Paul I. wrote:
My BIGGER concern NOW is that (I just discovered) that you can only have one device attached to your fuel sender to your tank. I did want to add one (NMEA2000), but the more think of it, I really don't need it!!! Between the GPS & the fuel flow monitor, the display should be able to show MPG/KPG. I thought, which BOTH Garmin & Lowrance do not state in there product manuals that you need one device per sender!!!!
I can't speak for the Garmin gear, but the Lowrance ones do use one sensor per fuel line. I have two, one for each engine. They're networked along one wire, using tee fittings. The Lowrance sensors are in-line with the fuel hose. They do not tap off the tank at all. They read the fuel flowing through them. I'd imagine anyone else's fuel flow sensors would do the same.
I think the only time you wouldn't use a sensor would be if the engine system itself was tracking the data. AND that data was made available on a field-connectable network. Note I'm emphasizing field-connectible. Just because a network exists doesn't mean you should connect to it. I have a NMEA2k network running between my engine computers. But I do not tap into it. The last thing I want is for any other network issues or traffic to interfere with the safe operation of the engines. I've read recently that NMEA2K bridges have started to come onto the market. I don't really need to get to the data present in the engine network (it's only transmission gear, rpms and throttle position). If there was more data, like oil and water temps, then I'd use a bridge to control extracting ONLY that data. But this is a technical conversation probably beyond the scope of this forum thread.
Back to my setup, two in-line fuel sensors spliced into the fuel hoses leading to each engine. Each one is wired to a tee. The tees are then wired up to tees for a single gauge, a GPS puck up on the arch, and then to the chartplotter. I'd imagine this would be similar to what any other vendor's setup would require. The gauge calculates MPG based on the flow rate and the GPS SOG (speed over ground).
If you're talking about a tank level sensor that's something entirely different. I don't know that I'd ever bother with one as the floats are notoriously unreliable. That and the level will fluctuate quite a lot based on the boat's attitude (rising up on plane) or pitching around in waves. Yeah, I pay attention to my tank levels on the analog gauges and if I had no choice other than to use a digital gauge I'd certainly go for it. But I'd never trust the tank levels on a boat, mine nor anyone else's.
Better to track flow rates, that way you KNOW what's been consumed. If you take the time to tell the gauge how much fuel you've just loaded then it can 'do the math' to calculate your remaining amount. As well as estimate all kinds of other values. I do not think you can accurately calculate MPG from anything other than flow rate and GPS. I mean, it's possible to use it for a guesstimate but that's nowhere near as accurate as using fuel flow rate.