Why Propeller Speed Matters
Boat speed depends on more than engine power. Propeller pitch, gear ratio, slip, hull load, and water movement all change the final number. A propeller acts like a screw moving through water. Each turn should advance by its pitch, yet water flexes and moves. That loss is called slip. A good calculator helps compare setups before a costly prop change.
Core Planning Ideas
The first value is shaft speed. It is engine RPM divided by gear ratio. The next value is theoretical speed. It assumes the propeller moves forward by its full pitch on every turn. Real boats never achieve that perfect value. Slip reduces the result. Extra weight, marine growth, trim angle, and a worn propeller can reduce it again.
Advanced Use Cases
This tool accepts pitch, diameter, slip, optional cup, load factor, and current. It also estimates propeller tip speed and advance ratio. These details help when comparing cruise settings, changing pitch, or checking if a propeller is likely too large. A low estimated RPM may suggest excess pitch. A high RPM with weak speed may suggest high slip or poor hull setup.
Reading The Result
Use the final speed as a planning estimate, not a sea trial certificate. Compare it with GPS speed, tachometer readings, and known engine wide open throttle limits. If real speed is much lower, inspect the propeller, hull bottom, trim, motor height, and load. Small changes can matter. One inch of pitch can change RPM and speed. Current can also hide true performance. Downstream numbers may look better than the boat actually performs through still water.
Best Practice
Run several scenarios. Keep one record for light load, one for normal gear, and one for heavy load. Save the CSV or PDF after each test. These notes make future propeller choices easier. They also help mechanics understand your setup quickly.
Safety Note
Do not select a propeller from one estimate alone. Check the engine maker RPM range. Watch water pressure and temperature during testing. Use calm water when possible. Record wind, crew, fuel, and trim. Stop testing if vibration appears. Damaged blades can stress shafts, bearings, and gear cases. When in doubt, ask a marine technician to inspect the setup.