Fuel Management and a bit on hardware

Some thoughts on (diesel) fuel management, especially as related to long coastal cruises in remote areas when fuel planning is of more importance than pottering around the Solent.

Obviously no one wants to run out of fuel, especially with a diesel that will require bleeding to get it going again and where a very low fuel level might cause crud in the tank to be drawn out. So, you need to know what the likely fuel burn is and how much you have.

The N coast of Devon & Cornwall is
very exposed and has no all tide
It is also important, especially sailing solo, to ensure or at least to know if you could motor to a place of safety if you or the boat are partially disabled (e.g. sailing with a broken arm would be problematical but probably OK under engine). If bad weather threatens it is also usefull to know how long you can motor, perhaps hove too under engine or heading for a safe haven. 

For example, a while ago I was at Milford Haven after doing a lot of motoring coming down from the Scottish islands, I had planned to refuel in the marina after riding out bad weather at anchor but things conspired against me and to do so would mean delaying departure by a day by which time the wind would likely have disappeared. I probably did not have sufficient fuel to reach Newlyn (120 NM) or the Scillies and to my view there are no all tide safe havens on the N Cornwall and Devon coasts, 

Knowing the likely fuel consumption and fuel remaining (much of it in cans so I knew that was accurate) I calculated I could go half the distance with a heathy reserve. Once clear of the Haven I switched to sail having decided that I would not motor until I had sufficient fuel to complete the trip. In an emergency or if I was faced with days of calm I would have sufficient fuel to turn back. In the event I made it with plenty of fuel to spare.

Maintain a Fuel & Engine Log.

A deep narrow tank will allow more accurate
measurement of the fuel but does have the
disadvantage of a higher centre of gravity.
First, if you have a fuel gauge, treat it with great caution, on some tanks a 1 cm error (about as good as you can get) in measuring the fuel in the tank can be as much as 3.4 Litres, and if the gauge sender or dip point is off centre and the boat is healing and / or out of trim fore and aft the error can be significantly bigger. And that is assuming that the method of measurement is accurate, many electrical systems aren't, see below.

When I moved from a bladder tank (with no way of measuring the amount of fuel) to a stainless tank I specified a large relatively deep tank, and bought the most accurate type of sender available with a NMEA 2000 converter so that the level can be displayed digitally on the boats instruments rather than on a small, rather vague analogue meter.

The tank holds 70 litres (60 - 65 litres assumed usable unless it is rough) and is 397 mm deep (inside), a 1 cm depth error due to the tank not being absolutely on the same level as when calibrated would result in a volume error of 1.75 Litres to which you have to add an allowance for the accuracy of the reading so even in good conditions the system could easily be in error by 2 - 4 Litres, c 4.5% or up to two hours 45 minutes /  15 miles motoring; and a lot more than that sailing and healing in a seaway. 

My fuel level metering is about as good as it gets and in practice is generally pretty accurate but I still maintain a fuel and engine log in Excel, it double checks the gauge, is a good back up if the electronics give up, tells me immediately my likely endurance and range and allows me to track actual consumption and trends.

Engine and Fuel Logs: 

  • Keep it manually separately from the main log or better, in a spread sheet.
  • Record level and engine hours at least daily, and more often if motoring or battery charging for long, this discipline should prevent surprises. If I am out of sight of features for an immediate visual position fix I make an entry in the navigation log every hour or so and include the fuel reading, then I transfer the end of day or leg readings to the engine log.
  • If the system allows set an alarm at a level above a safe reserve reserve. For some reason the Garmin will only set an alarm from a calculated use from a flow meter, why not give the option of using the fuel level meter????
  • If there isn't one, install an engine hours meter, they are easy to install, cost from c £37 and, unlike estimating or manually logging, is accurate so at least one variable can be relied on when doing calculations.
Navigation log to my design including a field for fuel remaining.
I don't record barometric pressure as usually recommended because
I have a barograph. Engine hours at the end of the day are noted
under comments before transfer to the spread sheet.
After keeping a detailed fuel log for 6 seasons with well over 500 days at sea, I know that on average the engine uses 1.4 litres an hour. That increased quickly to 1.8 litres for a brief while in 2023 when I had engine problems and a dirty hull (both fixed). Endurance could be increased by motoring at 5 knots or less but I don't know by how much or what effect it would have on range, nor do I know how much fuel is used when charging as I don't need to.

