Propeller Slip Calculator
Fill in the values and the answer updates as you type.
How the Propeller Slip Calculator works
Theoretical speed assumes the propeller screws through the water like a screw through wood, moving forward one pitch per turn. Speed in knots = pitch (in) × shaft rpm × 60 ÷ 72,913 inches per nautical mile, and shaft rpm = engine rpm ÷ gear ratio. Slip = (1 − actual speed ÷ theoretical speed) × 100. Use GPS speed in calm water with little current, and compare results only at the same load. Example: with Propeller pitch 21 in, Engine speed 5000 rpm, Gear ratio 1.85 : 1, and Boat speed (GPS) 38 kt, the result is Propeller slip (%) = 18.64%; Theoretical speed (kt) = 46.7; Speed lost to slip (kt) = 8.7.
Inputs and results
The Propeller Slip Calculator takes 4 inputs and gives 5 results.
What you enter
- Propeller pitch (in) — starts at 21.
- Engine speed (rpm) — starts at 5000.
- Gear ratio (: 1) — starts at 1.85.
- Boat speed (GPS) (kt) — starts at 38.
What you get
- Propeller slip (%)
- Theoretical speed (kt)
- Speed lost to slip (kt)
- Theoretical speed (km/h)
- What this means
Example: Propeller Slip Calculator with the starting values
With propeller pitch 21 in, engine speed 5000 rpm, gear ratio 1.85 : 1 and boat speed (gps) 38 kt, the calculator returns:
| Propeller slip (%) | 18.64% |
|---|---|
| Theoretical speed (kt) | 46.7 |
| Speed lost to slip (kt) | 8.7 |
| Theoretical speed (km/h) | 86.5 |
| What this means | A typical range for many planing boats |
Change any value above and the results update straight away.
Propeller Slip Calculator FAQ
How do I use the Propeller Slip Calculator?
Enter propeller pitch, engine speed, gear ratio and boat speed (gps) in the fields, then read propeller slip (%), theoretical speed (kt), speed lost to slip (kt) and theoretical speed (km/h) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Propeller Slip Calculator calculate?
Compare real boat speed with the theoretical speed of the propeller. It returns propeller slip (%), theoretical speed (kt), speed lost to slip (kt) and theoretical speed (km/h) from the 4 values you provide.
What values does the Propeller Slip Calculator start with?
It opens with propeller pitch 21 in, engine speed 5000 rpm, gear ratio 1.85 : 1 and boat speed (gps) 38 kt so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Propeller Slip Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Propeller Slip Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.