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What is screw race? Which is more prominent – screw race or transverse thrust?
What is wake current, frictional wake and girthing?

SCREW RACE
When the engines are working ahead, it causes the propeller to drive a spiral flow of water towards the rudder. The spiral flow of water is known as a screw race.
SPIRAL motion of water when propeller blades cut the water.
When engine works ahead spiral flow of water is thrown towards the rudder.
It is opposite to transverse thrust.
It gives better steerage
It increases as the ship’s speed increases, therefore it cancels the transverse thrust.
If tug line is cast off, the spiral race causes the line to be pushed away from the propeller blades
At low speeds transverse thrust more prominent
At high speeds screw race more prominent

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

FRICTIONAL WAKE/ SKIN FRICTION
When a vessel moves ahead, belt of water is drawn along the hull, which is called frictional wake.
This frictional wake creates a resistance to upper blades of propeller,
As a result, transverse thrust reduces.
Under sternway there is very little wake strength at the propeller and transverse thrust increases as speed increase.
What is girthing? What is capsizing moment? What are the conditions for it to happen?
It is the capsizing moment of the tug due to the sudden movement of the ship. The line is usually secured very near to the centre of floatation and for this reason the tug is liable to be girded. This phenomenon is known variously as girding, girthing or girting in different parts of the world.
It can be caused by one or both of the following.
– The ship turning independently and too quickly away from the tug.
– Excessive straight line speed with the tug made fast.
Position 1
In this area the tug is relatively safe and regardless of whether the ship’s speed is too high it does not result in any immediate problem, provided it remains within a small angle on the bow.
Position 2
It the tug is out in this position broad on the bow, the ship could, as a result of too much starboard helm or excessive speed or both outrun the tug which may have neither the time nor manoeuvrability to turn and keep up with the rapidly swinging or accelerating ship.
Position 3
Thi is the worst possible situation where the tug is being pulled around on the radius of the tow line and because of the position of its hook, is then dragged along with the tow line out on its beam. Due to the nature of the forces involved, it will also be pulled over to a dangerous angle of heel and unless the tow line breaks, or can be released immediately, the tug which is powerless to respond and already listing heavily may capsize.

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