Twice the speed, Ten times the fuel

425 HP Cummins QSB engine.

That's not a bad engine choice for a single screw boat- it can be operated at a low rpm without loading up (thanks to computerized fuel control), and it can ber pushed pretty hard to get more speed than one might otherwise expect from a boat in tht category.

Fuel consumption figures are taken from the engine-diagnostics screen installed with the Cummins QSB.

Traditonal trawler fans will recognize the performance figures at 1400 RPM, 8.2 kt at 2.4gph. On the other end of the performance spectrum, WOT is 3000 RPM where the boat makes 16.5 kt at 22.4 gph.

Makes a dramatic statement about fuel economy: Doubling "displacement" speed in this boat uses almost exactly ten times as much fuel.

And that is probably correct. The most likely reason for the difference is in the props but who knows. I'm using a rule of thumb approximation that assumes the engine is putting out peak horsepower at WOT.

BTW, 2400 RPM provides an extra 4.3 kt above the 1400 RPM operation. We made 12.5 kt at that setting. Those 4 knots are also pretty expensive; as the approximately 50% increase in speed results from close to 5 times the fuel consumption.

1400 RPM, 8.2 kt, 2.4 gph

2400 RPM. 12.5 kt, 12.2 gph

A lot of the manufacturers recommend cruising their latest engines at 200 below WOT, which is significantly faster than the old rule of thumb at 80%

I would doubt that the 4 banger you link to (86 HP at 2400 RPM) would be an ideal choice for a 24,500 pound boat that one wanted to cruise at 8 kts. Nor am I inclined to believe that wound up to 2400 RPM (where you *might* have a chance to get 8 kts out of it. maybe.) fuel burn would be down to 1.2 GPH, which is what it would need to be to come in at half the fuel burn of the QSB I referred to. And lastly, pushing that 4 cylinder with its shorter crankshaft and less torque hard enough to do the job would shorten its life expectancy considerably.

My last Perkins racked up almost 4000 hours before it croaked- and even then it was a manifold failure and not really anything that Perkins did when building the engine.

It's easy to agree that Perkins builds a very good engine. Older technology on most of the models- which was one of the reasons I stuck with Perkins when I had to repower.

There are some Krogens and Norhavns out there with very small single engine mains, probably for maximum possible range offshore, but slowed to 6 knots almost any engine is going to get pretty impressive fuel economy.