How to work out your boat’s range from a tote tank

The battery problem is easy; keep them on a two-year replacement cycle, ensure they’re always charged, and sell your old battery to help pay for the new one.

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But working out how much fuel you need is a bit more difficult if you don’t have the modern array of electronic instrument gauges available.

Not every boat uses a modern engine complete with electronic gauges. This is particularly important when your only source of fuel is from a portable tote tank that may not even have a fuel gauge on it.

How, then, do you work out how much of a tote tank’s fuel will get you? This is where some basic science helps.

Petrol floats on water, so we already know that a litre of fuel will weigh less than a litre of water. How much, though?

This is where we look at the specific gravity of one versus the other. The specific gravity is an indication of how much weight per unit a substance has compared with water, which has the benchmark specific gravity of 1.0 kilogram per litre.

Anything that floats in water has a specific gravity of less than 1.0, and anything that sinks has a specific gravity of more than 1.0.

Unleaded fuel has a specific gravity of around 0.739, or about the same as wood.

This is where we need to mess with a few numbers.

Just say we’re testing the fuel use of a two-stroke engine running off a 25.0-litre tote tank.

To start the test, get an ordinary set of bathroom scales and weigh the empty tote tank. This will be subtracted from the weight of our tank once it has 25 litres of fuel onboard so we know how much fuel has been used as the weight of the fuel burnt is lost.

In theory, 25 litres of fuel will weigh the number of litres of fuel multiplied by petrol’s specific gravity (25x0.739kg), which equates to about 18.5kg. However, because it’s a two-stroke engine, we’ve added 500mL of two-stroke oil, so we’ll round that number out to 18.9kg.

Now is the boring part. Go out and run the boat at a constant, fast speed for about 20 minutes, stop, and re-weigh the tote tank.

Say it is now 12.0kg once you’ve subtracted the weight of the empty tank, meaning in 20 minutes you’ve used 6.9kg of fuel.

Working backwards through the equation, 6.9kg of fuel equates to 6.9 divided by 0.739, or the equivalent of 9.3 litres of fuel. Now, because it’s only for a 20-minute interval, to convert it to litres per hour we need to multiply that figure by three.

Therefore, at a comfy cruising speed up on the plane, our two-stroke is using 27.9 litres an hour, meaning a full tank will last just shy an hour.

Build in a 5.0 per cent fuel reserve safety buffer, and that’s down to about 40 minutes of running time.

Repeat the test at a lower speed. Say in the next 20 minutes the weight of the fuel remaining in the tank use drops to 7.5kg, meaning you’ve used another 4.5kg of fuel in that test period.

Doing the maths again, 4.5kg of fuel equates to 4.5 divided by 0.739, or the equivalent of 6.1 litres of fuel. Multiply by three again to get the 20-minute interval up to an hour, and that’s the equivalent of a burn rate of 18.3 litres an hour.

We now have fuel figures equivalent to running for home ahead of an incoming storm and cruising out comfortably to the secret fishing spot.

The third part of the test is to run the boat at idle speed for 20 minutes – the equivalent of trolling.

The weight test shows us that in 20 minutes of running, we’ve used the equivalent of 1.2kg of fuel. Run the calculations, and this time we’ve used 1.8 divided by 0.739, or 1.6 litres of fuel – the equivalent of 4.8 litres an hour.

Rounding up to build in safety margins, we now have three benchmark numbers:

How do we use this? Keep an eye on the amount of time the engine is running and at roughly what speed.

If you get into the habit of giving the fuel tank a shake each time you prime the fuel line before starting the engine. This is because its weight will give an indication as to how quickly you’re burning through the fuel, and how much is left.

Working out how much fuel is left is easy; try and split it up in your mind by how many 3.0-litre cartons of milk you feel like you’re lifting, which should give you a fairly accurate gauge of how much fuel is left. Even listening to the noise the fuel makes as it sloshes around in the tank is a good gauge for how much is left.

When calculating fuel remaining, bear in mind that other factors such as windage – the movement of wind across the boat – the sea state, currents/tides and even load will all eat into fuel range, so always work on the conservative side of estimates.

Remember, too, that stored fuel will lose its octane rating over time, which means you will burn more old fuel to keep the engine at the same level of performance as if you were using fresh fuel.

Run old fuel through a ride-on mower; the two-stroke oil will help preserve the engine’s valves, and your lawn gets cut.