Here is part of my spread sheet for part of my 2023 late summers cruise through to the winter engine service, including a 20 hour dash @ > 6 knots back to the Hamble under engine for a family emergency.

The fields in yellow are entered manually and everything else is calculated automatically. The most important columns are:
  • Fuel remaining by calculation since the last time the tank was filled to the brim.
  • Fuel remaining in litres according to the fuel gauge, annoyingly reported as a % rather than in litres despite the plotter knowing the capacity.
Significant deviation between the two is highlighted; here the calculate fuel remaining varies from 5.7 litres more than the reported figure to 2.1  less. This after 163 hours since the tank was last completely filled. Not shown is the number of hours since the last service and this

The calculated endurance and range at a conservative
speed, in good conditions I would make c 6 knots.
The green gradually turns to red as the range drops.
The above is without a significant reserve but is at a conservative speed, if conditions are poor I can change the anticipated burn rate (in one field, independent of the normal usage rate shown) and it recalculates.

How much fuel?

Don't skim when buying cans, cheap ones
often leak through the spout to can junction
and red diesel stains can be difficult to remove.
This 20 Ltr "Scepter doesn't leak and has a vent
for quicker pouring, but with a small diameter 
Spout it is still slow. Avoid steel, they rust, are
heavy and noisy. Two 10 litre cans would be
easier on the boat but difficult on the trolley 
The decision on how much fuel to carry comes down to tank capacity, weight and space for cans in a locker or on deck. At a minimum you need a reasonable amount in the tank and with a small engine at least one 10 litre can to give c 7 hours running, more is better. If you have a fuel leak from the tank and you have fuel in cans, once fixed you can replenish or, if it is not fixable or the problem is the dreaded diesel bug, assuming the engine has a lift pump (almost all diesels do) it should be possible to put the fuel line and return directly into the can to get you going again. The cans can also be used for porterage.

A reserve should always be carried in at least one can + optionally a usable reserve in the tank + a generous allowance for unusable fuel which can be quite a lot in a choppy sea, see also comments on bladder tanks below. As mentioned above my reserve when coastal sailing, is enough to get to a safe port with much of that in cans.

World Sailing special regulations specify that all boats should start an event with sufficient fuel for all charging needs during the event plus 5 hours motoring slowly. In my view this is insufficient in a small boat on a long trip.

Around the Scottish Islands fuel can be hard to come by and I avoid marinas as much as possible; also to avoid needless nights at sea with no sleep I will use the engine when necessary to make a tidal gate or get to an anchorage at a sensible time. On the plus side I don't recall the last time I had to use the engine just to recharge batteries, my solar panels, wind turbine plus engine charging entering and leaving anchorages or harbours does the job.

Depending on the position of the filler large cans
 can be difficult to us, especially at sea.
So, I accept the weight of a lot of fuel and start a long cruise with a full tank of 70 litres and  a further 33 litres in cans, the cans would hold 35 but the 20 litre one full is heavy and it is difficult to avoid spillage transferring to the tank when it is full; a small pump would solve that but is too much hassle for a couple of litres but I may soon get one dedicated to fuel as I am finding the big tank increasing hard to use when transferring to the main tank.

One can is 5 litres which leaves some space in the locker for buckets etc., that is normally used to replenish the heater tank as a funnel is required with a bigger can.

With no reserve 103 litres in calm weather gives a range of about 400 nautical miles with a small allowance of 10 litres for unusable fuel.


Always use a fuel treatment to prevent diesel bug, and be generous with it, it is not possible to overdose with the ones I have used.

On the mooring I refuel by porterage, it is easier than moving the boat to the fuel berth and cheaper, at the time of writing white diesel from Tesco is £1.42 / litre and red diesel at MDL's Port Hamble fuel berth (with a 60:40 split on duty) is £1.48.

 If you have white diesel in the cans and a diesel car, at the end of the season you can cycle that fuel through the car and get fresh for next season. I did that but did not show it in the engine log above as I purchases sufficient fresh fuel to fill the tank for the winter - the normal recommendation to avoid condensation but now being challenged by some.

