Ship going astern

When a ship has a single right-handed propeller (rotating anti-clockwise when going astern), and a center-line rudder, the usual scenario is that the ship moves at low speed and high RPM when going astern.

In this case, the transverse thrust will be directed towards the port side causing the ship to turn to starboard.

This is due to the higher water pressure on the propeller blades in their deeper position. Additionally, the water pressure on the starboard quarter will be higher than on the port quarter since the water from the propeller blades is thrown upwards obliquely on the starboard side and downwards obliquely on the port side.

If the ship reduces speed from 5-6 knots to zero when going astern with a right-handed propeller, it will start turning to starboard unless counter forces from wind and current are stronger and cause a force to port.

However, no turn to starboard is noticed when going astern at higher speeds. The magnitude of the transverse thrust when going astern is around 10-14% of the total thrust on the propeller. Therefore, for a ship with an astern power of 7400kw, the transverse thrust is similar to the thrust from a bow thruster.


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