Advanced Prop Pitch Speed Form
Example Data Table
| Pitch | RPM | Gear Ratio | Slip | Theoretical mph | Actual mph |
|---|---|---|---|---|---|
| 17 in | 5600 | 2.00 | 10% | 45.08 | 40.58 |
| 19 in | 5200 | 1.85 | 12% | 50.59 | 44.52 |
| 21 in | 5000 | 1.75 | 15% | 56.82 | 48.30 |
Formula Used
Propeller RPM = Engine RPM ÷ Gear Ratio
Effective Pitch = Pitch × (1 + Pitch Correction ÷ 100)
Theoretical Speed in mph = Propeller RPM × Effective Pitch ÷ 1056
Estimated Actual Speed = Theoretical Speed × (1 - Slip ÷ 100)
Measured Slip = (Theoretical Speed - GPS Speed) ÷ Theoretical Speed × 100
The value 1056 converts inches per minute into miles per hour. It comes from 12 inches per foot, 5280 feet per mile, and 60 minutes per hour.
How to Use This Calculator
- Enter the stamped prop pitch and select its unit.
- Enter the engine RPM from a wide open throttle test.
- Add the lower unit or drive gear ratio.
- Enter expected slip, or provide GPS speed for measured slip.
- Use pitch correction when cup, wear, or tuning changes effective pitch.
- Press Calculate to view the result below the header.
- Use CSV or PDF export for records and comparison reports.
Prop Pitch Speed Guide
What Pitch Speed Means
Prop pitch speed is the distance a boat could move if the propeller advanced exactly one pitch length on every turn. It is a clean theoretical value. Real boats never match it exactly. Water is not solid. The hull also creates drag. The propeller must grip moving water, so some travel is lost.
Why Slip Matters
Slip is the gap between theoretical speed and actual speed. A small slip value usually shows a good setup. A large slip value can show ventilation, poor trim, heavy load, wrong engine height, or a propeller mismatch. Normal slip depends on hull type, speed range, and propeller design. A heavy work boat may show more slip than a light performance hull.
Using RPM and Gear Ratio
The calculator first converts engine RPM into propeller RPM. Gear ratio matters because the propeller turns slower than the crankshaft. A 2.00 ratio means the engine turns twice for each propeller turn. After that, pitch is applied to estimate theoretical travel each minute.
Pitch Correction and Diameter
Pitch correction helps when a prop has cup, repair changes, or a known effective pitch difference. A positive correction raises effective pitch. A negative correction lowers it. Diameter does not directly set pitch speed, but the pitch to diameter ratio helps compare prop styles. A very high ratio may suit faster hulls. A lower ratio may help load carrying.
Reading the Results
Theoretical speed is the perfect no slip value. Estimated actual speed applies your slip input. Speed loss shows how much speed is lost to slip. If you enter GPS speed, the calculator estimates measured slip from the real run. This makes the tool useful for tuning. You can test one change at a time, then compare results. Record weather, load, trim, water conditions, and engine height with each run. These details help explain why two tests can differ. Use the export buttons to save each setup before changing pitch, load, or trim. Good records make propeller selection easier and safer.
FAQs
What is prop pitch speed?
It is the theoretical boat speed based on prop pitch, RPM, and gear ratio. It assumes zero slip, so it is higher than real water speed.
Why is my actual speed lower?
Actual speed is lower because the propeller slips in water. Hull drag, load, trim, ventilation, and water conditions also reduce speed.
What is a normal slip percent?
Many planing boats may show about 8% to 15% slip at top speed. Heavy hulls, work boats, and poor setups can show higher values.
Should I use engine RPM or prop RPM?
Enter engine RPM. The calculator divides it by gear ratio to find propeller RPM automatically.
What does gear ratio mean?
Gear ratio compares engine revolutions to propeller revolutions. A 2.00 ratio means two engine turns create one propeller turn.
How does pitch correction work?
Pitch correction adjusts the entered pitch for cup, tuning, wear, or known prop changes. Use zero when you do not know a correction.
Can GPS speed estimate slip?
Yes. Enter GPS speed in the selected speed unit. The calculator compares it with theoretical speed and estimates measured slip.
Does diameter change the speed formula?
Diameter is not part of the basic pitch speed formula. It is included for comparison through the pitch to diameter ratio.