Marine Propeller Pitch & Slip Calculator
Determine apparent propeller slip percentage, theoretical engine distance, observed speed through water, and driveline efficiency from shaft RPM and pitch.
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Propeller Pitch, Theoretical Advance & Apparent Slip
A marine propeller acts as a hydrodynamic screw rotating in a fluid medium. The Pitch of a propeller is defined as the theoretical linear distance the propeller would advance through a solid medium during one complete 360-degree revolution. Because water is yielding and yields fluid slip, a propeller never achieves 100% of its theoretical pitch advance.
Propeller Governing Mathematical Formulas
- Theoretical Engine Speed (V_engine):
When pitch is given in meters:V_engine (knots) = (Pitch (m) × RPM × 60) / 1,852
When pitch is given in inches:V_engine (knots) = (Pitch (in) × RPM × 60) / 72,913.4 - Theoretical Engine Distance (D_engine):
D_engine (NM) = V_engine × Run Hours - Apparent Slip Percentage:
Apparent Slip (%) = [ (D_engine - D_observed) / D_engine ] × 100
Equivalently:Apparent Slip (%) = [ (V_engine - V_observed) / V_engine ] × 100 - True Slip vs. Apparent Slip:
Apparent slip uses ship speed relative to water or ground without adjusting for the wake current following the ship. Because the hull drags a boundary layer of water forward with it (wake fractionw), true slip measured against the localized inflow velocity of water entering the propeller disc is always higher than apparent slip.
Frequently Asked Questions
What is a normal apparent slip percentage for a merchant ship?
In calm deep water at clean hull condition, normal apparent slip ranges from 4% to 10% for fine-form container ships, and 8% to 15% for laden bulk carriers and tankers.
Can propeller slip ever be negative?
Yes. Apparent slip can become negative (e.g. -2% to -8%) when a vessel is assisted by a strong following current, heavy following seas, or strong following gale winds that push the ship forward faster than the propeller is theoretically screwing through the water.
What causes sudden high propeller slip (>25%)?
High slip occurs when the engine works against heavy external resistance: severe head seas, strong adverse currents, heavy hull biofouling (barnacles and algae), shallow water squat, towing another vessel, or physical propeller blade damage and cavitation.
Why do chief engineers track propeller slip daily in noon reports?
Tracking slip over time is the primary diagnostic method for detecting hull fouling. If slip gradually climbs from 8% to 18% at the same RPM over several months, it indicates that the hull needs underwater hull cleaning and propeller polishing.