I've just made this box (for my non-rib boat!). It is connected to a fuel flow sensor in the fuel line that sends a pulse for every 0.4 ml of fuel that is used. The number of pulses, GPS speed and lat/long get logged to an SD memory card once per second. The SD card also contains a programmable calibration factor to change the 0.4ml to whatever it needs to be to be accurate.
By setting the initial fuel quantity it counts down as fuel is used.
It also generates a little scrolling bar chart of flow rate as it varies with time:
(and shows GPS position, speed, etc.,)
The plan is to use this data to find out exactly how fuel consumption varies with speed (it gets told ho many people are in the boat and the amount of fuel, so total mass is guessable). With enough data, I hope to be able to get a good idea of fuel burn rate vs speed and, if I can log it, r.p.m.
I know wind, tide, etc,., will affect speed over the ground, which is why I will get as much data as I can and weight the data with the number of points at each speed. The aim is to build up a 'map' of " fuel consumption rate" vs "speed" (and also generate KML for each trip that can be viewed in Google Earth).
It might not work in practice, but I'm sure going to give it a try!
The sensor is fuel-proof, air-tight, able to cover a wide range of flow rates and has no metal-to-metal contact. With only 5V going to it, I am confident it won't blow up on me. I just need to get the boat wet now..