Marine & Nautical Calculators
Anchor Chain Length Calculator
Calculate the anchor chain length needed at a given scope ratio, water depth, and bow height, with common scope ratios for different conditions.
Apparent Wind Calculator
Calculate apparent wind speed and angle from true wind and boat speed using vector addition.
Bilge Pump Sizing Calculator
Calculate minimum recommended bilge pump capacity in gallons per hour based on boat length and beam.
Boat Draft Calculator
Calculate hull draft from displacement, waterline length, beam, and hull shape using Archimedes' principle, in either salt water or fresh water.
Boat Freeboard Calculator
Calculate a boat's freeboard - the hull height remaining above the waterline - from hull depth and draft.
Boat Fuel Consumption Calculator
Work out the fuel needed for a planned trip, or the range a full tank provides at a given cruising speed and burn rate, in either direction.
Dead Reckoning Calculator
Calculate an estimated vessel position using dead reckoning navigation from starting coordinates, heading, speed, and time.
Displacement Hull Speed Calculator
Estimate the practical speed ceiling of a displacement hull from its waterline length, the point where pushing a bow wave demands disproportionate power.
Fuel Range at Sea Calculator
Calculate boat cruising range and endurance accounting for a fuel safety reserve, following the boating 1/3 rule.
Knots to MPH Converter
Convert speed between knots and miles per hour using the exact 1.150779 ratio between the nautical mile and the statute mile, in either direction.
Marine Battery Capacity Calculator
Estimate battery runtime for a given load, or the battery capacity needed to hit a target runtime, accounting for voltage and depth of discharge.
Marine Distance Calculator
Calculate great-circle distance between two coordinates using the haversine formula, or find estimated time of arrival from distance and cruising speed.
Mooring Line Length Calculator
Calculate recommended dockline length for bow/stern, spring, or breast lines based on boat length.
Nautical Mile Converter
Convert precisely between nautical miles, kilometers, statute miles, and meters using the exact 1,852-meter nautical mile definition used at sea.
Planing Hull Speed Calculator
Estimate a planing hull boat's top speed from horsepower and boat weight using Crouch's planing speed formula.
Propeller Pitch Speed Calculator
Calculate theoretical and slip-adjusted actual boat speed from propeller pitch, engine RPM, and gear ratio.
Rule of Twelfths Tide Calculator
Estimate current tide height using the classic Rule of Twelfths mental-math shortcut mariners use for quick tide calculations.
Sail Area Calculator
Calculate total sail area from simple triangle dimensions or full sloop-rig measurements, using the standard I, J, P, E rig dimension convention.
Tide Time Calculator
Project the next high or low tide time from the last observed tide, or estimate current tide height using the navigator's Rule of Twelfths.
VHF Radio Range Calculator
Calculate estimated VHF marine radio line-of-sight range based on antenna heights at both stations.
The Physics of Moving Through Water
A boat's hull, unlike a car, has a physical speed relationship built into its length — there is a real ceiling on how fast a displacement hull can efficiently travel before it starts fighting its own bow wave. These ten calculators cover marine navigation and vessel performance math.
Finding Hull Speed, Step by Step
- Find the waterline length. Measure the boat's length at the waterline in feet, not overall length.
- Take the square root of that length. For a 25-foot waterline, the square root is 5.
- Multiply by 1.34. This constant comes from the physics of the bow wave a displacement hull generates as it moves.
- Read the result as the practical speed ceiling. 5 × 1.34 = 6.7 knots — pushing a displacement hull meaningfully faster than this requires disproportionately more power for very little speed gain.
Run this instantly with the Displacement Hull Speed Calculator, or convert the result to more familiar units with the Knots to MPH Converter.
Popular Marine & Nautical Calculators
- Boat Fuel Consumption Calculator — estimates fuel burn from engine size, speed, and trip distance.
- Anchor Chain Length Calculator — calculates recommended chain scope from water depth and conditions.
- Marine Distance Calculator — calculates nautical distance between two coordinates.
Related: Automotive Calculators, Aviation Calculators for other vehicle-performance math.