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D
eep in the bilge of the boat you’re designing, building,
surveying or repairing is her beating heart—her engine.
The recipient of endless tuning, cleaning, and fuss, it’s the
boat’s engine that drives her from anchorage to anchorage.
Engines, however, come in a wide array of sizes, shapes,
and flavors. Whether you’re repowering, determining which
propulsion-package option to install in a new boat, trying to
optimize performance on an
existing boat, or to under-
stand why an engine isn’t
achieving full rated RPM,
good information on engine
behavior can seem hard to
come by. The key to decipher-
ing engine performance is the
performance curves that are
included with the engine
manufacturer’s literature.
We’ll examine these curves
here.
Let’s take a look at a fairly
typical high-output diesel, the
Yanmar 6CX(M)-ETE, and see
what her curves will tell us. A
pair of these might drive a 35-
foot sportfisherman; or a sin-
gle engine could propel a 65-
foot motorcruiser. The info’
sheets for this engine are
handy because they happen
to have all the usual curves.
Some manufacturers don’t
include the torque curve or,
sometimes, the fuel-
consumption curve. At any
rate, there are five standard
performance curves:
1) Maximum output
power without reduction gear
2) Maximum output power after marine reduction gear
3) Propeller power curve
4) Torque curve
5) Specific fuel consumption
Between them, pretty much everything you need to know
about this engine’s performance is spelled out.
Maximum Output Power - BHP
The maximum output power curve is just what it says. It
shows the maximum power that the engine can produce
ideal conditions) at any given RPM. This is also called “br
horsepower” or BHP because in the old days it was meas-
ured on a gizmo termed a “Prony brake”— a form of dyna
mometer. These days other types of dynos are used, but t
result is the same. Note t
the brake horsepower is
maximum in every regard
tested on a bench in a sho
and before the reduction
gear. Real power in the ho
humid bilge of a boat may
somewhat lower. For our
420-HP CX Yanmar, the
maximum rated BHP is 4
HP, at 2,700 RPM. The un
for power on the graph ar
as you’d expect—horsepo
on the right and the metri
equivalent on the left, kilo
watts.
Output Power With Marin
Gear - SHP
Of course, almost all engi
engine have a reverse/
reduction gear mounted o
their tails. The reduction g
not only allows the boat t
back up (which I’m told is
useful), but it’s what allow
you, the designer, to matc
the torque characteristics
the engine to the optimu
propeller. All this is both
proper and also unavoida
Like all other machines, h
ever (and the reduction ge
is nothing more than a machine with lots of moving parts
reduction gears have built in power losses due to friction.
Standard marine gears fritter away about 3 percent of
power as they do their required job. This means that the
maximum “output power with marine gear” or the SHP is
reduced to 407.4 HP [420 HP x 0.97 = 407.4 HP.] This is
sometimes called “shaft horsepower” or SHP. It is what t
curve just below the maximum output power curve is sho
Understanding Engine Performance Curve
By Dave Gerr, © Dave Gerr 2012
Yanmar 6CX(M)-ETE Performance Curves