Cal:

You are going through exactly the analysis that I did before buying our new
trawler.

As you have noted there are true displacment trawlers- Krogens for example
and fast trawlers- Sabres, Mainships, Albins to name a few. The true
displacment trawlers have naturally aspirated diesels- the venerable Ford
Lehman, or lightly turbocharged, low rpm diesels- the John Deere in the
Krogen. The fast trawlers have somewhat larger engines, but mostly they are
heavily turbocharged with high boost pressures, common rail fuel injection,
etc which yields up to 70 hp per liter.

In my view if you don't go overboard, you can operate a fast trawler slow at
displacment speeds and enjoy the benefits of the fast engine when you need
it. Yes, the Krogen is a very efficient displacment hull and the John Deere
is a very efficient modern diesel. But a fast trawler such as my Mainship
34T with its 370 hp Yanmar can operate at displacement speeds just fine. I
run mine about half the time at displacement speeds and another half at
about 35 hp/liter which just gets me into planing territory at 12 kts.

The other consideration is running the high horsepower engine too slow and
at too light of a load. At some point- less than 10 hp per liter in my view,
a modern high output diesel will not reach operating temperatures. Then you
will get sooting, unburned fuel washing the cylinder walls, glazing, high
oil consumption, etc. But if you stay at 10 hp per liter or greater,
particularly if you run at fast cruising speeds for the last ten minutes of
the day, then you will be ok.

Some trawlers, the Sabres and Albins come to mind, put such large twin
engines that you can't really run them a displacment speeds and stay at the
10 hp per liter rule. These might work if you only ran one engine, however.

And back to your original question about engine efficiency. My Yanmar 370
peaks at about .05 gal/hp/hr at 2,800 rpm which is my fast cruising speed.
Slow it down to 1,700 rpm and it drops to .06 gal/hp/hr. I suspect there are
two reasons for this. First it takes a lot of parasitic horsepower to simply
turn over a big block and that parasitic horsepower is a bigger part of the
total at 1,700 rpm (60 hp) than at 2,800 rpm (200 hp). Secondly an injector
that can squirt enough fuel for 370 hp won't be very efficient at 60 hp. The
common rail piezo injectors have improved on this however.

An engine designed for displacment speeds only, say 3 liters and 100 hp
would be about 20% more efficient than the 370 hp Yanmar in my boat.

So, I probably answered more questions than you asked. Enjoy your search for
a new trawler.

David