consumption of a specific engine and applying it to a resistance curve of one’s boat. And comparing big engines to small engines the load will obviously be very different.
This question is very simple for FD boats but gets considerably more complicated for SD unless you choose the power for a specific speed of a SD boat. But most SD boat operators (here anyway) seem to want high speed capability. But that adds thousands of pounds of weight to the boat that you’re dragging alond almost all the time. It’s a choice.
Sometimes I go to extremes in comparing to answer questions like this. I may ask what if I put a Perkins 6-356 in my little Willard? Would I gain efficiency and burn less fuel? How-bout a 500 ci in Cat? I dont think so. But if I put a 100hp turbo Yanmar maybe so. But that’s a higher tech different type of engine. Apples to apples the little engine wins.
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Eric
North Western Washington State USA
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