Starlight Tools directory
Marine & Boating Calculators
18 free, browser-based tools. No account required.
- Free to use
- Runs in your browser
- No account required
Plan time on the water with private, browser-based boating calculators. Project boat loan payoff and recurring ownership costs; compare great-circle and rhumb-line distance and true bearing between coordinates; total engine horsepower and calculate loaded power-to-weight; estimate transient dock-line length and diameter; total loaded boat trailer weight and check towing limits; estimate bilge pump capacity from observed water accumulation and installation assumptions; estimate tide height and continuous passage windows from entered high and low waters; calculate sailing distance, average speed, or passage time; calculate loaded boat displacement and compare onboard weight with a published capacity; screen draft and under-keel clearance using depth, tide, squat, motion, and uncertainty allowances; calculate anchor rode; estimate displacement hull speed; compare propeller pitch and slip; or calculate fuel tank capacity, trip fuel, reserve, range, battery-bank runtime, and marina fuel cost. No account or uploaded vessel data is required.
Marine and boating calculator FAQs
How can I estimate boat loan payoff and ownership cost?
Project the current balance with the loan's interest rate, scheduled payment, and any principal-only extras. Keep recurring operating costs—such as insurance, storage, maintenance, fuel, registration, and taxes—separate from principal repayment so cost and cash outlay are not confused.
How is loaded boat trailer weight calculated?
Add the actual empty trailer, boat, separately listed engine, carried fuel, batteries, equipment and every other item on the trailer. Then compare the total with all vehicle, hitch and trailer limits and verify it on suitable scales.
How is bilge pump capacity estimated?
Add estimated incoming water to the flow needed to clear a standing volume in the target time, apply the selected capacity reserve, then divide by the expected installed-output fraction to estimate a pump label rating.
How is loaded boat displacement calculated?
Add the known base boat weight and every onboard load not already included: people, machinery, fluids, equipment, cargo, and gear. At static equilibrium, the total boat weight equals the weight of water displaced.
How is under-keel clearance calculated?
Subtract actual loaded draft from available water depth. A conservative screening estimate can also deduct separately assessed squat, vessel-motion, other draft, and depth-uncertainty allowances before comparison with the controlling minimum.
How can I estimate a tide-height passage window?
Enter three consecutive official high and low water times and heights, calculate the required tide height from depth and vessel inputs, and find where a simplified curve crosses that threshold. Treat the result as a screening estimate, not an official prediction or safe-passage decision.
How is anchor rode length calculated?
Add water depth, expected tide rise, and the height of the bow attachment above the water, then multiply that vertical distance by the selected scope ratio. The complete rode includes both chain and rope.
How is displacement hull speed estimated?
The common rule of thumb is 1.34 multiplied by the square root of loaded waterline length in feet, producing knots. It is an approximate wave-resistance threshold rather than a hard maximum speed.
How is sailing passage time calculated?
Divide planned route distance by a realistic average speed over ground. A 120-nautical-mile route at a steady average of 6 knots takes 20 hours before any separately assessed margin.
What does boat propeller slip mean?
Propeller slip is the difference between theoretical travel from pitch and actual travel through the water. Some slip is necessary for a propeller to produce thrust, and slip percentage is not the same as propeller efficiency.
Why is boat fuel use measured in GPH?
Boat resistance changes greatly with speed, hull, load, weather, and sea state. Gallons per hour or liters per hour therefore gives boaters a practical burn rate at a known operating condition.
Does the boat fuel calculator include a reserve?
Yes. You can choose a reserve percentage and see both expected trip burn and the total fuel recommended with that additional reserve.
How is boat battery-bank runtime estimated?
Calculate bank voltage and amp-hours from the series-parallel layout, reduce capacity for the selected state-of-charge range and battery health, then divide by battery-side current. An optional Peukert exponent can approximate discharge-rate effects.
How is marine fuel tank capacity calculated?
Choose the regular tank shape, enter internal dimensions, and apply the relevant rectangular, circular-cylinder, or elliptical-cylinder volume formula. A geometry estimate should be checked against the permanent tank label.
Can calculator results replace a passage or safety plan?
No. Treat results as estimates and verify them against engine data, measured fuel burn, tank usability, weather, currents, sea state, local requirements, and the judgment of the vessel operator.
About these marine and boating tools
Boating calculations need units and assumptions that road-trip tools usually omit. This collection is designed around nautical miles, knots, hourly fuel burn, multiple engines, usable tank capacity, and reserve planning. It is useful for early trip budgeting and for checking measured vessel performance.
Conditions on the water can change consumption significantly. Fouling, trim, load, wind, waves, current, engine condition, and throttle setting all matter. Use conservative values, keep an appropriate safety reserve, and rely on onboard measurements and official engine documentation for operational decisions.