Boat Speed Calculator
Predict the top speed of your boat by combining engine RPM, propeller pitch, gear ratio, and prop slip.
How to Use This Calculator
Enter your engine RPM
Use the maximum recommended RPM from your engine manual or tachometer readings during wide-open throttle.
Add propeller pitch and gear ratio
Pitch is stamped on the prop hub, while the gear ratio is provided by the outdrive or lower unit manufacturer.
Estimate prop slip
Start with 10–15% for most planing hulls. High-performance setups may see 5–8% slip.
Press calculate to get your top speed
Review the results in mph, knots, and km/h to plan prop changes or confirm real-world GPS readings.
Formula
Speed (mph) = (RPM × Pitch × (1 − Slip)) ÷ (Gear Ratio × 1056)
Where: Pitch is in inches, slip is entered as a decimal (e.g. 12% → 0.12), and 1056 converts inches-per-minute to miles-per-hour.
Example with 5,200 RPM, 19" pitch, 2.15 gear ratio, and 12% slip:
Speed = (5200 × 19 × 0.88) ÷ (2.15 × 1056) = 43.3 mph
Knots = 43.3 × 0.869 = 37.6 kn
About the Boat Speed Calculator
Propeller-driven boats translate engine rotation into forward motion. This calculator models the relationship between engine RPM, propeller pitch, gear ratio, and prop slip to estimate how fast your hull will travel through the water.
When to Use This Calculator
- Prop selection: Compare pitch options before purchasing a new propeller.
- Tuning for towing: Ensure you stay within safe speeds for wakeboarding or tubing.
- Performance benchmarking: Validate GPS readings or diagnose underperforming setups.
- Engine swaps: Predict speed changes after replacing an outboard or sterndrive.
Why Use Our Calculator?
- ✅ Instant conversions: Get speed in mph, knots, and km/h without extra math.
- ✅ Slip awareness: Factor in real-world losses from cavitation and hull drag.
- ✅ Mobile friendly: Make adjustments on the dock or at the ramp.
- ✅ Free and flexible: Experiment with settings as often as you need.
Common Applications
High-performance boating: Dial in racing setups by targeting specific RPM and slip values.
Fishing and cruising: Ensure your prop choice keeps you within efficient speed ranges for long trips.
Watersports: Choose props that deliver the exact tow speed for skiing or wake surfing.
Tips for Best Results
- Measure RPM with a calibrated tachometer for accuracy.
- Use actual GPS speed to back-calculate slip and refine future estimates.
- Remember that heavy loads and rough water increase slip and reduce top speed.
- Test several props to find the best balance between holeshot and top-end speed.
Frequently Asked Questions
What is propeller slip?
Prop slip is the percentage difference between theoretical and actual distance traveled. It accounts for inefficiencies caused by water drag, cavitation, and hull dynamics.
How do I find my gear ratio?
Gear ratio is typically printed on the outboard or sterndrive data plate or listed in the owner's manual. You can also divide engine RPM by propeller shaft RPM.
Why does my calculated speed differ from GPS speed?
Real-world conditions introduce variables like wave action, wind, hull cleanliness, load, and altitude. Adjust the slip value until the calculator matches your measured speed.
Can this calculator help with prop selection?
Yes. Test different pitch values to see how they change theoretical speed and RPM. Remember to keep your engine within its recommended operating range.