Cruising
Efficiently and Saving Dollars
With talk of expensive oil in our future, I’ve been looking at
ways to get the most out of a gallon of gas. I’m not talking about camping
at the marina vs. on the hook with the genset running, or treating guests to
Subway vs. going down to Tumbleweed, or adding a mast, solar panels and a fuel
cell motor. My objective is to demonstrate how to tweak things to operate more
efficiently over a given distance with a given boat, both with and against
the current.
I started thinking about this over
the winter when I considered taking the boat to Pittsburgh
for my vacation in May. Do we really get .65 mpg at
cruise as we were told, or is it .40 or .70 or 1.00?
Maybe we should have bought the diesel?
Everybody knows that by staying off
plane, you can extend your range in calm waters. You’ve
seen the graphs in magazines before. The numbers are
usually best at idle, if you can stand going that slow,
then decrease as you approach “displacement speed,” or “theoretical
maximum hull speed.” Once you start to plow,
your efficiency decreases dramatically, until planing,
where you are riding on top of the water with less
friction and at a higher speed.
So how can we determine the theoretical
maximum hull speed?
There are several factors involved,
such as hull type and beam. To generalize, a deepV
hull with a large deadrise will be affected more by
drag than a boat with a flat bottom or a displacementtype
hull. Without getting too much into the science, as
you push the water in front of the vessel, a waveform
is generated. The key is to determine the maximum speed
that you can operate without getting into the drag
caused by overriding the wave. The formula commonly
used is as follows:
V = 1.34 * Square Root(LWL)
Where,
V is the solution, in knots
LWL is the length at the waterline, in feet
I’ll save you the math: 28’ LWL
yields 7.09 knots, while 36’ LWL yeilds 8.04
knots.
Let's examine a local BrandX 37
for example, with a LWL of 34'. The solution yeilds
7.69 knots, or 8.84 statute mph. Practically speaking,
you could cruise efficiently up to 8.8 mph in that
boat, but if you want to go faster, you’ll be
using more fuel per mile. I used to think 1200 RPM
= 1400 RPM = 1800 RPM = 3400 RPM (planing) in terms
of efficiency, but the data below doesn't support that.
Current will also make an impact, which we will examine.
What about planing? Many boats will
see their mpg increase again at planing speeds. Tabs
up helps, but you need instrumentation to know for
sure. For example, used to operate a mid80’s
Sea Ray 340 Sundancer on Lake Erie that was equipped
with fuelflow meters and a fancy sumlog. It would
use the same fuel per mile at 3000 RPM with tabs (1/4
deployed to improve the running attitude) as it would
at 3400 RPM clean. At 3000 RPM without tabs, the Sundancer
would fall off plane. Once the secondaries opened up,
of course, the efficiency decreased dramatically. It
all depended how fast you wanted to go or how much
noise you wanted to make.
The BrandX 37 described above gets
slightly better efficiency on plane with a 3 second
burst of trim tabs out at 3400 RPM (as indicated by
GPS speed changes). Generally speaking, tabs are used
in rough seas to control the running attitude with
head or following seas, but in this case we're describing
the use of a small amount to improve running attitude
and efficiency.
Applying a little geekfactor here,
I considered installing a fuelflow meter on my current
boat so that I could monitor fuel flow, improve my
operating efficiency and mitigate the increasing cost
of fuel. Captain George East, who has some experience
with drag racing and dynos, suggested the following
as an alternative:
While fuel flow meters are a very
good idea for optimum efficiency, the cost may turn
some people off. There is another way that is not
as precise but will yield good results: Go to any
boat magazine that has boat tests that give gal./hr.
figures for various RPMs. Be sure to use an engine
that has the same displacement as the ones in your
boat i.e. 5.7L 350 CI, 7.4 L 454 CI, 8.1L 496 CI,
etc. This data that is given in gph is fuel burn
under a load and will not significantly vary from
boat to boat provided that the engines in the tests
turn a similar full throttle RPM that the boat engines
do.
