The short, honest answer
If you're asking how much fuel your boat uses, you already have the right instinct. Fuel is the one consumable that decides whether a good day on the water stays a good day, and it's the number most likely to get a boater into trouble when it's guessed instead of known.
The honest answer is that it depends. But “it depends” isn't useful on its own, so it helps to separate two different numbers. The first is a ballpark from published rules of thumb, which is fine for a quick sanity check. The second is your boat's own measured burn, which is what you actually plan with. This guide covers both, and how to turn the first into the second.
What GPH means (and why MPG matters more)
GPH is gallons per hour: the rate your engine consumes fuel at a given RPM and speed. Hold a steady throttle and you'll settle into a steady GPH. It's the number fuel-flow gauges show you, and it's the easiest way to compare one RPM to another.
But time isn't what runs out on a passage; distance is. To plan a trip, convert GPH into miles per gallon, or nautical miles per gallon (nmpg) for coastal and offshore work. The relationship is simple: nmpg equals your speed in knots divided by your GPH. Run 24 knots while burning 12 gph and you're getting 2 nmpg.
That nmpg figure is what sets your range. Multiply your usable fuel by your nmpg and you get the distance you can realistically cover. Note usable fuel, not total tank capacity: you can't safely draw the last gallons off the bottom of a tank, and gauges get vague and optimistic down low.
Why there's no single answer
Fuel burn is the sum of a lot of moving parts. Change any one of them and the number moves, which is why two identical-looking boats at the same dock can post very different consumption:
- Boat size and weight. More displacement to push means more fuel. The same hull loaded with full water, fuel, crew, gear, bait, and ice burns noticeably more than it does running light.
- Hull type. Planing hulls burn a lot climbing onto plane, then settle down once they're up. Displacement hulls, like trawlers, are effectively capped near hull speed and sip fuel below it. Semi-displacement boats live in between.
- Speed. This is the single biggest lever. Once a planing hull is up there's a sweet spot just above the threshold where it's most efficient; push past that toward the top of the range and drag climbs steeply, so each extra knot costs outsized fuel. Displacement hulls hit the same steep penalty as they approach hull speed. Either way, easing a couple of knots off the top usually buys a lot of range.
- Load and trim. A bow-high attitude or too much weight aft plows water and wastes fuel. Trim tabs and motor trim dialed in for the conditions can hand a chunk of it back.
- Sea state and wind. Head seas, chop, and a headwind or foul current all push your burn up. A following sea and fair current help. The trip out and the trip back are rarely the same number.
- Bottom growth. A fouled hull and running gear add drag. A season's worth of slime and barnacles can measurably cut your MPG without you noticing day to day.
- Prop and engine condition. Wrong prop pitch, a dinged blade, or a tired, poorly maintained engine all move the number, usually the wrong way.
Rules of thumb: use them as a sanity check, not gospel
You can estimate a rough ceiling straight from horsepower. Every engine has a brake specific fuel consumption (BSFC), measured in pounds of fuel burned per horsepower per hour. Combine that with the weight of the fuel and you get gallons per hour: GPH is roughly (BSFC × horsepower) divided by the weight of a gallon. Gasoline weighs about 6.1 lb per gallon; diesel about 7.1.
For a gasoline engine near wide-open throttle, BSFC runs around 0.45 to 0.50 lb/hp-hr, which works out to roughly horsepower × 0.08 gph. As a rule of thumb, figure about a gallon per hour for every 10 to 12 horsepower flat out. So a 300 hp gas outboard at WOT lands somewhere near 25 to 30 gph.
Diesels are more efficient. A marine diesel near full load typically runs a BSFC around 0.38 to 0.42 lb/hp-hr; with heavier fuel that works out to roughly horsepower × 0.055 gph, or about a gallon per hour for every 18 or so horsepower. A 300 hp diesel flat out lands near 15 to 17 gph.
Read those with two big caveats. First, they're wide-open-throttle figures, which is a ceiling, not your cruise number; at an efficient cruise you'll burn well under them. Second, your engine is not the textbook. Age, tune, prop, and altitude all shift it. Treat rules of thumb as a way to check that your measured numbers are in the right ballpark, never as the plan itself.
Rough ballparks by boat class (illustration only)
These are wide, illustrative ranges to set expectations, not values to plan a trip with. Your boat, your load, and your conditions will land somewhere of their own:
- Small single-outboard center console or bay boat (150 to 250 hp): very roughly 4 to 12 gph at cruise.
- Mid-size gas cruiser or express with twin engines: roughly 20 to 40 gph at cruise.
- Offshore center console with triple or quad outboards: roughly 25 to 60-plus gph at cruise.
- Semi-displacement diesel cruiser: roughly 8 to 20 gph, heavily dependent on how fast you push it.
- Full-displacement trawler at hull speed on a single diesel: often just 2 to 6 gph, which is the entire point of the type.
How to estimate your boat's real burn
The only number worth planning with is the one you measure on your own boat. There are three good ways to get it, and they work best combined.
Fill-to-fill (works on any boat)
Top the tank off at the fuel dock, using the same pump position and the same automatic “click” shutoff so your starting point is consistent. Then run the boat the way you actually run it: a normal mix of cruising, idling, and maneuvering, not a special economy run.
Note your engine hours or the distance covered for the outing. At the next fill-up, top off again to the same click. Gallons added divided by hours run is your real average GPH. Nautical miles covered divided by gallons added is your real nmpg. Do it over a handful of trips and you'll have a working average that's far more trustworthy than any brochure.
Fuel-flow at cruise
If you have digital engine gauges or a NMEA 2000 fuel-flow sensor, you can read GPH directly. Settle at your normal cruise RPM in reasonably flat water and note the steady-state GPH alongside your speed. That's your baseline cruise burn and the nmpg you'll plan around.
Flow meters are excellent for comparing RPMs and finding your sweet spot, the cruise setting with the best nmpg. They can also drift over time, so cross-check what they tell you against the fill-to-fill method now and then.
Track it over trips
One reading from one calm day isn't the whole story. Log your burn across a range of trips and conditions: loaded versus light, calm versus snotty, clean bottom versus late-season growth. Keep the pessimistic numbers, not the flattering ones.
Planning around your bad-day burn instead of your best-day burn is what keeps a range estimate honest, because the day you lean on it hardest is rarely a flat-calm one.
Always plan with a reserve: the one-third rule
Even a well-measured average is still an average. The day you need range the most, building seas, a headwind on the way home, a longer route around weather, is exactly the day your burn runs high. That's what reserve fuel is for.
The time-tested guideline is the one-third rule: one-third of your fuel to get out, one-third to get back, and one-third held in reserve. It's simple, conservative, and it has kept boaters out of trouble for generations.
Treat it as a floor, not a target. Add margin for a fouled bottom late in the season, for a headwind or foul current on the return, and for the fact that fuel gauges and tank senders are notoriously optimistic near empty. Reserve isn't fuel you wasted. It's the fuel that gets you home when the plan doesn't go to plan.
