Running short of fuel is rarely caused by one dramatic mistake. More often, several small assumptions stack up: the gauge looked healthy, the return journey was expected to be quicker, the tide was stronger than forecast or the fuel berth was closed when you arrived.
Good fuel planning removes most of that uncertainty. Before leaving, you should know roughly what your Motorboat uses, how much fuel is actually available, what the complete trip is likely to require and how much you want left if the plan changes.
Know how much usable fuel you actually have
Start with the fuel capacity of the boat, but do not automatically assume every litre in the tank is usable.
Tank shape, fuel pickup position and the attitude of the boat can affect how much fuel the engine can actually draw. On an unfamiliar Motorboat, check the manufacturer’s information and avoid planning around the absolute maximum capacity alone.
Fuel gauges also deserve some caution. They are useful indicators, but readings can change as the boat moves or the fuel shifts around the tank. A gauge showing half a tank does not necessarily mean half your original range remains.
The more familiar you become with your own boat, the easier it is to compare the gauge with fuel added and engine hours rather than treating the display as the only figure that matters.
Work from real fuel consumption
Manufacturer figures can be helpful when you first acquire a boat, but your own records become more useful over time.
Note how many litres you add after a trip and how many engine hours you have run. If your electronics provide reliable fuel-flow data, record that too.
Consumption will change with:
- Cruising speed
- Hull condition
- Boat loading
- Sea state
- Wind
- Tidal stream
- Engine condition
- Time spent manoeuvring at low speed
A Motorboat that produces one figure in flat water with two people aboard may use noticeably more fuel when heavily loaded or working into poor conditions.
Using an average based on your normal boating gives you a much better starting point for planning.
Calculate for the whole trip
Do not calculate fuel only to the destination.
Include the return journey, leaving and entering marinas, reduced-speed areas and any expected waiting time. Then consider what could reasonably make the trip longer.
If your normal fuel consumption at cruising speed is 25 litres per hour and you expect four hours of engine running, the basic requirement is 100 litres.
That is only the starting figure.
You still need enough fuel for a sensible reserve and any realistic changes to the passage.
Fuel should therefore sit alongside weather, tide and route choice in your wider passage planning, rather than being checked separately just before the lines come off.
Keep fuel in reserve
A reserve is there for the journey you did not expect to make.
You may need to divert because a berth is unavailable, spend longer avoiding traffic, return by a more sheltered route or run against a stronger tidal stream than planned.
There is no single reserve percentage that works for every Motorboat and every passage. The appropriate margin depends on how accurate your consumption figures are, how far you are travelling, weather conditions and how many realistic alternatives are available.
Some boaters use the rule of thirds as a conservative starting point: roughly one third for the outward journey, one third for the return and one third retained as reserve.
It can be a useful reminder not to plan too tightly, but it should not replace proper calculation. The outward and return legs may have very different tidal conditions, speeds and fuel requirements.
A more useful test is simple: if reaching the planned destination already depends on using the fuel you intended to keep in reserve, the plan needs changing.
Allow for weather and tide
Fuel consumption and passage time are closely connected to conditions.
A favourable tidal stream may increase speed over the ground without requiring more engine power. The same passage against the stream can take longer.
Sea state matters as well. A rough return may require repeated speed changes or a different route. You might slow down considerably for comfort and control, increasing the total engine-running time even if the hourly fuel consumption falls.
Wind against tide can also create short, uncomfortable seas in areas of the Solent and South Coast, which may change both your speed and route.
Check the conditions expected for the return journey rather than basing the fuel calculation solely on what you see when leaving the berth.
If the forecast makes the later part of the trip uncertain, carrying a larger fuel margin gives you more options.
Know where you can refuel
Do not wait until fuel is becoming a concern before deciding where to stop.
Check fuel availability while planning the route. Look at opening hours, access, depth restrictions where relevant and whether there is another realistic option nearby.
A fuel berth can be occupied, temporarily unavailable or awkward to approach in the conditions when you arrive. Your plan should allow you to leave and use another option rather than making the manoeuvre because you have no fuel margin left.
Around the Solent, fuel facilities may appear close together on the chart, but that does not mean each one will suit your boat or be available at the time you need it.
Refuelling earlier can sometimes be the better decision simply because it keeps the rest of the day flexible.
Check fuel before leaving the berth
Fuel belongs in the pre-departure routine.
Confirm the amount aboard, check that the correct tank is selected where relevant and look for any obvious signs of leakage around accessible fuel lines and connections.
If the boat has fuel filters or water separators intended for routine inspection, follow the engine manufacturer’s guidance.
Do not ignore signs of contamination or an unexplained change in fuel consumption. An engine that is using considerably more fuel than normal, running poorly or repeatedly collecting water in the fuel system needs investigating.
That links directly with the wider engine checks before departure that help identify problems while the boat is still alongside.
Keep a simple fuel log
One of the most useful things a Motorboat owner can do is keep basic fuel records.
You do not need an elaborate spreadsheet. Record:
- Engine hours
- Fuel added
- Approximate distance travelled
- Usual cruising speed
- Anything unusual about the conditions
After several trips, patterns become much clearer.
You may discover that one cruising speed gives noticeably better range, or that the boat uses considerably more fuel with a full crew aboard. You can also spot unusual consumption more easily because you have a normal figure to compare it with.
These records become particularly useful when planning longer passages or deciding whether a planned fuel stop is actually necessary.
Refuel while you still have options
There is little advantage in running a Motorboat close to the limit of its range simply because the tank should contain enough fuel on paper.
Fuel planning should leave room for the parts of boating that cannot be predicted perfectly: weather, tide, traffic, delays and changes of destination.
Know what your boat normally uses, calculate for the complete journey and keep a reserve that reflects the passage you are making.
The right time to refuel is before the remaining fuel starts dictating the decisions you have to make.
FAQs about planning fuel for a motorboat trip
How can you plan fuel for a motorboat you have not used before?
Start with the manufacturer’s available consumption information and allow a cautious margin rather than assuming the most economical figure will apply. Shorter initial trips can then help you establish how the boat performs with your usual cruising speed, loading and operating conditions.
Should generator fuel be included in your passage fuel calculation?
Yes, if the generator draws from the same fuel supply as the engines. Domestic equipment can therefore reduce the amount available for propulsion, particularly during longer trips or overnight stays. Understand which tank supplies each system before calculating the fuel available for the passage.
How should you plan fuel on a motorboat with more than one tank?
Know the capacity and usable fuel in each tank as well as how the fuel system is arranged. Do not assume that the combined total can always be accessed in every configuration. Familiarise yourself with tank selection and any transfer arrangements fitted to your particular boat.
What should you do if the fuel gauge and fuel-flow display disagree?
Treat the difference as something to investigate rather than simply choosing whichever reading is more reassuring. Compare both figures with fuel added, engine hours and previous consumption records. A persistent or significant discrepancy may need checking before you rely on either display for a longer passage.
Is it better to calculate a fuel reserve in litres or engine-running time?
Both can be useful. A reserve expressed in litres shows how much fuel remains, while thinking in terms of additional engine-running time can make it easier to judge what that reserve would actually allow you to do. Whichever method you use, base it on realistic consumption for your boat and the conditions expected.