"Shortwave Sportfishing" wrote in message
...
I had a debate with a friend this morning concerning miles vs gallons
per hour.

These two items are different than the ones listed in the header. The text
above quotes MPH vs. GPH. The header quotes GPH vs. MPG. I'm sure you
realize the MPH and MPG are entirely different calculations, but perhaps
your end of the debate would be strengthened by consistency. Given the
discrepancy, I'm not sure if you are arguing GPH vs. MPH or GPH vs. MPG.
Either way, I'm not sure I understand the basis for the debate altogether,
as each tells a different thing, presumably to be used for different
purposes. GPH will tell you fuel consumption over a fixed period of time,
but as it has no accounting for speed, it has no direct accounting for
distance traveled in that period of time, so by itself is not reasonable
measurement of fuel consumption over distance traveled. MPH is simply a
measurement of speed, with no accounting for fuel consumption at all. MPG
is perhaps the more useful measurement of the three, because it calculates
fuel consumption for a given distance traveled, but may not be as relevant
as GPH for some.



I contend that gallons per hour is a more reliable method of
determining how far and fast a boat can/should go. Obviously, my
friend took the opposite viewpoint.

What is the collective wisdom concerning these measure of fuel
efficiency?



Which of these is more appropriate for you will depend much on your boat and
the way you use it. For instance, if your boat is a trawler type, without a
great variation of speed, or if the majority of your engine hours are at
idle or trolling speeds, it may be that GPH is the most relevant measurement
of fuel consumption. for you. On the other hand, if your boat has
significant variations in speed, and is used to travel large distances, MPG
would more likely be the consumption measurement of choice.

My own case offers an interesting study. I have a 29' twin gas I/O cruiser,
with a top speed of 41 MPH, a typical cruise speed of 30 MPH, and I also
spend a significant amount of time gunkholing at idle and leisuring cruising
at about 8 MPH. Whenever I buy fuel, I always fill the tank completely, and
maintain a detailed fuel log with engine hours since last fill, gallons
used, and distance traveled. Distance traveled is generated from the GPS,
which has a resetable odometer function, which I reset at every fueling.
The boat never moves without the GPS being turned on and therefore recording
accumulated distance traveled.

Depending on how the boat was used for that particular tank of fuel, my GPH
will fluctuate dramatically. My fuel log (going back 5+ years) shows a GPH
high of 18 and a low of 3.6. Quite a range. I have two entries showing
18.0 GPH, with both of them being non-stop long distance cruises, using the
logged fuel in a matter of hours. One was a run of 133 miles using 109
gallons, and the other was a run of 67 miles using 49 gallons. I was at a
high speed cruise the entire time on both runs. At the other end of the
range, the 3.6 GPH entry logged 141 miles using 102 gallons, but over a four
month period of time. This was a period of time where the boat never really
went very far at any one period of time, and consisted of mixed usage at
high cruise speed and quite a bit of gunkholing at low speed cruising. I
also have many log entries with GPH readings anywhere between the 3.6 and
18.0 GPH readings. So for me, GPH isn't a very meaningful statistic by
itself.

On the other hand, and I've always found this somewhat fascinating, even
with GPH readings all over the place, my MPG readings are remarkably
consistent. MY MPG high and low readings are 1.41 and 1.22, with the vast
majority of them hugging around 1.3 MPG. For instance, the 18.0 GPH
readings resulted in 1.22 and 1.38 MPG readings. The 3.6 GPH reading
resulted in a 1.39 MPG reading. What this tells me is that my boat gets
about 1.30 MPG whether I'm cruising at 3500 RPM and 30 MPH or at 1700 RPM
and 8 MPH. Obviously the former has a much higher GPH reading than the
latter, but the MPG readings seem to equalize. I would have never guessed
that both those speed would offer the same MPG, but it has been offered up
way to many times to be denied.

So for me, GPH is pretty much irrelevant, while MPG is highly relevant. If
my MPG readings started to consistently show less than 1.2, I would start to
suspect something amiss with one of the engines. GPH readings would give me
no such clue. So I guess that puts me on the other guy's side of the
debate. But, as they say, your mileage may vary.

RG