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?


there may be an effect from
currents.

Didn't think GPS cared about currents.
If you want to know how fast you're going toward your destination, GPS
can tell you.
If you want to know how fast the water is moving past your speedo
pitot, the boat speedo tells you that - assuming it's working
correctly.
This is what I've heard, anyway.
The graphic (DIP switches) in the link I provided shows the Yamaha
speed sensor can be set to speedo or GPS (NMEA0183.)
Fuel metering DIP can be set for GPH and l/h only.
Looks like no translation to MPG.
All info is good if you make use of it.
So if you assume the boat owner knows his burn per hour (I'd trust
engine rpm more than a fuel flow meter *usually*) and is making a long
run toward a destination, say 100 miles, the GPS speedo is valuable in
calculating whether his fuel will get him there. If it did the
translation and was accurate it would be better.
On some engines a fuel pump/regulator diaphragm can develop a leak and
lose fuel to the intake manifold without real obvious signs. A flow
meter might come in handy for detecting that.
Personally I've found just counting the gallons pumped into the tank
gets me a good enough handle on car mileage. A flow meter seems good
for boats to determine most economical rpm.
From what I've heard the car/boat flowmeters are better at finding the
"best" spot than they are at actually measuring the flow spot exactly.
For instance a guy is reporting he's getting 34 mpg at 65mph on his
car and 31mpg at 75 mph according to his flowmeter.
He's really getting 32 and 29mpg. But the meter does accurately tell
him he's using less fuel at 65mph.
If all or any of the that is wrong let me know.
Your question about a 5kt current and 10kt speed for example.
I think you have to add "apparent" or "true" to the 10kt part to
answer that, but I'm not sure.
Already got a headache.

--Vic