Originally Posted by Pikey Dave
inline fuel flow sensors are notoriously inaccurate, especially at low speeds.
They are sufficiently precise (consistent) at high flow rates/speeds for determining efficient boat running setup - for example obtaining optimum engine trim and rev range for fuel economy. They can also highlight problems with the engine or hull that would otherwise go unnoticed.
I had a seam on my tubes that was coming unglued and billowing out at planing speeds but not at rest. I first knew something was not right as the fuel consumption rate was 20% higher for the same cruising speed and load than I was used to.
Depending on the purpose of your boat, I could imagine one burns well under 10% of fuel at low speeds; the rest at high speeds, therefore the accuracy at low speeds is mostly irrelevant.
I wouldn't rely on one for a calculated tank level reading if you are the type of boater to start a trip with anything other than a full tank. If you only ever add the odd few litres here and there, and you are down to the last 1/10th of a tank on the 'gauge', I wouldn't presume there was any fuel left at all!
Having said this, my EP-60r after calibration returns an error of +-1 litre of fuel on 70 litres of use from a 90litre tank through a 100HP outboard. This is an accuracy of +-1.5% and has been within this margin of error over 40 or so trips to the pump. When fuelling the boat on the water I can't judge when the tank is 'full' to within this margin of error anyway. so with proper trip planning and safety margins, the range/duration remaining value calculated using one of these fuel sensors could be useful. I still rely on my Mk1 eyeball fuel level sensor before setting off as I can see inside my tank, and would get level sensor if not, in order to verify tank level.
As with all these gadgets, if you know their limitations, then they can still give useful information, just not perhaps the full picture that you desire.