So, you’re having a yacht built…or perhaps, undertaking a
major refit and repowering of an older vessel. And you’re naturally considering
not only which engine make and model to choose, but grappling with the decision
as to selection of power rating.
Well, let me tell you that a significant percentage, if not
all of the advice that you will commonly receive may be misinformed or
misguided. One of the most common “old wives’ tale” you will be exposed to is:
“Higher HP engines always consume more fuel
than those with lower ratings.”
Fuel consumption is directly related to power production. An
internal combustion engine only produces power in response to a load. All other
factors held constant, an engine with a higher HP rating will not produce any
more power than one with a lower HP rating, when both engines are faced with
the identical load, provided that each engine is sufficiently powerful to
adequately respond to the load in question.
Horsepower is a measure of work accomplished. As such, it is
the product of torque and engine rpm. Torque is produced by combustive force in
an engine’s cylinders acting through its chain of pistons, connecting rods and
crankshaft. Assuming similarly efficient fuel burn between different engines (a
reasonable assumption in most cases), combustive force is determined by the
quantity of fuel burned. The upshot is that for a given load — or we may say,
for a given application in a specific yacht — the fuel consumption of two engines
with different HP ratings will be effectively the same.
HORSEPOWER RATING VERSUS COST
For this reason, in terms of fuel consumption and range,
choosing an engine with a lower HP rating may not be advantageous. If anything,
it is likely that, of two engines able to handle the load that will be placed
upon them, the one with the lower HP rating will have a lower first cost. Thus,
if you need, say, a 500 HP engine, selecting one with a similar weight, but a
maximum 700 HP rating will probably cost more — although not ultimately in
terms of fuel consumption.
Against that, however, you need to consider the effect of
maximum HP rating on longevity or MTBF (mean time between failure). If you run
an engine consistently for long periods of time at 95% to 100% of its rated
horsepower output, you are almost certainly going to seriously reduce its
operating life. So the idea is to select an engine or engines with power
ratings which assure that the bulk of your operating hours will be at 80% to
85% of maximum rated output. For by increasing MTBF, you will actually be
reducing the ultimate cost per hour of running time of your engines.
NO DIETS FOR YACHTS
You should also consider that no yacht ever loses weight as
it ages. What we refer to in the industry as “weight gain” is a never-ending,
never-reversing process that begins when the vessel moves from design and
engineering to actual construction, then to outfitting and commissioning, and
finally to and during real-world use. This life cycle is always, yes always,
accompanied by a growth in the overall, loaded weight of a vessel, and
consequently with an increasing load for her propulsion engines to face. All
due to the fact that we always put things onto and into a yacht — tenders, jet
skis, dive tanks, air compressors, and all other manner of gear and equipment —
but we rarely, if ever take anything off.
If the engines you select start out life in an application
that is initially projected to see them loaded to 95% or more of their rated
power, you can almost certainly expect that somewhere during their life in the
yacht in question, they will end up running overloaded, that is, producing
power at levels above their rated maximum. This will mean accelerated stress,
wear, and tear; decreased MTBF, and ultimately higher cost per hour of running
time than if you had specified and initially paid for engines with a higher
power output rating.
But that’s not the only problem with engines whose power
production ratings are only marginal for the application at hand. My experience
in yacht construction is that a significant number of vessels specified with
engines whose rated maximum power output is very close to the design loads
initially projected, end up underpowered for their performance targets, by the
time their construction is completed.
YACHT CONSTRUCTION IN THE REAL WORLD
As construction and final engineering progress in a yacht
build project, previously unanticipated weight growth may force you to accept
lower vessel performance than originally targeted. In such cases, there may be
remedial steps that can be taken while the build is still in process. For
example, one might be able to boost projected thrust by upsizing the vessel’s
propellers. However, because of the higher loading produced by such larger
propellers, this kind of move will only be successful if the engines selected
at the early stages of project development and specification have a sufficient
margin of reserve power available.
In my experience as a yacht builder, it is prudent to select engines whose power
production ratings are 10% to 25% higher than the designing naval architects
and engineers originally anticipated would be necessary. Provided you watch to
see that the overall weight of the higher powered engines is not significantly
higher than those originally specified (and in many cases they won’t be), you
can provide for a margin of reserve power, without incurring and penalties in
terms of fuel consumption and range, if it turns out that the additional power
is not required. A bonus will be that the engines with the higher power
ratings, when run at a lower percentage of rated maximum power, will last
longer and be more reliable.
My next post will deal with why total fuel consumed provides a better basis than engine hours for scheduling maintenance.
