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ing—the maximum power at each RPM, minus 3 percent.
In reality, this isn’t true shaft horsepower. There is still an-
other source of loss due to friction—the shaft bearings. Gen-
erally, you lose about 0.5 percent of power for each bearing,
so—with one or two shaft bearings—the true power the pro-
peller sees is about 96 percent of brake horsepower or 403
HP for this engine. (If there’s a remote V-drive, subtract an-
other 2 percent of power for it.)
Propeller Power Curve
So far so good, but the propeller power curve makes things
more entertaining. What is it showing? Well, remember that
the BHP curve is generated by testing the engine in a lab to
get the maximum power that the engine can deliver at each
RPM. The word “can” is crucial. The fact is that the power
the boat’s propeller demands or absorbs increases or
changes at a very different rate relative to RPMs than does
the output power
that your engine
can deliver. What
the propeller power
curve shows is
power that a stan-
dard propeller
would be using at
any given engine
RPM. You can see
just how different
the shape between
the two curves is—
between the BHP
(or SHP) curve and
the propeller curve.
This is unfortunate
in a way as it con-
trols a lot of things
about propeller se-
lection. If you in-
stalled a propeller
that was too large
in diameter or that had too much pitch, then the propeller
curve (Propeller Power Curve A) would be shorter and
steeper and would intersect the engine power curve at some
point less than maximum 2,700 RPM, perhaps at 2,100
RPM. In this case, the propeller would be overloading the
engine and lugging it down. This would limit speed, and
would be bad for the engine. In fact, most engine warranties
require that the propeller allow the engine to spin up to
maximum RPMs or nearly so, otherwise any engine damage
is likely to be blamed on overloading the engine and the
warranty considered void.
Conversely, if the propeller had too little diameter or pitch
the propeller power curve would flatten and extend out be-
yond and/or below the engine power curve (Propeller Power
Curve B). In this case, the engine would spin up to max RPM
with ease, but the prop would be too small to do useful work
and, again, wouldn’t drive the boat effectively.
The goal is to have the propeller sized and selected so its
maximum power demand exactly matches the maximum
power (shaft horsepower) produced by the engine and m
mum rated RPM. Because the curves are such different
shape, they can’t meet at any other point, so this is a co
promise, but the only one possible, and it’s one that work
The way to make the power curves match up more closel
at other RPMs is to use a controllable-pitch propeller. Thi
quite useful for vessels that operate under varying loads
run for long periods at different speeds, but the extra ex-
pense is not called for on most average boats.
The Missing Power Mystery
Okay, you may ask, but what about all that extra power t
the engine is pr
ducing? If you lo
at the 420 CX Y
mar curves, it in
cates that prop
ler power at 2,3
RPM is 250, but
that the engine
putting out abo
380 HP at that
same speed. W
happened to th
missing 130 ho
power? The an-
swer is that the
engine isn’t gen
ating it. A diesel
engine’s power
any RPM is con-
trolled by how
much fuel is me
tered into the in
jectors. This eng
could produce 3
HP at 2,300 RPM, but since the propeller is only absorbin
250 HP, less fuel is being injected into the cylinders and
less fuel means less power—even at the same RPMs.
Of course, if you added an auxiliary load (perhaps a high-
output alternator) then this could add another 15 HP of l
above the propeller load. In this case, more fuel would au
matically be injected to keep engine at our throttle-set
2,300 RPM and add an extra 15 HP of output power. You
could keep on adding loads at this RPM until you reached
the maximum rated 380 HP, at which time the engine
would be overloaded, and you’d have to increase RPMs.
Eventually, as you increase RPMs the propeller curve gra
ally rises until it crosses the engine power curve and ther
no more extra power available.
In fact, the extra power we’ve been discussing here is a
Propeller Power Curve Variations