Propeller Pitch & Slip Calculator
Local Apparent Slip Arithmetic for Inboard, Sterndrive, and Outboard Marine Review
Free source-aware propeller pitch and slip screen for marine mechanics, boat owners, and propeller shops who need a local arithmetic prompt before reviewing current engine, drive, and propeller data. Enter pitch, gear ratio, engine RPM, diameter, measured speed, and speed unit. The screen returns theoretical speed, measured speed conversion, apparent slip percentage, pitch/diameter prompt, and source-boundary warnings.
Propeller slip is the difference between theoretical travel from pitch and RPM and the measured boat speed. Mercury manufacturer sources describe slip as necessary for thrust and useful for setup review, but every boat is different. This page keeps the result as a prompt because hull form, loading, trim, propeller model, blade condition, drive height, current, wind, tachometer calibration, GPS timing, and speed-through-water measurement are not modeled.
Use this screen to organize sea-trial notes and decide what evidence needs review. Do not change propellers, alter drive setup, exceed engine limits, or troubleshoot on the water from this output alone. Current manufacturer data and qualified marine review control actual product and operating decisions.
Size the DC wiring from the battery to your engine systems
ABYC DC Wire Sizing Calculator →Screen vessel battery capacity assumptions before manufacturer and ABYC review
Marine Battery Bank Sizing Calculator →Determine sacrificial anode sizing for hull and running gear protection
Sacrificial Anode Calculator →Calculate gear ratios for your marine transmission
Gear Ratio Calculator →How It Works
-
Enter Propeller Data
Input the propeller diameter and pitch in inches from the hub stamp, product data, or shop measurement. Keep the exact model, blade count, cup, rake, hub, material, and rotation available for review.
-
Enter Gear Ratio
Input the transmission or lower-unit gear ratio from the current engine or drive manufacturer data. If the ratio is uncertain, treat the result as a measurement-review prompt only.
-
Enter RPM and Measured Speed
Input the engine RPM and measured boat speed from a consistent sea-trial record. GPS speed over ground can differ from speed through water because of current, wind, and timing.
-
Review Source Boundaries
Use the local apparent-slip output, pitch/diameter prompt, source pointers, and residual data gaps to prepare for manufacturer, dealer, prop-shop, or qualified marine review.
Built For
- Marine mechanics organizing RPM, speed, gear-ratio, and propeller notes before current manufacturer data review
- Propeller shops using a transparent apparent-slip prompt alongside blade scan, product data, and sea-trial observations
- Boat owners comparing consistent sea-trial records after load, trim, bottom, or equipment changes
- Surveyors and service writers documenting unresolved data needed before product or operating decisions
Features & Capabilities
Apparent Slip Arithmetic
Calculates apparent slip as the percentage difference between theoretical speed (pitch x shaft RPM / 1056) and measured speed. Results stay local prompts, not acceptance criteria.
Speed Unit Conversion
Accepts measured speed in miles per hour or knots and shows both outputs so sea-trial notes can be compared consistently.
Local Review Bands
Shows broad manufacturer-context prompts for negative, low, common, elevated, and high apparent slip while keeping measurement and setup gaps visible.
Source and Safety Warnings
Surfaces manufacturer source pointers, NIST unit context, and unresolved data gaps for engine limits, propeller product details, sea state, instrumentation, and qualified review.
Frequently Asked Questions
Learn More
Propeller Pitch, Slip & Performance
How to review apparent propeller slip arithmetic, measurement limits, manufacturer data gaps, and sea-trial boundaries before product decisions.
Related Tools
ABYC DC Wire Sizing & Voltage Drop Calculator
Size marine DC wiring per ABYC E-11. Calculates minimum AWG for voltage drop and ampacity with engine room derating.
Sacrificial Anode Weight Calculator
Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.
Marine Battery Bank Sizing Calculator
Size house and starting battery banks for boats. Calculates CCA, Ah capacity with DoD limits, temperature derating, and chemistry comparison.