Improving Fuel Economy
wisdom of dockside sages; the protocol must
unequivocally follow the manufacturer’s literature.
Stuffing boxes, particularly those of the dripless variety,
often have requirements regarding cooling while
trailing a shaft, particularly above certain speeds.
Again, you must check with the manufacturer.
The alternative to freewheeling calls for securely
locking the shaft. If you choose this route, the lock
must incorporate two principles. First, it must be
rugged and capable of withstanding the loads placed
upon it. Second, it should include an electrical
interlock that prevents the engine from being started
unless the lock has been disengaged. (One commercial
Top: Intercoolers and aftercoolers need maintenance, too.
Even a small amount of oil or dust fouling will reduce engine
efficiency. Check your engine manufacturer’s guidelines for the
proper intercooler service frequency. If your turbo has or had
an oil leak and your engine is equipped with an intercooler,
it needs to be cleaned. Above: Poor turbo maintenance leads
to poor fuel economy. Leaking turbo compressor seals foul
intercoolers, injectors, and combustion chambers, all of which
reduce efficiency.
unit I know of automatically unlocks when the engine
is started and the transmission is engaged, making the
ignition interlock unnecessary; see shaftlok.com.) This
is easier than it sounds; a small contact switch is all
that’s required to prevent the potential disaster that
could follow shifting a locked shaft into gear. Of
course, feathering props, which swing into a low-drag
position when the engine is not running, also are
extremely efficient alternatives for those who are
willing to incur the initial expense of the hardware.
If you intend to trail regularly, a feathering prop may
be worth exploring.
Beyond saving fuel, there’s yet another advantage to
trailing an engine. If you run your boat on one engine,
or on a wing engine for that matter, you’re much more
likely to optimally load that engine. Think of it this
way: Running on two engines that are both operating
at 30 percent load creates all of the aforementioned
negative side effects of overcooling, carbon formation,
sludge, and so on. On the other hand, running one of
these engines at 60, 70, or 80 percent load eliminates
or considerably reduces these detrimental effects.
Perhaps of greater significance is the efficiency at
which a diesel engine operates when heavily loaded.
The amount of “energy” produced by a diesel engine
per gallon of fuel burned increases with load. Lightly
loaded engines are especially inefficient; in fact, a
recent side-by-side test showed that lightly loaded
generators are terribly uneconomical in terms of
kilowatts produced per gallon of fuel burned. It
therefore makes good sense to run one engine at
50–80 percent load rather than two engines at 30
or 40 percent load. Additionally, heavily loaded
diesel engines typically produce less in the way of
harmful emissions, such as carbon monoxide and
nitrogen oxide.
Find Your Sweet Spot
If you’ve ever owned or operated a boat powered
by a large, carbureted gasoline engine, you may be
familiar with the term “secondaries.” These devices that
live within a carburetor are tasked with delivering large
amounts of fuel when the operator pushes the throttle
beyond a certain point. The change in the engine’s
tone is quite noticeable, going from a rumble to a
throatier growl or roar, depending on how well the
engine compartment is insulated. Equally noticeable
is the lightening of your wallet from the higher fuel
bill. The only easy way to counteract this effect is
to back off the throttle. Balancing fuel consumption
with desired performance is often called “finding the
sweet spot.”
