Boating calculators Boat Card
Work the numbers before you cast off.
How far out can you go and still keep a third of the tank in reserve? How much anchor line goes over the bow in 12 feet of water? Weldon Marine does the arithmetic for small-boat skippers, and a free Boat Card turns your fill-ups into your boat's real, measured fuel burn.
No account, no email. The calculators are free and store nothing.
Worked example one-third rule
60 gallons usable, 6 gal/h at 24 knots
- Endurance
- 10 h
- Full range
- 240 nm
- Furthest out
- 80 nm
A 70 nm run out needs 52.5 gal with the reserve: within the rule. Open it
The legend
Four calculators
Free, nothing stored, no account.
- No. 1
Fuel and range
Endurance, range, and how far out you can go under the one-third rule. Checks a planned trip: within the rule, into the reserve, or beyond the tank.
- No. 2
Passage time and ETA
Any two of speed, time and distance give the third. Departure time gives the arrival time; burn gives the fuel for the leg.
- No. 3
Anchor rode and swing circle
Line to let out for 5:1, 7:1 or your own scope, measured from the bow and the deepest the water will get, and the circle you will swing through.
- No. 4
Prop slip
Theoretical speed from RPM, pitch and gear ratio, and the slip against your GPS speed.
The Boat Card
Your boat's real burn, from your own fill-ups.
The card is private: one tap makes it, and a private link (no account, no email) is the way back in from another device.
- 01
Save the boat
Usable fuel, cruise burn and speed, length, bow height, prop and gear ratio. Every calculator can fill itself in from it.
- 02
Log fill-ups
Each time you fill to full, enter the engine-hours reading and the gallons it took. Nothing else.
- 03
See the measured burn
Gallons divided by engine hours between fills, per fill and on average, and the fuel calculator uses it instead of the brochure figure.
Prices
Free to use. A Helm Pass for the whole log.
Free
$0
No account, no email
- All four calculators: fuel and range, passage, anchor rode, prop slip
- A private Boat Card for one boat
- Measured fuel burn from the newest 6 fill-ups
Helm Pass
$1.95 a month
or $19.50 a year (two months free)
- Up to 10 boats on your Boat Card
- Up to 1,000 fill-ups per boat, with measured burn over all of them and season by season
- A printable Helm Card for each boat: one-third rule table and anchor rode table
- CSV download of every fill-up
Free during preview: every Helm Pass feature is on for everyone and nothing is charged.
What this is not
Planning arithmetic, not a navigation app: no charts, positions, weather or routing, and no promise that a trip is safe. Weldon Marine is made and run by WeldonPC.com; it does not sell, repair, broker, insure or charter boats.