Boat Speed Calculator

Updated Sep 2026 Used 0 times

Choose the speed modelHull, propeller, and trip speed answer different questions.

Calculation mode

Choose the model that matches the available measurements.

Enter model inputsOnly inputs for the chosen method remain active.

Waterline length

Use length at the waterline, not overall length.

Hull-speed coefficient

1.34 is the familiar displacement-hull reference. Change it only with a defensible vessel-specific source.

Engine speed

Enter engine crankshaft RPM at the modelled operating point.

Gear ratio

For 2:1, enter 2.0: the engine turns twice for one propeller revolution.

Propeller pitch

Use nominal pitch stamped on the propeller.

Estimated propeller slip

Use a measured or defensible estimate. Slip varies with hull, load, trim, speed, and propeller.

Distance travelled

Use chart or GPS distance and select its unit.

Elapsed time

Use underway time for speed over the route, or total elapsed time for overall trip average.

Guest calculations stay on this device. Signed-in results sync privately.

Your result

Estimated boat speed

Enter your values, then calculate to see a verified result.

Inputs usedReview the information used for this result.

Full calculation and sourcesOpen Full calculation and sources to inspect the selected marine formula.

Versioned calculationFormula v1.0.1

Three models, not one

Hull, propeller, and observed trip speed are clearly separated.

Marine unit bridge

Knots, mph, and km/h are always displayed together.

Visible uncertainty

Slip and hull coefficient remain visible assumptions.

Choose the modelMatch the calculation to the available data.Propeller estimate from RPM and pitch

Use measured valuesLoad, RPM, pitch, slip, and distance must describe one scenario.GPS speed at the same RPM

Read the method labelDo not compare unlike speed models as if they were measurements.Hull reference versus trip average

Interpretation

One knot is one nautical mile per hour.

Propeller pitch speed is reduced by the entered slip fraction.

Real performance changes with current, sea state, trim, hull condition, and load.

Use this result

Compare the correct modelReview hull, propeller, and observed-trip formulas before comparing results.Account for real conditionsKeep load, trim, slip, current, and sea-state limitations with the estimate.

Keep exploring

4 available tools

Swim Pace CalculatorCross-check swim pace, finish time, speed, and split intervals.Arrow Speed CalculatorCompare measured or estimated FPS with kinetic energy, momentum, and grains per pound.Golf Club Length CalculatorEstimate a static club-length starting point from height and wrist-to-floor.DLS Target CalculatorCalculate a transparent Standard Edition par score from resource percentages.

Public verification recordFormula v1.0.1

Review scope: Displacement Hull Reference, Propeller Pitch-speed Estimate, Observed Trip Speed, And Marine Speed-unit Conversion.
Review team: CalculatorGeek Algorithmic Team
Verified: 2026-09-26
Next source review: 2027-09-26
Automated fixtures: 6 cases

Review method

The 1056 propeller constant was independently derived from inches per mile and minutes per hour; NOAA conversions, slip boundaries, metric inputs, and distance-time identities were cross-checked.

Known limitations

Hull speed is a displacement-hull reference, not a universal maximum for planing hulls, semi-displacement hulls, or multihulls.
Propeller slip is not constant across RPM, trim, load, hull state, or sea conditions.
Nominal pitch may differ from effective pitch, and tachometer or gear-ratio errors can materially change the result.
Trip speed can be speed over ground and therefore includes current; speed through water may differ.

Primary sources

Latest update

2026-09-26 · Restored unified Use this result cards, four related-calculator cards, and authored contextual links with valid root-relative destinations.

Published calculation checks

Case	Inputs	Expected result
36 foot displacement hull	mode=hull, waterline_length=36 ft, hull_coefficient=1.34 coefficient, engine_rpm=6000 rpm, gear_ratio=2 ratio, prop_pitch=24 in, prop_slip=12 percent, distance_nm=30 nm, time_hours=2 h	speed_knots=8.04, speed_mph=9.25126676192, classification=hull-reference
Propeller estimate 6000 RPM	mode=propeller, waterline_length=36 ft, hull_coefficient=1.34 coefficient, engine_rpm=6000 rpm, gear_ratio=2 ratio, prop_pitch=24 in, prop_slip=12 percent, distance_nm=30 nm, time_hours=2 h	theoretical_mph=68.18181818181819, propeller_mph=60, speed_knots=52.13857032722602, classification=propeller-estimate
Thirty nautical miles in two hours	mode=trip, waterline_length=36 ft, hull_coefficient=1.34 coefficient, engine_rpm=6000 rpm, gear_ratio=2 ratio, prop_pitch=24 in, prop_slip=12 percent, distance_nm=30 nm, time_hours=2 h	speed_knots=15, speed_mph=17.26169172, speed_kmh=27.78, classification=observed-trip
Metric waterline normalization	mode=hull, waterline_length=10.9728 m, hull_coefficient=1.34 coefficient, engine_rpm=6000 rpm, gear_ratio=2 ratio, prop_pitch=24 in, prop_slip=12 percent, distance_nm=30 nm, time_hours=2 h	speed_knots=8.039999999704415

