Suggestions for fuel level indicator please

[rob2]

I have a sight glass - designed for heating fuel tank and available from BES. It needed the bottom fitting adapted to suit connection to copper fuel pipe, but has a press to open tap at the base so in the event of an engine fire, only the fuel coloumn in the tube will be released. Of course, it is only simple to install if you already have a take off from the bottom of the tank.

A similar boat to mine has a similar design concept, except that in place of a sight glass, he fitted a vertical copper pipe of suitable size to take a vertical sliding gauge - if either of these interest you, don't forget that the top of the tube must be "open", so needs a breather led to a safe height.

I'm assuming the tank is not of regular cross section as you propose filling by increments to calibrate a dipstick (same applies to a sight glass) otherwise a linear scale of increments would serve. A drill mounted pump will pump out the lot in a couple of minutes. We used a cheap plastic dustbin to temporarily store our diesel, then ladled it back in with a jug of known volume wearing marigolds unless you want to remember for a week.

Rob.

 

[Hadenough]

1 litre per hour per 10 horse power used.

So if my engine is rated at 90hp @ 2600 RPM, how do I calculate the output at cruising revs, say 1200 RPM?

 

[LittleSister]

There may well be a graph in the manual or sales bumph giving max output by revs, if so look it up. The consumption you calculate from this will only ever be a very rough guide as (1) your engine will only draw the fuel it needs to maintain the 1,200 revs (you might be in smooth water or rough, or strong head or tail wind (going downhill? ) and may not be producing the max power available at those revs, and (2) it takes no account of your particular gearbox/prop etc inefficiencies.

 

[lw395]

One boat I know has a small day tank that the engine runs from.
Every time you push the button to fill that from the big tank, that's 3 gallons or whatever.

 

[jdc]

So if my engine is rated at 90hp @ 2600 RPM, how do I calculate the output at cruising revs, say 1200 RPM?

The answer is it depends on the boat, not on the engine. All diesel engines use the same amount of fuel per kWhr of energy delivered, and in fact it hardly depends on the size of engine nor the revs (look at BSFC graphs or Wiki if you want corroboration). This is near as dammit 0.3 litres per kWhr (BSFC is usually stated in grams per kWhr, so you have to divide by density, around 0.85, to get volume per kWhr).

So at 90HP you'd use 90 * 0.746 = 67kW * 0.3 = 20 litres per hour. BUT despite you having a 90HP engine it's only going to develop this at maximum revs and if your prop and boat are perfectly matched to the engine and gearbox.

A better way to estimate consumption is to calculate the drag of the boat as a function of speed, which at lowish speeds is surprisingly small, and, knowing the gearbox ratio, prop pitch and diameter calculate the thrust as a function of rpm and boat speed, and solve to get boat speed as a function of rpm, and, assuming a reasonable value for prop efficiency, the HP actually used as a function of rpm. Then HP actually used can be converted to fuel consumption.

I have developed a computer simulation which calculates this for a variety of monohulls, and here is the graph for my boat (42', heavy and traditional shape). The engine is 50HP, but that makes no odds at all, it's the consumption vs speed which is all that counts. The red crosses are measured and the blue lines the predictions from the simulation program.