Slip is the difference between a theoretical distance the ship’s propeller should advance in one revolution and the actual distance the ship travels through the water. It is an important indicator of propeller efficiency, hull condition, engine performance, and overall propulsion effectiveness. A small amount of slip is normal; excessive slip suggests problems such as fouling, propeller damage, shallow water effect, or heavy weather resistance.
What is Theoretical Advance or Pitch Speed?
Each propeller has a pitch. The pitch is the distance in which the propeller would move ahead in one revolution, if there were no water resistance and no loss of thrust. A 7-metre pitch means, for every revolution, the propeller should theoretically advance 7 metres. This ideal theoretical distance converted into speed is called pitch speed. Pitch speed is always higher than actual speed and the difference forms the slip.
Pitch Speed in knots = Propeller pitch in meters × RPM × 60 / 1852
Types of Slip
There are two types: Apparent Slip and Real Slip.
Apparent Slip uses the speed through water given by the ship’s log.
Apparent Slip (%) = ((Pitch Speed – STW) ÷ Pitch Speed) × 100
Real Slip uses GPS speed over ground (SOG) to remove effects of current.
Actual Slip (%) = ((Pitch Speed – SOG) ÷ Pitch Speed) × 100
Example Calculation
Assume the following:
Propeller pitch = 7.0 m
Propeller RPM = 90
Speed through water (STW) = 13 knots
Speed over ground (SOG) = 12 knots
Step 1: Calculate the pitch speed.
Pitch Speed = 7 × 90 × 60 / 1852
Pitch Speed = 37800 ÷ 1852 ≈ 20.41 knots
Step 2: Apparent Slip Calculation
Apparent Slip = ((20.41 – 13) ÷ 20.41) × 100
Apparent Slip ≈ 36.33%
Step 3: Calculate Real Slip.
Actual Slip = ((20.41 – 12) ÷ 20.41) × 100
Real Slip ≈ 41.12%
This means that the propeller loses about 36% of its theoretical efficiency through water, and when current is taken into account, the real slip is around 41%.
Factors Affecting Slip
Hull fouling adds resistance and slip.
Propeller damage or cavitation reduces thrust and increases slip.
Shallow water reduces propeller efficiency and increases slip.
Slip is increased by heavy weather, head seas, swell, and strong winds.
Overloading or deep draft increases slip.
Low RPM or the wrong pitch-RPM combination can increase slip.
Current following reduces the real slip, while head current increases it.
Importance of Slip Monitoring
The slip helps assess the actual state of the hull and propeller.
It helps in tracking changes in ship performance over a voyage.
Slip values enable the engineers to benchmark engine power versus delivered speed.
It aids in the early detection of underwater fouling, especially on long voyages.
Slip guides decisions on the optimization of speed and fuel consumption.
It helps the ship operators to identify whether any cleaning or maintenance is required.
