Re: Diesel Fuel Consumption vs. Horsepower
A few days ago there was a thread about the relationship between diesel
fuel consumption in gallons per hour (gph) vs. horsepower (hp) produced by
the engine. Paul Kruse and others mentioned a figure of 20 hp for each
1 gph.
My boat is a 32' Grand Banks which, courtesy of the previous owner, was
repowered about 800 hours ago with a 135 hp Ford diesel (Dover series),
marinized by Northeast. In the owners manual for the engine is a graph
showing Brake Horsepower vs. Engine RPM. At 1500 rpm (roughly 7 knots, my
normal cruising speed) it shows an output of about 78 hp.
Applying the above numbers, my fuel consumption should be 78/20 = 3.9 gph.
In practice it is much lower, more like 1.35 gph (computed by dividing
gallons used by engine hours for the interval between fill ups). Something
does not compute.
Working in reverse, my 1.35 gph figure would indicate I am only using
20*1.35 = 27 hp, which is a lot different than the 78 hp figure shown in
the owner's manual for 1500 rpm.
Can someone explain this apparent discrepancy?
Chuck Taylor
SEASCAPE GB 32-2
Everett, WA, USA
The numbers that have been circulated are just "rules-of-thumb" to get into
the ball park. The real estimate of fuel is best obtained from a curve that
the engine manufacturer puts out. It is called, "The Specific Fuel Curve"
or something close. It shows the fuel burned in pounds of fuel per hour at
maximum load for each engine RPM. The curve is never flat, but some modern
engines have one that is flatter than in the past.
It is important to note that this curve is for the maximum horse power the
engine can generate at the RPM in question. We will seldom run our engines
at that load, and we really don't have a curve to tell us what our engine
burns at a given RPM at reduced load. Good yacht manufacturers will provide
you with a speed/power curve obtained in a sea trial. It will allow you to
get a real indication of what fuel your yacht will burn.
The curve that came with my KK-42 showed better than 20 hp per gallon at
cruising RPM, but worse than that at top RPM. In the finally analysis, it
is best to construct your own curve from the way you will load and run your
own yacht.
Working in reverse, my 1.35 gph figure would indicate I am only using
20*1.35 = 27 hp, which is a lot different than the 78 hp figure shown in
the owner's manual for 1500 rpm.
That is not surprising at all. It's better to find out what you did than
the other way around. Loads on Trawlers are generally inflated.
CaptnWil, Ret
40 Pier Pointe
New Bern NC 28562
(252) 636-3601
captnwil@coastalnet.com
It sounds like the graph showing hp vs rpm was for hp at full load. On an
engine with a propeller, the engine is under full load only at peak rpm. At
lower rpm the engine is not under full load and hence will draw less than
the full load graph. Some graphs include a so-called "propeller load" line
which is under the full load line at all points except maximum rpm.
Hal
-----Original Message-----
From: owner-trawler-world-list@samurai.com
[mailto:owner-trawler-world-list@samurai.com]On Behalf Of Chuck Taylor
Sent: Sunday, July 25, 1999 9:11 PM
To: trawler-world-list@samurai.com
Subject: Re: Diesel Fuel Consumption vs. Horsepower
In the owners manual for the engine is a graph
showing Brake Horsepower vs. Engine RPM. At 1500 rpm (roughly
7 knots, my
normal cruising speed) it shows an output of about 78 hp.
Applying the above numbers, my fuel consumption should be
78/20 = 3.9 gph.
In practice it is much lower, more like 1.35 gph