The pump price isn't the number that matters
Fill a 100-gallon tank at a fuel dock charging $5.00 a gallon and the receipt says $500. That number scares more new boaters than anything else about the sport, and it's the wrong number to fixate on. A fill isn't a trip — the tank is a reservoir you draw down over days or weeks. What boating costs is set by the gallons you burn, and the gallons you burn are set by your boat and your throttle, not by the size of the tank.
Two numbers do the real work of answering what it costs to fuel a boat. The first is cost per hour — what an hour of running costs at a given throttle setting. The second, and the more useful one, is cost per nautical mile — what covering distance costs. Both fall out of figures you can measure on your own boat: burn rate in gallons per hour (gph), speed in knots, and the price on the pump. This guide works through both, with honest numbers, and shows why the throttle — not the fuel dock — is where most of the money actually goes.
Marina fuel vs. road fuel: the premium is real
Fuel at the dock costs more than fuel at a road station, reliably and nearly everywhere. At typical US coastal marinas, expect gasoline and diesel to run roughly fifty cents to two dollars a gallon over the local street price; remote docks, resort harbors, and anywhere fuel arrives by barge can run higher still. Prices also vary from one harbor to the next far more than road stations do, so it pays to look before you tie up.
The premium isn't mostly gouging. A fuel dock may sell in a year what a busy highway station sells in a few weeks, so its fixed costs spread across far fewer gallons; over-water storage tanks, spill containment, and dock staff on payroll all get priced into the gallon; and many docks sell ethanol-free gasoline, which costs more at road stations too — part of the gap you see is apples-to-oranges.
What you can do about it depends on your boat. Trailer boats can fill at road stations on the way to the ramp — plenty of owners do, using ethanol-free fuel where the engine maker recommends it. Bigger boats can shop docks: on a 300-gallon diesel fill, a 75-cent spread between two harbors is $225. But be honest about the ceiling on all of this. The price per gallon is a lever you barely control; gallons burned is the one you do. And whichever price you pay, use that price in your math — a plan built on street-price arithmetic and dock-price fills flatters itself.
From price per gallon to cost per hour and per mile
Cost per hour is the easy one: multiply your burn rate by the price. GPH times price per gallon equals cost per hour. A boat cruising at 14 gph on $5.00 marina gas is spending $70 an hour. The same boat at wide-open throttle burning 30 gph is spending $150 an hour. A trawler easing along at 3 gph on $5.50 diesel is spending $16.50. Cost per hour is the right number for time spent running the engine without going anywhere in particular — but time isn't what a trip is made of. Distance is.
For distance, you need your fuel efficiency in nautical miles per gallon: nmpg equals speed in knots divided by gph. Run 28 knots on 14 gph and you're getting 2.0 nmpg. Cost per nautical mile is then just the fuel price divided by nmpg — $5.00 a gallon at 2.0 nmpg is $2.50 per nautical mile. From there a trip estimate is one more step: distance divided by nmpg gives gallons, gallons times price gives dollars, plus a few gallons of allowance for the idling and maneuvering that earn no distance.
All of it leans on knowing your boat's real burn — measured, not brochure. If you don't have that number yet, our guide to how much fuel a boat uses covers getting it, either fill-to-fill at the pump or off a fuel-flow gauge. The dollar math is only as honest as the gph you feed it.
Two worked examples
The numbers below are illustrative — chosen to show the method, using $5.00 a gallon for marina gasoline and $5.50 for marina diesel as working figures. Your dock, your boat, and your season will land somewhere of their own; the arithmetic won't change.
A mid-size center console at 2 nmpg
Take a 26-foot center console with a single 300 hp outboard and a 100-gallon tank. At an efficient cruise it runs 28 knots on 14 gph — 2.0 nmpg. On $5.00 gas that's $70 an hour and $2.50 per nautical mile.
Now price a normal day: a 60 nm round trip to the grounds and back, plus about 3 gallons of idling, trolling, and no-wake work. The running legs take 60 divided by 2.0, or 30 gallons; with the allowance, call it 33 gallons — about $165 for the day. Note what the tank size had to do with it: nothing. Whether the fill that morning came to $150 or $500, the trip cost $165.
Run the same day in a hurry and the price moves fast. At 40 knots the burn is around 30 gph, which is 1.33 nmpg — $3.75 per nautical mile. The same 60 nm now takes 45 gallons of running fuel; with the same 3-gallon allowance, that's 48 gallons and about $240 for the day. The throttle bought back 39 minutes (2 hours 9 minutes at 28 knots versus 1 hour 30 at 40) and it cost $75 — roughly two dollars a minute. Sometimes that's worth it. The point isn't never to run fast; it's to know what fast costs, so it's a decision instead of a surprise.
