Short Wave Sportfishing wrote:
On 7 Jan 2007 10:25:41 -0800, "wf3h" wrote:

OK here's a little topic for discussion

If you're going from point A to point B it seems you'd want to know how
much gas you're going to use. And that leads, by definition, to MPG (or
in the case of boats, GPM).

To an extent.

Most fuel curves, however, are for GPH...how many gallons per hour a
boat burns. That makes sense if consumption....MPG...is relatively flat
(which for many boats, once they're on plane, it, relatively speaking,
is).

Not really.

So why GPH instead of MPG? Also, the value that seems to make the MOST
sense is G/MPH...that is, how much gas you burn at a certain speed, per
hour. That combines most of the info you'd need to get from point A to
B if the consumption curve is flat.

GPH as a measure to MPG is useful for a number of reasons. For one
thing, you may not be traveling anywhere with your engines running for
one reason or another - like trolling in a circle for instance or if
you are sitting in a busy seaway or in heavy weather with your engines
at idle.

Additionally, MPG even for big cruisers can be deceptive as, as Chuck
said, set and drift seriously affect boats. Say 1 gallon per mile
used for instance in a recent discussion on diesel economy. If you
travel 10 miles in an hour, that's 10 gallons per hour. But what if
at the same RPM, you only travel 7 miles in that hour due to set and
drift. You still burned ten gallons in that hour, but you only
traveled 7 miles.

GPH can be specific because it eliminates a number of variables and
you can guesstimate fuel usage because at a set RPM, you use X amount
of fuel in an hour.

Miles per hour works just fine on land, it's not moving. But on the
water, the water can be moving, you have current to deal with.
Sometimes the current works with you, sometimes it works against you
(speed over bottom vs speed through the water) and sometimes their is
no current. But fuel usage, gallons per hour, stays pretty constant,
your either going a little faster, slower or your normal speed. In a
boat you need to know how much fuel you need to go from point A to
point B. If you know at what rate your burning fuel, at a given RPM's,
and how fast your going, you know how much fuel you need (five hours
running time, ten gallons per hour, fifty gallons).

A lot of new boaters like to use MPG because it's what they're used to
using, they don't know any better, and they get themselves in trouble
from time to time because of it. If you use miles per gallon, and you
don't have a real good "fudge factor" built in, your light on fuel, and
the current (and wind) is working against you, you might end up in
trouble. When you hear someone talk about MPG, it's a dead give away
their not experianced.