the data for one to one-and-a-half minutes depending on rpm; 2,500 rpm and
below were at one minute, and 3,000 rpm
and above were at one-and-a-half minutes.
The runs were conducted over flat water
and I traveled in two directions over the
same course and averaged the results to
eliminate any effects due to tidal currents,
though in the area I was running the tests,
there was very little current to begin with.
To monitor the fuel consumption, I
used a FloScan 800 fuel meter that gave me
a series of pulses proportional to fuel flow.
Along with speed from my GPS, I then
input the flow rate into my laptop using a
program called LabVIEW. Using LabVIEW
and Microsoft Excel, I was able to view the
data in real time and also graph it to get a
better appreciation of the data. The photo
shows my laptop with the LabVIEW program that I wrote to gather the data. This
wasn’t meant to be a super quantitative
analysis of efficiency, but kind of an arm’s-length indication of how I might get the
best fuel economy.
The best prop set turned out to be the
DP with the B5 prop set, and the worst was
the SP with an aluminum prop and a pitch
of 15 by 19. Not only will the DP prop
sets get you where you’re going with better mileage, they will get you there faster,
handle additional weight better, and have
flatter, less peaky optimal efficiency range.
So, what did I learn? Was there a silver
bullet? In a nutshell, no silver bullet, but
some interesting takeaways:
Keeping the rpm between the
2,800 and 3,500 sweet spot has a dra-
matic effect on mileage. The mileage falls
off rapidly when going too slow or too fast.
Weight can have a significant
effect, particularly at the slower speeds.
The DP unit handles weight better
than the SP unit. I didn’t see a huge dip
in fuel economy with the DP unit when I
kept the rpm in the 3,000 to 3,500 range.
The DP lives up to its reputation
for better mileage, and has a wider, flatter
mileage curve than the single-prop unit.
The aluminum B5 prop
set was the top performer, edging out the stainless-steel C4 set.
In all, there wasn’t as much difference
as I had expected between the different
setups, but for my money, I’ll go with the
C4 as a good compromise between mileage
and durability. I’ve had trouble with aluminum props being damaged in the past.
My next steps will be to look at the
engine, and what I might be able to do
there. The famed Holley Carburetor makes
a marine throttle body injection system
as an aftermarket kit that’s supposed to
improve mileage, so this winter I’ll do the
installation and give it a shot next year.
Beyond the engine are some more technical hull modifications that ultimately I’d
like to couple with a diesel-jet, but that
might be a ways off. For now I’ll try to keep
the boat light, and keep the rpm in the
Ed Caldwell is a mechanical engineer who’s worked
on designs for sonar systems, medical ultrasound
devices, printers, and
bicycle accessories. He
boats as much as he can in the Pacific
Many boats receive a coat of bottom
paint year after year. On a seven-year-old
boat, that can translate to between seven
and 10 coats of paint. Depending on the
paint used, each coat can contribute to
hull roughness. There’s been significant
research on the effect of hull roughness in
the commercial marine area. Commercial
vessels often travel from 250 to 300 days
a year, 24 hours a day, and according
to a study conducted by the National
Shipping Company (VSCSA), the difference between an old, rough antifouling
paint surface and a new application of
smooth antifouling can mean as much
as six percent reduction in fuel consumption. Some coatings that reduce
surface roughness or become smoother
with use can reduce fuel consumption up to 10 percent in the first year,
according to a study conducted by Force
Technology in Denmark.
How does this translate to pleasure-boat use? Think “smoother is better.”
Rather than having five to eight coats of
paint on the bottom to chip away from
paint-film thickness, start with a smooth
bottom by removing all of the old dead,
spent paint film. Then select a quality self-polishing antifouling paint that
washes away like a bar of soap rather than
becoming more rough and porous with
time as does hard bottom paint. This will
reduce surface roughness, prevent the
accumulation of fouling, and help the
hull move through the water with less
surface drag. Some brands have a low
level of biocide; others will have stronger
formulations, yet still comply with EPA
guidelines for the safe use of biocides in
water. Recycled copper has been the primary biocide used in antifouling paints
for decades. A clean bottom creates less
drag, long-term fuel savings, reduced
greenhouse-gas emissions, and corresponding carbon-footprint reductions.
Combine that with using the correct
prop, and finding the RPM sweet spot,
and your hull will consume less fuel.
Smoother Is Better
By Steve Schultz
With 30 years experience
businesses, Steve Schultz
is head of North American
operations for the Italian
paint manufacturer Boero