UnitConv

Boat Propeller Calculator

Calculate theoretical speed, actual speed, slip and the required pitch from propeller pitch, engine RPM and gear (reduction) ratio.

in
rpm
:1
%

On calm water, slip is generally around 10–20%. It varies with hull type, load and propeller condition.

Theoretical speed
53.5 mph
86 km/h · 46.5 kn
Actual speed
45.4 mph
73.1 km/h · 39.5 kn
Actual speed vs. theoreticalSlip: 15%
15% slip
ActualTheoretical
Slip health
0%10%25%40%+

Healthy (10–25%) — a good range

Propeller RPM: 2,688 rpm

Source: estimates based on the standard propeller-speed formula (theoretical speed = pitch × RPM / (reduction × 1056)) and typical marine slip guidelines.

What is the Boat Propeller Calculator?

This calculator works out a boat's theoretical speed, actual speed and propeller slip — and the pitch you need to hit a target speed — from the propeller pitch, engine RPM and the gear (reduction) ratio. Theoretical speed is the speed you would reach in an ideal world with zero slip, where each turn of the propeller drives the boat forward by exactly its pitch. In real water the propeller slips against the water, so the actual speed is lower than theoretical. Three modes (speed, slip and required pitch) let you plan a re-prop or fine-tune your setup.

How to use

1. Pick a mode: Find speed, Find slip, or Find pitch. 2. Enter the propeller pitch (inches), engine RPM and the gear (reduction) ratio. 3. In speed mode add the slip percentage; in slip mode add your measured speed; in pitch mode add your target speed. 4. Read the theoretical speed, actual speed and slip (or the required pitch), with slip health shown as a bar.

How it is calculated

Theoretical speed = pitch × RPM / (reduction × 1056) Slip (%) = (theoretical − actual) / theoretical × 100 Actual speed = theoretical × (1 − slip / 100) Pitch is in inches, RPM is engine speed and reduction is the gear ratio. The constant 1056 converts inches per minute to miles per hour. Propeller RPM = engine RPM / reduction.

How to read the results

Theoretical speed is the zero-slip ceiling; actual speed is the realistic figure once slip is subtracted. On calm water a slip of about 10–20% is generally healthy. Under 10% can mean the prop is too large (over-pitched) or the load is light, while over 25% points to ventilation, overloading, or a damaged or mismatched propeller. In pitch mode you also see the pitch needed for your target speed plus the nearest off-the-shelf size.

FAQ

What is propeller slip? ▾

Slip is the difference, as a percentage, between how far a propeller should theoretically advance in one turn (its pitch) and how far the boat actually moves. Because water is not solid, a propeller always slips a little. Slip is not wasted effort — some is needed to generate thrust.

What is a healthy slip percentage? ▾

On calm water, roughly 10–20% is the usual guideline. A well-set-up planing boat often sits in the low teens. There is no single correct number — it depends on hull shape, load, trim and the type of propeller.

What is pitch? ▾

Pitch is the distance, in inches, a propeller would travel in one turn through a solid. A higher pitch tends to give a higher top speed but sacrifices acceleration and low-speed grunt; a lower pitch favours acceleration.

What does the gear ratio mean? ▾

The gear (reduction) ratio is the ratio of engine RPM to propeller RPM. For example, 1.86:1 means the engine turns 1.86 times for each turn of the propeller. A larger reduction turns the prop more slowly and lets you swing a bigger, higher-pitch propeller.

What causes slip that is too high? ▾

Common causes are ventilation (the prop drawing in air), overloading or too little power for the hull, poor trim or prop mounting height, a damaged or worn propeller, and choosing the wrong propeller for the job. If slip is well above 25%, check these.