A displacement trawler: a gallon a mile, or less than half of it
Now a 40-foot displacement trawler with a single diesel and about 36 feet of waterline, which puts hull speed near 8 knots. Pushed right up at hull speed she burns around 7 gph to make 8 knots — 1.14 nmpg, close enough to a gallon a nautical mile. On $5.50 diesel that's $38.50 an hour and about $4.80 per mile: sportfish money out of a boat the type promises will be cheap.
Throttle back to 6.5 knots and the same hull needs only about 2.5 gph — 2.6 nmpg, $13.75 an hour, roughly $2.10 per mile. That isn't a rounding difference. It's the same boat, the same day, at well under half the cost per mile.
Over a 100 nm passage the spread is stark: about 87 gallons and $480 pushed at hull speed, against roughly 38 gallons and $212 at 6.5 knots. The slow passage takes 15.4 hours instead of 12.5 — call it three extra hours to keep $270. Most trawler owners take that trade without blinking, which is the entire point of the type. The lesson generalizes: on a displacement hull, the last knot before hull speed is the most expensive fuel you'll ever buy.
Slowing down is the biggest cost lever
Both examples are the same curve wearing different numbers. Fuel burn doesn't rise in a straight line with speed — it rises on a convex curve, steeper than linear, because the drag a hull has to overcome grows with roughly the square of speed and the power to push through it with roughly the cube. Near hull speed on a displacement boat the wall is steeper still. Since cost per mile is gph times price divided by knots, wherever burn climbs faster than speed — which is most of the throttle range — each extra knot costs more per mile than the one before it. That convexity is also why easing off the top hands money back in outsized chunks: the first couple of knots you give up are the most expensive ones you were buying.
One honest caveat before you treat slower as always cheaper: on a planing hull it isn't. Efficiency improves as you come down toward the sweet spot just above the speed where the hull holds plane. Pull back past that and the boat falls off plane into a bow-high plow that burns more per mile, not less. If you truly need to go slower than efficient cruise, come all the way down to a level displacement speed instead of wallowing at the hump. Displacement hulls are simpler — slower is genuinely cheaper the whole way down. Either way, know your boat's best-nmpg cruise and, when the budget or the tank is tight, run at it.
It's worth noticing that cost per mile and range are the same arithmetic with different units on the answer. The throttle setting that stretches your dollars is exactly the one that stretches how far your boat can go on the fuel aboard — cheap miles and long range come from the same place on the curve.
Budgeting fuel for a season
A season budget is trip math run forward, and there are two honest ways to do it:
- By hours: expected engine hours times your average gph times price. Your average over real days — idling, trolling, no-wake zones, cruise — runs well below your steady cruise burn; the 14-gph-at-cruise center console above might average 8 or 9 gph across a season of mixed running. A 100-hour season at 9 gph and $5.00 is $4,500.
- By trips: the outings you realistically expect times your typical trip cost from the log. Twenty-five days like the $165 example above is about $4,100.
- Sanity-check one against the other. The two methods should land in the same neighborhood — if they don't, one of the inputs is a guess dressed up as a number.
- Add a band, not a point. Fuel prices move year to year and within a season, and a fouled bottom pushes burn up in the back half of it. Budget the estimate plus 15 to 20 percent and the bad case is planned for instead of discovered.
Log your fills and the cost history builds itself
Both season methods are only as good as the per-trip figure they're built on, and that figure comes from the pump, not a spreadsheet. Every fill-up hands you a perfect data point: real gallons and real dollars, printed on the receipt. Pair each one with the engine hours and nautical miles run since the previous full fill — the discipline a ship's log makes routine — and you have your actual cost per hour and per mile — averaged over the way you really use the boat, with the idling and the chop and the full load included, which no calm-day flow-meter reading can claim.
The discipline is the same fill-to-fill routine used for measuring burn: top off to the same automatic click each time, write down gallons, price, hours, and distance, and let several intervals accumulate before you trust the average. Treat flattering intervals with suspicion — a fill that implies a wildly low burn usually means the tank didn't top off the same way, not that the boat found free fuel. Keep the pessimistic numbers. After a season, the log turns "roughly $2.50 a mile" into your number, per trip and per season, and next year's budget stops being a guess and becomes arithmetic.
