Twice the speed, Ten times the fuel
 

Just reviewing my notes from yesterday's sea trial of a 37-foot
semi-displacement trawler.

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.

Twice the speed, Ten times the fuel
 

I wasn't recommending running anything at WOT. Merely noting that doing
so produced twice the speed of the 1400 rpm operation while burning 10
times the fuel.

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%

Twice the speed, Ten times the fuel
 

"Chuck Gould" wrote in
ups.com:

Traditonal trawler fans will recognize the performance figures at 1400
RPM, 8.2 kt at 2.4gph.


http://www.perkins-sabre.com/PR/Engi.../Equipment.cfm
8 knots on half that fuel. The Perkins will be running after he's dead.

Twice the speed, Ten times the fuel
 

Larry wrote:


http://www.perkins-sabre.com/PR/Engi.../Equipment.cfm
8 knots on half that fuel. The Perkins will be running after he's dead.

I own a late model Perkins engine. 6354 NA, 130 HP. Like it a lot.

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.

Twice the speed, Ten times the fuel
 

On Tue, 31 Oct 2006 17:00:26 -0500, Harry Krause
wrote:



425 HP Cummins QSB engine.


On the other end of the performance spectrum, WOT is 3000 RPM where the
boat makes 16.5 kt at 22.4 gph.


Who the hell runs a diesel at WOT? I'll bet at a more reasonable engine
speed, say about 2400 rpm, that 22.4 gph drops to about 10 or 11 gph.



2400 = 80% of WOT = 64% of max hp = 272 = approx 15 gph.

Twice the speed, Ten times the fuel
 

Wayne.B wrote:
On Tue, 31 Oct 2006 17:00:26 -0500, Harry Krause
wrote:



425 HP Cummins QSB engine.


On the other end of the performance spectrum, WOT is 3000 RPM where the
boat makes 16.5 kt at 22.4 gph.


Who the hell runs a diesel at WOT? I'll bet at a more reasonable engine
speed, say about 2400 rpm, that 22.4 gph drops to about 10 or 11 gph.



2400 = 80% of WOT = 64% of max hp = 272 = approx 15 gph.

For some reason or another, the reading we got on the engine diagnostic
screen was 12.2 gph at 2400.

Twice the speed, Ten times the fuel
 

On 31 Oct 2006 19:15:59 -0800, "Chuck Gould"
wrote:

2400 = 80% of WOT = 64% of max hp = 272 = approx 15 gph.

For some reason or another, the reading we got on the engine diagnostic
screen was 12.2 gph at 2400.

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.