Fuel planning is more than a full tank. Consumption has to be estimated realistically, a reserve set aside, and fuel stops placed along the route; on a motor yacht in particular, the sum decides the safety of the passage.
How consumption is worked out
- Maker's curve
- The engine maker's consumption curve gives litres per hour at cruising rpm.
- Sea trial
- Actual consumption over a known distance at known rpm is the most reliable figure.
- Logs
- Fuel taken on and engine hours on earlier passages.
- Per mile
- Litres per hour divided by speed at that rpm. The same boat burns very differently per mile at different rpm; the economical cruising rpm is found this way.
What increases consumption
- Headwind, sea and current.
- A foul bottom and propeller.
- Load: full water tanks, equipment, people.
- Running the generator throughout.
Reserve and refuelling
A common approach at sea is the rule of thirds: a third to get there, a third to return or divert, a third in reserve. On a long route, fuel stops are set in advance with the fuel berth's hours and depth.
Before departure
- Tank level. Gauge compared with a dipstick or the filling record.
- Filters. Spare filters aboard; water separator drained.
- Tank bottom. On a boat left standing, check for water and sediment.
- Refuelling plan. Fuel stops along the route, and alternatives.
How is the reserve set?
A common habit on motor boats is to divide fuel into thirds: one third out, one third back or for diversion, one third reserve. The rule does not fit every route exactly, but it shows well why a reserve is needed. Running into wind and sea, current, a diversion to a refuge or an unexpected wait raise consumption quickly above plan.
Where the consumption figure comes from
The most reliable data is the boat's own record: every refuelling and the engine hours run in between. Maker's figures give an idea, but the boat's weight, bottom condition and propeller set-up change real consumption. On a boat without records, readings at different rpm on a sea trial and watching tank levels provide a starting figure.
Choosing speed
On motor boats, consumption rises disproportionately with speed. A boat that can plane may use far less fuel per mile when slowed to displacement speed. On a long delivery, a small drop in speed increases range markedly and can remove the need for a fuel stop.
Fuel stops
Ports where fuel can be taken along the route are set in advance, and their opening hours and suitability for the boat's draft are checked. In some areas fuel points are sparse and may be closed out of season.
Fuel quality
To be sure fuel taken at an intermediate port is clean, the filter bowl is watched after refuelling; if water or sediment appears, action is taken early.
In the calculator
The delivery calculator uses standard consumption assumptions; for your boat, your own consumption figures give a better estimate, and we use them in the written quote.
Under way, tank level, rpm, speed and sea state are logged at intervals. If consumption runs above plan, speed is reduced or the next fuel stop brought forward; the decision is made when the difference first shows, not when fuel runs low.
The plan allows for weather waits and diversions; the more realistic it is, the fewer surprises at sea.
Deciding early keeps the reserve intact.