Propeller Slip

Propeller Slip in Marine Propulsion

Propeller slip is a fundamental concept in marine propulsion analysis that describes the difference between a propeller’s theoretical advance and the ship’s actual forward motion. In ideal conditions, a propeller with pitch P rotating at a given RPM would advance exactly one pitch length per revolution. In reality, hydrodynamic losses, wake effects, hull interaction, and flow separation reduce the effective advance, resulting in slip.

Slip is therefore not a defect by itself, but a physical consequence of how propellers generate thrust in water. Understanding and monitoring slip is essential for evaluating propulsion efficiency, diagnosing performance issues, and comparing design and operational conditions.

Definition of propeller slip

Propeller slip is commonly defined as:

Slip = (V_t − V_s) / V_t

where V_t is the theoretical speed derived from propeller pitch and rotational speed, and V_s is the actual ship speed through the water. Slip is typically expressed as a percentage.

A positive slip means the ship advances less than the theoretical distance per revolution, which is the normal operating condition for marine propellers.

Theoretical speed from pitch and RPM

The theoretical advance speed is computed from propeller pitch and rotational speed as:

V_t = P × RPM × 60 / 1852

where pitch P is given in meters, RPM is revolutions per minute, and the conversion factor 1852 converts meters per hour into knots. This represents a purely kinematic speed, assuming no losses and no slip.

Typical slip ranges and interpretation

Slip values vary depending on vessel type, loading condition, speed, and propeller design. As a general rule of thumb:

• 5–15% — typical for efficient displacement ships at service speed
• 15–30% — common at lower speeds, heavy loading, or off-design operation
• >30% — may indicate fouling, overload, cavitation, or mismatch
• Negative slip — usually indicates measurement inconsistency or strong following current

Because slip depends on speed, comparisons should always be made at similar operating points (RPM, draft, and environmental conditions).

Ahead vs astern operation

In astern operation, propeller inflow conditions are highly disturbed and efficiency is significantly reduced. Slip values in astern are therefore much higher and should not be compared directly with ahead-running slip. This calculator allows explicit selection of direction to avoid misinterpretation.

Why slip matters in practice

• Performance monitoring: Increasing slip at constant RPM often signals hull fouling or propeller degradation.
• Propeller matching: Excessive slip at service speed may indicate an under-pitched or overloaded propeller.
• Sea trials analysis: Slip helps interpret speed–RPM curves and propulsion efficiency.
• Operational diagnostics: Sudden slip changes can point to damage or environmental effects.

Limitations and correct use

• Slip calculations assume accurate pitch, RPM, and ship speed measurements.
• Ship speed should ideally be speed through water, not speed over ground.
• Wake fraction, thrust deduction, and propeller efficiency are not explicitly included.
• Slip alone does not define propulsion efficiency; it must be interpreted with power and thrust data.

Related propulsion & power calculators

Propeller slip is typically evaluated alongside other propulsion performance indicators:

• Propeller Efficiency (η₀) – evaluates how effectively thrust power is converted from shaft power.
• Delivered Power (PD) – converts resistance into required delivered power at the propeller.
• Brake Power (PB) – estimates engine brake power based on drivetrain losses.
• Speed–Power Curve Generator – visualizes how power demand evolves with speed and highlights slip-related trends.

Tip: When analyzing slip trends, always compare results against historical data at similar drafts, trims, and environmental conditions. Absolute slip values are less meaningful than consistent changes over time.