Gori Propeller Speed & Slip Calculator
Use this calculator to estimate shaft RPM, theoretical speed, and estimated real-world speed for a Gori propeller in normal pitch and overdrive pitch.
Estimated Results
What This Gori Propeller Calculator Does
A Gori propeller calculator helps you estimate whether your current setup is in the right range before you spend time and money on prop changes. With a few values (engine RPM, gear ratio, pitch, and slip), you can quickly see how your boat should perform in both normal and overdrive modes.
This is especially useful for cruising sailors trying to balance fuel economy, engine loading, and practical cruising speed. If your measured speed is far from calculated expectations, that can be a clue to investigate fouling, prop sizing, RPM limits, or drivetrain assumptions.
How the Calculation Works
1) Convert Engine RPM to Shaft RPM
The transmission reduces engine speed before it reaches the propeller:
Shaft RPM = Engine RPM / Gear Ratio
2) Convert Pitch and Shaft RPM into Theoretical Speed
Pitch is the forward distance (in inches) the propeller would move in one revolution in a perfect medium with zero losses.
Theoretical Speed (knots) = (Pitch × Shaft RPM) / 1215.223
3) Apply Slip for Real-World Estimate
No propeller achieves perfect advance. Slip accounts for losses:
Estimated Speed = Theoretical Speed × (1 - Slip/100)
How to Use This Tool
- Enter your normal cruising engine RPM.
- Enter gearbox ratio exactly as specified by your drivetrain.
- Enter normal pitch and overdrive pitch for your Gori setup.
- Choose a realistic slip value based on your hull and conditions.
- Click Calculate and compare predicted values to sea-trial speed.
Typical Slip Ranges (Practical Guidance)
- Light to moderate sailboats in flat water: 15% to 25%
- Heavier cruising boats: 20% to 35%
- Adverse chop, heavy load, dirty bottom: often above 30%
If your numbers only work with very low slip, the prop may be underpitched (or your assumptions are off). If slip must be very high to match real speed, check hull fouling, loading, and whether the engine reaches rated RPM.
Normal vs Overdrive on a Gori Propeller
Many Gori users think of normal pitch as the all-around mode and overdrive as a lower-RPM cruising tool once the boat is already moving. In practical terms:
- Normal pitch: stronger acceleration and easier engine response at lower speed.
- Overdrive pitch: lower engine RPM for a given speed under favorable conditions.
- Tradeoff: overdrive can lug the engine if engaged too early or under heavy load.
Interpreting Results Like a Skipper
Use calculated values as targets, then validate with real data: tachometer, GPS speed over ground, and calm-water test runs in opposite directions. Average your readings to reduce current/wind bias. If your measured speed consistently differs, adjust slip in the calculator until estimates align with reality.
That tuned slip percentage becomes your personal baseline and is far more useful than a generic number from the internet.
Common Mistakes to Avoid
- Using an incorrect gear ratio.
- Comparing through-water assumptions to raw GPS speed in strong current.
- Evaluating prop performance with a dirty hull or overloaded cruising trim.
- Running overdrive in conditions where the engine cannot stay in a healthy power band.
FAQ
Is this an official Gori calculator?
No. This is an independent estimation tool for planning and education.
Do I need exact pitch values?
Yes, as close as possible. Even small pitch differences can noticeably change calculated speed.
Can this replace sea trials?
No. Use it to narrow decisions, then confirm performance on the water under controlled conditions.