Analyze propeller pitch, RPM, boat speed, and slip. Review advance ratio and travel estimates clearly. Get clear engineering outputs for smarter setup choices today.
Enter all known values. Leave the selected output field blank.
| Pitch (in) | Engine RPM | Gear Ratio | Slip (%) | Speed (MPH) |
|---|---|---|---|---|
| 19 | 4800 | 2.00 | 12 | 38.0000 |
| 21 | 5200 | 1.86 | 10 | 50.0367 |
| 17 | 4500 | 2.30 | 15 | 26.7725 |
| 23 | 6000 | 1.75 | 8 | 68.7013 |
1. Shaft RPM
Propeller Shaft RPM = Engine RPM ÷ Gear Ratio
2. Zero-Slip Speed
Zero-Slip Speed (MPH) = (Pitch × Shaft RPM × 60) ÷ 63360
3. Actual Speed
Actual Speed = Zero-Slip Speed × (1 - Slip Fraction)
4. Propeller Pitch
Pitch = (Actual Speed in MPH × 63360) ÷ (Shaft RPM × 60 × (1 - Slip Fraction))
5. Engine RPM
Engine RPM = (Actual Speed in MPH × 63360 × Gear Ratio) ÷ (Pitch × 60 × (1 - Slip Fraction))
6. Slip Percentage
Slip % = [1 - (Actual Speed ÷ Zero-Slip Speed)] × 100
7. Effective Advance per Revolution
Effective Advance = Pitch × (1 - Slip Fraction)
8. Advance Ratio
J = V ÷ (nD), where V is speed, n is revolutions per second, and D is propeller diameter.
Propeller pitch and RPM work together. They control how far a boat moves per revolution. Pitch is the theoretical forward travel of the propeller. RPM shows how fast the system turns. Higher pitch can increase speed potential. Higher RPM can also raise theoretical speed. Real boats never reach perfect travel. Water drag, hull load, and slip reduce the final number. That is why engineers compare pitch, shaft RPM, gear ratio, and slip together. A single value rarely explains full marine performance.
Engine RPM is not the same as propeller shaft RPM. The gearbox changes that relationship. A 2.00 gear ratio means the engine turns twice for one propeller revolution. This reduction affects speed estimates directly. Many tuning mistakes happen here. Users sometimes compare pitch against engine RPM only. That can mislead setup decisions. Always convert engine speed to shaft speed first. Then apply pitch and slip. This method gives a cleaner performance estimate. It also helps compare setups across different lower units and powertrain combinations.
Slip is the gap between theoretical travel and actual travel. Some slip is normal. It appears because water is not a solid surface. Hull weight, trim, blade design, water condition, and engine height all affect it. Lower slip often means a more efficient setup. Very low or negative slip can suggest bad readings. It may also point to current assistance or tachometer error. Engineers use slip to judge setup quality. When the slip percentage drops into a sensible range, the propeller usually matches the application better.
This calculator helps with setup planning and troubleshooting. You can solve for speed, pitch, RPM, or slip. You can also review zero-slip speed, effective advance, travel per minute, and advance ratio. Those values help during propeller selection. They also support test-day comparisons. Use the same unit system every time. Record actual results from water trials. Then compare calculated and observed numbers. That process improves propeller tuning. It also supports better engineering choices for fishing boats, performance craft, and work vessels.
Propeller pitch is the theoretical distance a propeller moves forward in one revolution. It acts like thread movement on a screw. Real travel is always lower because of slip.
Gear ratio converts engine RPM into propeller shaft RPM. The propeller does not turn at crankshaft speed. Without gear ratio, speed and pitch calculations become inaccurate.
Normal slip depends on hull type, load, and propeller design. Many planing boats operate within roughly 5% to 20%. Testing under real conditions gives the most useful number.
Yes. Enter actual speed, engine RPM, gear ratio, and slip. The calculator then estimates the pitch that matches those operating conditions and assumptions.
Negative slip usually suggests a measurement issue. It may also happen with strong current assistance, wrong tachometer readings, or speed data taken under unusual conditions.
Zero-slip speed is the ideal speed if every propeller revolution moved the full pitch distance. It is a useful benchmark, but boats rarely achieve it in practice.
Advance ratio compares boat speed, propeller size, and shaft speed. It helps engineers evaluate operating condition and propeller loading across different setups and test points.
That depends on engine power, load, hull drag, and target use. More pitch may raise speed potential. More RPM may improve acceleration if the engine stays within its safe range.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.