On this page

Direct answer

The boat speed calculator separates three different questions: displacement hull-speed reference, propeller pitch-speed estimate, and observed trip speed.

Each mode shows knots, mph, and km/h, but the interpretation changes by mode. Hull speed is a displacement-hull rule of thumb; propeller speed depends on entered slip; trip speed is simply distance divided by time.

For pool pace and finish-time planning, use the Swim Pace Calculator. For equipment launch speed with energy and momentum, use the Arrow Speed Calculator.

How to use the calculator

Choose displacement hull, propeller estimate, or observed trip mode.
Enter waterline length, propulsion data, or distance and time for the selected mode.
Check knots, mph, and km/h together.
Read the mode-specific limitations before comparing with GPS or manufacturer data.

Inputs, outputs, and result meaning

Inputs

Calculation mode: Choose the model that matches the available measurements.
Waterline length: Use length at the waterline, not overall length.
Hull-speed coefficient: 1.34 is the familiar displacement-hull reference. Change it only with a defensible vessel-specific source.
Engine speed: Enter engine crankshaft RPM at the modelled operating point.
Gear ratio: For 2:1, enter 2.0: the engine turns twice for one propeller revolution.
Propeller pitch: Use nominal pitch stamped on the propeller.
Estimated propeller slip: Use a measured or defensible estimate. Slip varies with hull, load, trim, speed, and propeller.
Distance travelled: Use chart or GPS distance and select its unit.
Elapsed time: Use underway time for speed over the route, or total elapsed time for overall trip average.

Outputs

Boat speed: the primary answer for the selected calculation.
Speed in mph: a supporting value used to interpret or cross-check the result.
Speed in km/h: a supporting value used to interpret or cross-check the result.
Zero-slip theoretical speed: a supporting value used to interpret or cross-check the result.
Propeller shaft speed: a supporting value used to interpret or cross-check the result.
Entered slip speed difference: a supporting value used to interpret or cross-check the result.

Formula and methodology

Displacement hull reference: coefficient x square root of waterline length in feet, reported in knots.

Propeller estimate: (engine RPM / gear ratio) x pitch in inches x (1 - slip) / 1,056, reported in mph before conversion. The constant 1,056 is 63,360 inches per statute mile divided by 60 minutes per hour.

Observed trip speed: nautical miles / hours.

Worked examples

Propeller speed

At 6,000 RPM, 2:1 gearing, 24-inch pitch, and 12% slip, theoretical speed is 68.18 mph and estimated actual speed is 60.00 mph, or about 52.14 knots.

Hull speed reference

For 36 feet of waterline and coefficient 1.34: 1.34 x sqrt(36) = 8.04 knots.

Interpretation and limitations

Hull speed is a displacement-hull reference, not a universal maximum for planing hulls, semi-displacement hulls, or multihulls.
Propeller slip is not constant across RPM, trim, load, hull state, or sea conditions.
Nominal pitch may differ from effective pitch, and tachometer or gear-ratio errors can materially change the result.
Trip speed can be speed over ground and therefore includes current; speed through water may differ.

Frequently asked questions

What is hull speed?

It is a familiar displacement-hull wave-speed reference calculated from waterline length and a coefficient. It is not a universal top-speed limit.

How is propeller boat speed calculated?

Divide engine RPM by gear ratio, multiply by pitch, divide by 1,056 for mph, then multiply by one minus the slip fraction.

Why is boat speed measured in knots?

A knot is one nautical mile per hour, the navigation speed unit used with nautical charts.

What does propeller slip mean?

It is the percentage shortfall from the no-slip pitch-speed estimate. It is expected because a propeller works in a fluid.

Can I use this result for navigation safety?

No. Use approved navigation instruments, charts, weather information, fuel reserves, and seamanship practices.

Sources and transparency

Sources establish the governing definition, measurement context, or documented estimate. CalculatorGeek independently recomputed the public fixtures on 2026-09-26. An estimate remains an estimate even when the arithmetic is exact.