BoatCost estimates fuel burn from engine math, not from a brochure. The trip fuel calculator and the ownership fuel line share the same functions in lib/fuel.ts. This guide is the plain-language version, with the constants visible so you can disagree with them on purpose.
Disclosure. This is an SFC estimate, not a sea trial. Props, hull fouling, load, altitude, and a heavy right foot will move the number. Confirm with a fuel-flow meter when it matters.
The SFC formula
gph at WOT = SFC × horsepower × engines / fuel specific weight
BoatCost uses:
- Gasoline: SFC 0.50 lb/hp-hr, 6.2 lb/gal
- Diesel: SFC 0.40 lb/hp-hr, 6.9 lb/gal
A 300 hp gasoline outboard is about 24.19 gph at wide-open throttle (0.50 × 300 / 6.2). The same horsepower in diesel is about 17.39 gph (0.40 × 300 / 6.9). Those figures are in the test suite. Two engines double the result.
Cruise burn is WOT times a load factor (default 0.65). That is a planning average, not your GPS speed and not “cruise RPM” from a dealer sea trial. If you ski and tube, try 0.75. If you troll, try 0.35. The ownership and fuel calculators both expose the control.
Gasoline also shows the old rule of thumb: horsepower / 10 at WOT. A 300 hp gas engine is 30 gph on that rule and 24.19 on SFC. Use the rule as a sanity check. If your mental budget was “it sips a gallon an hour,” both numbers should startle you.
From gph to trip and annual cost
Trip fuel = cruise gph × hours × dollars per gallon.
Annual fuel on the ownership page is the same formula with hours per year. The state default gasoline and diesel prices are planning numbers. Override $/gal when you know the marina board.
Worked pattern — example inputs, not a fuel receipt:
- 300 hp, 1 engine, gasoline, load 0.65
- Cruise ≈ 15.7 gph
- 4 hours × $3.50/gal ≈ $220 for the trip
- 80 hours/year ≈ $4,400 if the price holds
A twin 300 hp gasoline pair is twice the gph. Cabin-cruiser defaults on this site start with two engines for that reason. A 30 hp diesel sailboat auxiliary is a different universe; switch type or horsepower instead of scaling a center console in your head.
Open the fuel calculator and tap a Yamaha, Mercury, or Honda preset to start. Presets only fill horsepower and brand context; they do not certify burn.
What the formula ignores
- Prop pitch and diameter
- Hull fouling and bottom paint
- Weight of people, ice, and fuel
- Wind and current
- Elevation (inland mountain lakes)
- Whether “hours” are engine hours or clock hours at the dock
Those omissions are why the page repeats not a sea trial. They are also why a 10% change in load factor is a more honest stress test than hunting a brochure gph to two decimals.
If you have a flow meter, compare a measured hour at a typical RPM to the calculator’s cruise gph. Then set load factor so they match. That calibrated load is more useful than the default 0.65.
Diesel vs gasoline without folklore
Diesel SFC and fuel weight in this model are lower burn at the same horsepower, not a promise that diesel is cheaper. Diesel often costs more per gallon, and the engine may cost more to buy and to repair. Enter the pump price you actually pay. Do not take a gasoline 300 hp number and “shave 20% because diesel is efficient” on top of the formula — the formula already used 0.40 vs 0.50.
Mixing one gas outboard and one diesel generator is outside the tool. The calculator is one fuel type at a time.
How fuel shows up in ownership
On the ownership calculator, fuel is a variable line with maintenance. Fixed costs (loan, insurance, storage, winterization, registration) do not care about gph. That is why a boat can be expensive when you stay home and still grow a large fuel line when you fish 80 hours.
Cost per outing divides the whole annual bill by outings, not fuel alone. A cheap-gph sailboat with a large slip can still lose per outing to a trailered bowrider.
See true cost of boat ownership for the fixed vs variable split, and maintenance schedule for the other variable pile.
State fuel defaults
Each state row has a gasoline and diesel planning price. They move the default bill when you open the center console hub or a page like center console in California vs center console in Texas. They are not EIA series and not a promise. Override them.
Pros and cons of SFC budgeting
Pros
- Transparent constants
- Scales with engines and hours
- Same math on fuel and ownership pages
- Easy to stress-test load
Cons
- Not a sea trial
- Easy to forget twins
- Easy to leave hours at a default that is not your life
- Diesel vs gas is a constant swap, not a full combustion model
Planning a coastal hop vs a lake afternoon
A four-hour clock trip is not always four engine hours. Current, idle in a no-wake zone, and a wait for a bridge add time at low load. The default 0.65 is a blend. For a long no-wake river, drop it. For a wide-open lake with skis, raise it. Run both on the fuel calculator and pick the one that matches the day you are actually taking.
If you are comparing two listings, keep hours and load identical first. Then change horsepower. That is how you see the engine decision without mixing in a fantasy schedule.
Who this is for
- Shoppers comparing 150 vs 300 horse on the same hull
- Owners planning a long coastal hop
- Anyone who was told “it burns about a gallon”
Keep a notebook of gallons added and hours run for one month. Divide. That measured gph, however crude, is a better load-factor teacher than an argument about SFC constants. Then set the calculator to match so trip planning stays consistent.
Alternatives
- Fuel-flow meters and tank soundings (best)
- Manufacturer cruise charts (better than folklore, still not your hull)
- Rule of thumb hp/10 for gas WOT only
- Buy vs rent if fuel-plus-ownership makes a club look rational
FAQ
Why 6.2 and 6.9 pounds per gallon? They are the site’s gasoline and diesel specific-weight constants. If you prefer different weights, the code is the place to change them — the page will not take a custom density.
Does ethanol change gph? Real-world energy content can. The model does not have an ethanol switch.
Can I enter gallons per hour directly? Not as a separate box. You can raise or lower horsepower, engines, and load until cruise gph matches a measured number, or treat fuel as an override by folding a known annual fuel spend into hours × price.
Why does a pontoon with 300 hp match a bowrider with 300 hp? Because the formula is horsepower-based. Hull type is not in the SFC line.