If the preliminary fuel filter does not have a water trap with a clear window to monitor, install a dedicated water trap in the fuel line and check it regularly, this is easy with red diesel or a mix, with white you may need to look for a meniscus at a water / fuel boundary and it there isn't one use the drain tap to check that it is all diesel not water.  
This preliminary filter has a sight glass & drain at the bottom, it is
all red so there is no significant water present. This type is difficult
to put back together after changing the filter, a  turbine "Racor type"
Filtration system is much better, but I don't have room for one. The
genuine ones are very expensive but cheaper versions are available
It is best to have a separate tank for diesel heaters, taking the fuel from the main tank will make a nonsense of the fuel log and if you find yourself weather bound in cold weather you could find you have insufficient fuel left for the engine. It is also an additional point of failure, it has been known for a poorly installed defective unit to syphon fuel from the tank or, if the pipe comes from the bottom of the tank or from pipework below, a simple leak can drain all the diesel into the bilge.
My dedicated 7.5 litre tank for the heater, sufficient for 30 hours
operation at maximum (2Kw) output. More powerful heaters can
use three times as much or more. I treat this as for the engine so if
push came to shove I could use anything in it for the engine.


A few notes on hardware in the fuel system.

Bladder tanks

Are convenient and easy to fit and, as they are rubber and collapse as fuel is used, condensation introducing water into the system is non-existent. However there are some significant drawbacks:

  • They are difficult to secure and vulnerable to damage, especially in a knock down which puts a lot of pressure and big shock loads on the tie down points, especially if it is half full. They are therefore banned from all events under World Sailing's special regulations (although a rigid contained tank can have a flexible liner).
  • The usable fuel can vary and may not be as advertised due to the shape of the compartment, the one installed in Sancerre had a nominal capacity of 100 litres, the boat details said it held 50 but when I removed it I found it would hold 80 but due to a fold in the base and the shape of the compartment the amount usable varied significantly in a lumpy sea, probably from 40 - 55 litres and I had been carrying around almost 30Kg of fuel that couldn't be used.

Fuel gauges

A true and tested method of measurement is a dip stick though some of the inaccuracies mentioned above and below are still present there, it is often inconvenient but there is little to go wrong if used properly but carry a spare! It may also be appropriate to have one as a back up to other methods.

A reading from a meter like this,
only a couple of inches in
diameter, is not going to be very
accurate, even if the sender is.
A sight tube is another traditional method but will not be practical in many installations. It is generally reliable but crud can enter the tube stopping it from reading. A sensible precaution against damage is to have a tap at the bottom, turned on just when measuring. The top needs to be vented, preferably back into the tank but certainly well above the maximum fuel level.

Electric and mechanical gauges come in several varieties, the two main one are:

  • A float on a swinging arm, either mechanically linked to an analogue gauge or to a variable resistor the resistance of which shows the fuel level. This type is notoriously inaccurate on a boat and is rarely seen these days except on old installations.
  • A float on a "pillar" or "pole", the float activates reed switches inside the pole, the accuracy will depend on the spacing of the switches, the quality of the components and the shape of the tank. My WEMA high precision version pole sender resolves to 10mm.
Electrical gauges are typically small analogue devices which measure the current through the sender which is proportional to the resistance and therefore fuel level. Being small they tend not to be that precise and depend on the stability of the voltage for a consistent reading, a decent one will supply that regulation.

The fuel level showing top right, clearly easier
to read and precise, but for safety assume the
actual level is somewhat less.
A more precise method is to have a device to measure the resistance and convert it to a digital form for display on a dedicated digital meter or to  NMEA 2000 or the older NMEA 0183 standard for display on compatible instruments. I have a Garmin GFL NMEA2000 converter and my Garmin plotter and GMI display will display this in analogue and / or digital form, the cheaper GNX display will not.


ALWAYS carry spare preliminary and primary fuel filters and associated seals, preferably more than one, especially for the preliminary, if you get fuel bug you will need them. Make sure you know how to change them and how to bleed the engine after to get the engine working again.

Ensure you can access the filters easily in harbour and at sea, when I acquired Sancerre the preliminary filter was well behind the engine and accessible only with difficulty after a lot of wood work, drawers and the companion way steps were removed. It is now in a cockpit lock as shown above.

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