Terms related to ship handling

 

Transverse thrust:

bullet Athwartship component of propeller thrust.

 

bullet When engine works ahead of a right screw vessel,
bullet Helical discharge from propeller creates larger pressure on port side of rudder.
bullet Slight upward flow from the hull into the propeller puts more pressure on down sweeping propeller blades.
bullet Flow of water in propeller area is uneven in velocity.
bullet Net result is a right handed propeller gives small swing to port.

 

bullet When engine works astern of a right screw vessel,
bullet Helical discharge of propeller splits and passes forward, towards either side of the hull.
bullet On port quarter, it is inclined down and away from hull.
bullet On stbd quarter, directed upwards and on to the hull.
bullet Flow striking on stbd quarter can be substantial to swing stern to port/ bow to stbd.

 

 

Screw race:

bullet When engines works ahead, a spiral flow of water is thrown towards rudder.
bullet It is opposite to transverse thrust.

 

Wake current:

bullet When a vessel moves ahead, a cavity is created at stern.
bullet Water from sides flow and swirl to fill the cavity, which is called wake current.
bullet Steering will be adversely affected as the rudder works in partial vacuum.
bullet Propeller works in disturbed water, speed will be lost, vibration will set up.
bullet Wake current increases with speed.
bullet In a finely sterned vessel, wake current is less.
bullet When engines work astern, wake current is less and propeller is not affected.

 

Frictional wake:

bullet When a vessel moves ahead, belt of water is drawn along the hull, which is called frictional wake.
bullet This frictional wake creates a resistance to upper blades of propeller.
bullet As a result, transverse thrust reduces.

 

Updated: 22-Sep-2007