"John H." wrote in message
...
On Fri, 4 Jul 2008 11:18:42 -0400, "Jim" wrote:


"Richard Casady" wrote in message
. ..
On Fri, 4 Jul 2008 06:29:26 -0400, "Jim" wrote:


"Graham" wrote in message
news:ypjbk.54744$Jx.31786@pd7urf1no...
Does anyone know if these gauges display miles per gallon (or litres
per
km) or just miles travelled and gallons consumed?

Do they need any special interface to work with standard GPS's like a
Raytheon chart plotter?

Thanks, Graham

The GPS reports miles per hour over land. It would be useless to use
those
calculations in determining real miles per gallon over the surface of
the
water.

It reports MPH everywhere equally well. It doesn't know sea from land
unless you get it wet. It reports speed over the land and if the land
is flooded, at sea or on a river,






THIS IS THE WHOLE POINT. If you are heading against a 5 kt current at
10
kts, what speed will the GPS report?

10 knots. But, without the current, you'd be making 15 knots all else
being
equal.
Wrongo old bonged one. The GPS sees you doing 5MPH over ground. What matters
on a boat is your speed thru the water. Speed thru the water is determined
by your throttle position which has a relationship to RPM and GPM. These
things are predictable and repeatable. When you want to know ground speed
toward destination you need to take into consideration current and drift.
Same thing in an airplane. There is ground speed and air speed. They're
different.
Sorry you got bonged. I wonder why Harry got less jail time for a more
flagrant rule violation?