Range starts with one simple equation
"How far can my boat go?" is the question hiding inside every float plan, and the math that answers it is refreshingly simple. Cruising range equals your usable fuel multiplied by your nautical miles per gallon. Two numbers, one answer — though, as you'll see, the honest version of it is the theoretical ceiling, and the number you plan around is smaller.
The nmpg half is your fuel efficiency at a given speed — knots divided by gph. We walk through how to find and measure it in our guide to boat fuel consumption, so we won't re-derive it here. If you've already logged your boat's real burn, you're holding the harder of the two numbers.
The trap is what you do with the other one. Reach for total tank capacity and multiply, and you'll get a big, comforting figure that no careful skipper would ever plan a trip around. The number that keeps you out of trouble is smaller, and this is how you get to it.
Usable fuel is less than your tank size
Range math tempts you to plug in the capacity printed on the tank. Don't. What sets your range is usable fuel — the amount you can actually, and safely, draw and burn. That's always less than the nameplate number, for a few practical reasons:
- The fuel pickup sits above the very bottom of the tank, so the last inch or two is unreachable by design.
- Water and sediment collect at the bottom, and that's the last thing you want feeding the engine as the tank runs down.
- In any kind of chop the fuel sloshes and the pickup can suck air well before the tank is truly empty.
- Fuel gauges and tank senders get vague and optimistic near E, right where accuracy matters most.
A rough working figure
As a rough starting point, plan on no more than about 90 percent of stated capacity as usable — and treat that strictly as an unverified ceiling, not a safe assumption, because plenty of tanks give up noticeably less. The honest way to pin it down is to note what your gauge reads at various levels and compare it against how many gallons the pump actually takes to fill from there; do that a few times and you'll know your real usable number instead of guessing at it. Until you have it, lean low and let your reserve carry the difference.
Theoretical range vs. the range you can actually use
Multiply usable fuel by your nmpg and you get theoretical range: the distance to bone dry, in ideal conditions, running your efficient cruise. It's a useful ceiling and a terrible plan, because it assumes you'd coast into the dock on the last cupful of fuel with nothing left for a surprise.
Safe range is that number minus a reserve you never touch. The time-tested way to set it is the one-third rule — one-third of your usable fuel to get out, one-third to get back, and one-third held in reserve. So the distance you can actually travel and still get home is two-thirds of your usable fuel, converted to miles.
That held-back third isn't slack in the plan. It's the fuel for the headwind that wasn't forecast, the detour around a squall, the day the engine isn't running its best. Plan around your reserve, not through it, and it's there when you need it most.
Slow down and your range grows
Speed is the single biggest lever on range, and it's worth understanding why. Because range is usable fuel times nmpg, and nmpg swings hard with speed, a small change in throttle moves your range a lot — often more than boaters expect.
Every hull has a sweet spot: the cruise setting that returns the most nmpg. Push past it toward the top of the rev range and drag climbs steeply, so each extra knot costs outsized fuel and quietly shrinks your reach. Ease a couple of knots off the top and the range you get back is usually far larger than the time you give up.
Where that sweet spot sits depends on the hull. A displacement boat pays a steep penalty as it approaches hull speed — near the limit the last knot can climb toward doubling the burn and cutting the miles you get per gallon roughly in half. A planing hull is least efficient — most fuel per mile — while plowing at the transition hump, and its nmpg is poor again at wide-open throttle; its best cruise is the setting just above where it settles onto plane. Either way, the move is the same: know your best-nmpg cruise, and when range is tight, slow down to it or below.
Conditions quietly eat the rest
Even a carefully measured nmpg is a fair-weather number. The real world subtracts from it, and the subtraction is largest on exactly the days you're leaning on your range the hardest:
- Head seas and chop force the hull to work harder for every mile.
- A headwind does the same, especially on a boat with a lot of freeboard or a tall tower.
- A foul current is pure lost ground — you burn fuel making way you don't keep.
- A heavy load of fuel, water, crew, gear, and ice pushes the burn up all day.
- A fouled bottom late in the season adds drag you can't see from the helm.
The return is the expensive leg
The most important thing conditions teach you is that the trip out and the trip back are rarely the same number. A fair current and a following sea on the way out can flip to a foul current and a head sea on the way home — and that harder, thirstier leg lands right when your tank is at its lowest. Plan the return as the costly one, not the trip out.
Think in range rings, not one big number
A single range figure invites a one-way mistake: you spend it all reaching a spot and then discover you can't get back. The better question is how far out you can go and still return with your reserve intact. That distance is your radius of action, and it's roughly half of your safe travel distance — with the one-third rule, about (usable fuel divided by three) times your nmpg.
Drawn on a chart, that's a ring around your departure or fuel point. Anything inside the ring you can reach and round-trip with your reserve untouched; anything outside means you refuel along the way or you don't go. It turns an abstract number into a line you can actually navigate against.
Real range rings aren't tidy circles. Wind and current stretch them — you can always reach farther downwind and down-current than you can claw your way back upwind and up-current. So when the conditions favor the trip out, shrink the ring in your head, because the return is the leg that decides whether the plan holds.
Two worked examples
The numbers below are illustrative, chosen to show the method — not values to plan a trip with. Run the same steps with your own measured burn and your own usable-fuel figure and you'll get an answer that fits your boat.
Gas outboard center console
Say the boat carries a 100-gallon tank, so you plan on about 90 gallons usable. At an efficient cruise it runs 28 knots and burns 14 gph, which is 28 divided by 14, or 2.0 nmpg. Multiply usable fuel by nmpg and the theoretical range is 90 times 2.0, or 180 nautical miles to dry.
Now hold a reserve. Two-thirds of 90 gallons is 60 gallons of actual travel, which at 2.0 nmpg is 120 nm on the water — a radius of action of about 60 nm out and 60 back, with roughly 30 gallons (another 60 nm) still aboard when you tie up.
Push the throttle to 40 knots and the burn jumps to around 30 gph, dropping you to 1.33 nmpg. Same fuel, but the radius of action shrinks to about 40 nm. Wanting to get there faster just cost you a third of your reach.
Single-diesel trawler
Now a full-displacement trawler with a 400-gallon tank, so plan on roughly 360 gallons usable. At a comfortable hull-speed cruise it makes 8 knots on just 3 gph — that's 8 divided by 3, about 2.7 nmpg. Theoretical range is 360 times 2.7, close to 960 nm.
Holding a third in reserve leaves two-thirds, or 240 gallons, for travel — about 640 nm on the water, a radius of action near 320 nm. That long, patient reach is the entire reason the type exists.
But nudge her to 9 knots and watch the penalty. Near hull speed the burn can roughly double to about 6 gph, nearly halving nmpg, from about 2.7 to 1.5. Theoretical range falls from 960 nm to about 540, and the radius of action drops from around 320 nm to roughly 180. On a displacement hull, the last knot is the most expensive fuel you'll ever buy.