Using a GPS, do a two way average
to determine speed at these test RPM settings. Simple
math will yield the answer on mpg. Also, the physics
of optimum fuel burn for internal combustion engines
are .44.45 lbs of fuel per horsepower per hour for
gasoline and .35.36 lbs of fuel per horsepower per
hour for diesel. Modern engines have fuel injection
systems that meter fuel more precisely through out
the entire power range better than carburetors on
older gasoline engines.
Using George's suggestion, I ran the
numbers on the 37 BrandX Convertible equipped with
a pair of 454 fuelinjected Crusaders. It's a very
nice, beamy boat, with the interior space of a small
houseboat. The Crusader factory rep sent me information
regarding fuel burn at given RPMs on a very similar
boat, which happened to have the same wideopenthrottle
RPMs. On a nowind morning, I plotted GPS speed data
at 200 RPM increments from 1000 RPMs to WOT. Although
I took numbers on the river going both directions,
it soon became apparent that the current that day was
a healthy and constant 2.4 knots. I compensated the
speeds to reflect nocurrent conditions. For each RPM,
I took the speed divided by fuel flow to determine
the specific range, or miles per gallon (mpg). These
numbers were converted to statute measurements, as
the inland waterways are measured as such.
For the BrandX, the following numbers
were obtained with 3/4 fuel, 1/2 water, two passengers,
and a dirty bottom that needed new paint after 3 years.
Obviously, new bottom paint would improve the numbers.
Looking at the above graph from the
BrandX, the numbers near idle give the best fuel
over distance. The mpgs decrease closer to planing
speeds, but on this particular boat, the mpg numbers
did not improve more than a little bit while on plane.
In this case, it becomes a function of hull design.
I know from flying that a tailwind or headwind can
have a large effect on range, so given that we're focusing
on the river, I ran additional numbers accounting for
a 2.5 mph current either downbound (DB) or upbound
(UB). The difference is greatest at slow speeds. On
the above chart, for example, you can see how much
the efficiency improves going with the current at slow
speeds (i.e., 1200 RPM), but going upriver, it doesn't
make sense not to go 1600 RPM or more, as the efficiency
is almost the same.
What about the "theoretical maximum
hull speed" that we started with? Sure enough,
on the BrandX, 8.8 mph (no current) occured at 1800
RPM. You can see that the efficiency drops very close
to that point, supporting the math and physics.
Regarding the cost of cruising, some
people say “you don’t want to know,” or “if
you have to ask, you can’t afford it.” With
good numbers for your engine and a GPS for speed, it's
pretty easy to run a spreadsheet and come up with the
specific range for your boat. You could go one step
further and determine the cost per mile, but that's
not as much fun at today's prices. You can easily figure
out, however, where your boat performs best for the
way you are operating on a given day. The difference
can be measured in dollars per mile, which will add
up over a distance.
Captain Eric
Eric Grubb
Founder, Port KY
Licensed Master

Eric grew up around boats, trading summers on board his parents' Sea Rays for many manhours of swabbing the decks. He grew up by the little town of Sewickley, Pennsylvania, overlooking the the Dashields Locks and Dam. He has traveled the Great Lakes, Lake Huron's North Channel, Gulf of Mexico and several rivers to include the Ohio, Allegheny, Monongahela, Kanawha, Mohawk (Erie Canal), Tennessee, Tombigbee, Black Warrior and Mobile Rivers.
As a commercial pilot, Eric flies jets and is a flight instructor. He has owned recreational boats ranging from PWCs to most recently, a flybridge convertible that he keeps in a Louisville marina (MM 590). You can also find him with his family on the "Escape Pod," an 18' fishing boat. His most memorable journey was aboard the J. S. Lewis, a 155' towboat in service since 1931.
Eric is a USCG Licensed Master with a Commerical Tow Assistance rating, and is a member of the Sons and Daughters of Pioneer Riverman and the Louisville Sail and Power Squadron. After moving to Louisville, he conceived the idea for Louisville's Port KY website while searching for information to help him become a safer and more knowledgable local boater. He has worked hard over the years to educate other boaters by promoting safety classes through Port KY and by hosting captain's classes and related events.

