Fuel planning for offshore runs: the rule of thirds

Running out of fuel offshore is one of the most common reasons boats need a tow, and 50 miles out it can turn into something far worse. The rule of thirds is the simplest way to make sure it doesn’t happen to you.

The rule

Split your usable fuel into three equal parts:

1. One third to get out.
2. One third to get back.
3. One third in reserve.

The reserve isn’t for fishing. It’s for the things you didn’t plan for: a head sea on the way home, a long chase after a school, helping another boat, a detour around weather, or an engine running rough.

Work out your burn

You need to know how much fuel your boat uses at cruising speed. The best number comes from your own boat:

• A fuel flow meter on the engine display gives you gallons per hour (GPH) live.
• Or fill up, run a trip, and fill up again, then divide the gallons by the engine hours or the miles run.

Turn it into nautical miles per gallon:

Miles per gallon = cruising speed (knots) ÷ fuel burn (GPH)

A worked example

Say your boat has a 150-gallon tank, cruises at 25 knots and burns 20 gallons per hour.

Step	Number
Miles per gallon	25 ÷ 20 = 1.25 nm/gal
Usable fuel (don’t count the last bit in the tank)	about 135 gal
One third	45 gal
Range out (one third)	45 × 1.25 = about 56 nm

So this boat can go about 56 nautical miles out on the rule of thirds, before any fishing.

Then take off the fishing

Fishing burns fuel too, and it comes out of the “out” and “back” thirds, not the reserve.

• Trolling at 6 to 8 knots for a few hours.
• Running and gunning between schools and paddies.
• Idling and holding position on a drift or a kite.

If you expect a few hours of trolling or running around, cut your range out to make room. In the example, two hours of trolling at 6 GPH is 12 gallons, so the real range out drops to about 40 nm.

What eats into it

• Head seas. The afternoon wind usually blows from the west and northwest, so the ride home is often into it. You’ll go slower and burn more per mile.
• Load. A full bait tank, ice, fish, fuel and people all add weight.
• A dirty bottom or damaged prop.
• Running at wide-open throttle. Most boats burn far more per mile at top speed than at a comfortable cruise.

Habits that keep you safe

• Fill up before every trip, even if the gauge says you have enough.
• Know your turnaround point before you leave, and stick to it.
• Check fuel used against miles run during the day. If it’s worse than planned, turn back sooner.
• Twin engines? Plan what happens if one quits. One engine is often slower and less efficient.
• Tell someone your plan, including how far out you’ll go.

Quick table

Range out on the rule of thirds, with no fishing time, for a few common setups:

Usable fuel	Burn at cruise	Cruise speed	Range out
75 gal	12 GPH	25 kt	~52 nm
135 gal	20 GPH	25 kt	~56 nm
200 gal	30 GPH	28 kt	~62 nm
300 gal	40 GPH	26 kt	~65 nm

Use your own boat’s numbers. These are only examples.

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