Burn is a curve, not a number
Most fuel calculators take one gallons-per-hour figure and multiply. Real boats don't work that way: fuel rate climbs steeply and non-linearly with speed, and the fuel you burn per mile is U-shaped — poor at a crawl, best near an efficient cruise, and punishing toward the top of the throttle. A single flat number applied to every speed can be off by two to four times.
So Far Enough fits a convex curve through whatever it knows about your boat — one point at minimum — and reads the burn at the speed you actually plan to run. The curve's shape comes from your hull type: planing hulls, semi-displacement, and displacement boats each climb at a different steepness, which is why the same throttle change costs them very different fuel.
One deliberate safety rule is baked into that curve. Below your reference speed, a naive curve would claim a planing hull gets more economical per mile as it slows off plane — the opposite of reality, where the boat falls into a bow-high, plowing attitude that burns more per mile, not less. The model floors the burn there so slowing down can never look like free range on a planing or semi-displacement hull. Displacement hulls, which genuinely do sip less as they slow, keep the honest gain.
The horsepower physics estimate
If you've never measured your burn, the app can still give you a defensible starting point — from your engine. Every engine has a brake-specific fuel consumption (BSFC): the weight of fuel it burns per horsepower per hour. Marine gas four-strokes run around 0.50 lb/hp-hr (two-strokes richer, around 0.60), diesels around 0.40. Combine that with the weight of a gallon — about 6.1 lb for gasoline, 7.2 for diesel — and a typical cruise load of roughly 70% of rated power, and horsepower alone yields a believable cruise gph.
This estimate is deliberately labeled what it is: physics, not measurement. It gets the widest uncertainty band of any source in the app (more on the band below), and the moment any real data about your boat exists — a logged fill-up, a fuel-flow reading — it steps aside.
Conditions push the estimate the honest way
A calm-water number is a best case, so the model adjusts for the day — and every adjustment moves in the direction that costs you fuel when it matters:
- Sea state raises the burn (up to roughly 25% in rough water) and slows your speed through the water at the same time — so the same distance takes more hours at a higher rate, exactly as it does on the boat.
- Wind and current are applied per leg, and the return leg is computed separately from the trip out. The breeze and current that helped you out are flipped against you coming home — the canonical way boaters get stranded — and the app warns you when the way back burns meaningfully more.
- Hull fouling and a heavy load each add their own multiplier, because a season of growth and a full cockpit both quietly raise the bill.
- Unknowns coerce conservative. If a conditions input is missing or unreadable, the model doesn't guess kindly: an unknown sea state is treated as rough, an unknown hull condition as fouled, a missing reserve setting as the recommended 20%. Bad data can make the estimate more cautious — never more optimistic.
Fill-to-fill calibration distrusts flattering data
The best burn number is the one your own boat writes. Log a fill-up to full with your engine hours and the app has a real, independent measurement: gallons added divided by hours run. As those fill-to-fill intervals accrue — weighted toward the most recent — they become a measured average that outranks both the physics estimate and the rated number you typed.
But calibration is built to be hard to fool, because on a range tool an understated burn is the dangerous direction. Your rated (or physics) figure acts as a floor: if your measured burn comes in higher than rated, the app believes it immediately and fully. If it comes in lower — the flattering direction — the estimate only eases down gradually, as several consistent intervals earn it.
Suspicious fills are deliberately distrusted. An interval whose implied burn is physically implausible, or far below your rated figure — the signature of a typo in the hours or gallons — is dropped from the average entirely rather than allowed to quietly stretch your range. Slow-trolling intervals are excluded too, since a day of idling would understate what a cruise leg really burns. And with no rated figure to anchor against, a single fill is never trusted on its own; the app waits for a second before it calibrates at all.
The go/no-go reads the pessimistic end
No fuel model deserves to present a single precise number, so Far Enough doesn't. Every estimate carries a confidence band whose width depends on where the burn figure came from: a live fuel-flow reading off the boat's own network earns the tightest band (a real measurement — though still one noisy sensor's point on the speed curve, so it keeps a margin), a measured speed-and-burn table is the tightest of any source you enter by hand, a calibrated average tightens as real fill-ups accrue but never claims full precision, and the physics or rated-number fallbacks stay wide no matter what — logging fill-ups without engine hours is not calibration, and the band won't pretend it is.
Here's the part that matters: the go/no-go verdict is computed from the high end of that band — the pessimistic fuel estimate — against the fuel actually aboard, with your reserve already held out and untouchable. A trip only reads as a go if it clears the cautious math, and "tight" is shown as tight, never rounded up to comfortable. There is a separate, clearly-labeled line for what a full tank would change, but the headline verdict always reflects the fuel you really have.
What it does not do
An honest tool states its limits plainly. Far Enough is a planning aid: it is not a chartplotter or navigation system, and it is not a substitute for official charts, working fuel gauges, or prudent seamanship. It doesn't track your voyage, notice you're overdue, or call for help — that work belongs to a real person ashore holding your float plan. Weather and tide figures are modeled, not measured at your hull.
And even calibrated, a fuel estimate is still an estimate — engines age, bottoms foul, seas build. That irreducible uncertainty is exactly why the confidence band and the untouchable reserve exist. You're the skipper; the app's job is to make sure the fuel question gets answered conservatively before you leave the dock.