There is a particular kind of silence that falls over a cockpit when the fuel gauge dips below a quarter and the next harbour is still forty miles away. For UK offshore powerboating, whether you are running a twin-outboard RIB across the Irish Sea or a diesel sportscruiser down the Channel, fuel management is not just about economy. It is about safety, passage planning and knowing your boat intimately.

Know Your True Fuel Burn

Manufacturer figures are a starting point, not a promise. A 250hp outboard might claim 1.2 litres per nautical mile at 30 knots, but add a following sea, four crew, a full load of gear and a fouled hull, and that figure can rise by 20 to 30 per cent. Before any long passage, run a measured mile or use a flow meter to establish your real burn at different rpm bands. Note the difference between planing cruise (often 3,500–4,500 rpm), fast cruise and wide-open throttle. Wide-open throttle can double your consumption per mile. For diesel engines, the same applies: a 300hp inboard at 2,800 rpm might sip 40 litres per hour, while at 3,400 rpm it gulps 75.

Calculate Range With a Realistic Reserve

Range is not tank capacity divided by best-case burn. Work out your usable fuel—typically 90 per cent of tank capacity, as you never want to run a diesel dry or starve a petrol engine in a seaway. Then apply a reserve. For offshore work, a 30 per cent reserve is sensible; 20 per cent is the absolute minimum for coastal hops with multiple bail-out options. Rule of thumb: plan to arrive with one-third of your fuel remaining. That gives you margin for weather, a missed harbour entrance or a shift to an alternative port.

• Example: 400-litre tank, 90 per cent usable = 360 litres. At 1.5 litres per nautical mile, safe range with 30 per cent reserve is about 168 nautical miles.
• If you must push into a head sea, reduce that by a further 15 per cent.
• Carry a written fuel plan with legs, distances, expected burn and alternate stops.

Plan Refuelling Stops Around the UK Coast

The UK has a wonderful but uneven network of refuelling points. The Solent, Plymouth, Falmouth, Milford Haven, the Clyde and the east coast ports are well served, but stretches like the Bristol Channel, the north Cornish coast and the Scottish islands demand careful thought. Tidal access is the big one: many harbour fuel berths dry or have limited windows. Check tide tables and fuel dock opening hours before you leave. Always phone ahead to confirm fuel availability and payment methods. Some small harbours only take cash or have a minimum quantity. Petrol is less common than diesel, so plan those stops meticulously. If you carry jerry cans, secure them properly and decant through a filter funnel.

Monitor Consumption Underway

Once on passage, watch your burn like a hawk. A fuel flow meter is the single best investment for long-range powerboating; it gives instantaneous and cumulative consumption. Without one, log engine hours and refill volumes at every stop to build a running average. Note how trim, sea state and heading affect burn. A head sea can add 10–15 per cent; a following sea can reduce it. Reset your trip log at each refuel and calculate remaining range every hour. If your burn rises above plan, slow down. Dropping from 30 knots to 24 knots can cut fuel consumption per mile by a quarter, at the cost of only a modest increase in passage time.

Maintenance and Contingency

Fuel problems offshore are almost always preventable. Water in the fuel, clogged filters and degraded petrol are the usual culprits. Service your water separator and primary filter before every long passage. Carry spare filters and know how to change them in a rolling sea. For petrol engines, use fresh fuel and consider a fuel stabiliser if the boat sits between trips. Diesel bug is a real threat; keep tanks full to reduce condensation and use a biocide as recommended. Carry at least 20 per cent of your fuel capacity in approved jerry cans for true offshore passages. And have a plan for rationing: if the weather turns, reduce speed, motor-sail if you have sails, or divert to the nearest safe harbour. Running dry is not just inconvenient; it is dangerous.

With honest burn figures, a sensible reserve and a flexible refuelling plan, you can explore the UK’s coast with confidence. The sea rewards preparation. Know your range, watch your consumption, and keep a little in the tank for the unexpected.