Ship handling Flashcards
What ship characteristics affect course stability?
Length to breadth ratio (high ratio has good course stability)
Block coefficient - large coefficient has poor course stability but turns easily
Where is the pivot point when moving ahead and astern?
1/4 of ships length from bow or stern respectively
How does the UKC influence the pivot point?
With a small UKC, the additional resistance from the restricted water moves the pivot point further aft creating a smaller turning lever and making turning more difficult and sluggish.
What is the “ship generated current”?
As a ship moves, the area of previously displaced water must be filled by surrounding sea. This generates a current in the direction of the ships movement. If the ship stops all propulsion, this current will continue to push the ship for a period.
How does the centrifugal force act during a ships turn?
As the ship turns, the centrifugal force carries the ship in a direction away from the direction of turn.
As ship turns to starboard, it will have lateral force to port
As ship turns to port, it will have lateral force to starboard
How does transverse thrust turn a vessels bow when moving astern?
A right handed propellor turns the bow to starboard (Right)
A left handed propellor turns the bow to port (Left)
How does transverse thrust work?
As the propellor turns, it pushes water from one location to another creating a positive pressure point where there is a build up of water and a negative suction where the water has been removed from. The positive pressure point pushes the stern towards the suction side.
It is most pronounced during astern manoeuvring due to the water being pressed against the hull from the propellor instead of being pushed away to open sea.
Why should care be taken during a crash stop due to transverse thrust?
During a full astern manoeuvre, maximum transverse thrust will occur swinging the bow. Care must be taken that there is sea room for this swing.
How does the wind force interact with the pivot point?
The wind force will create a turning moment around the pivot point, gradually reducing the lever until the wind force acts in the same plane as the pivot point and the turning moment is zero.
How is wind force related to wind speed?
Wind force increases with the square of the speed.
Double the wind speed and the force increases x 4
What is the “limiting speed” in shallow water?
Displacement vessel speed is limited in shallow water due to the additional resistance and inability for water to flow any faster.
Limiting speed = 4.5 x square root of the water depth
How do you calculate the wind force to calculate tug requirement/ vessel capability?
(Wind speed /18,000) x windage area of the vessel
How do you approach a berth port side to on a single screw ship?
Approach at a large angle and the transverse thrust will bring the stern in as the vessel stops. Kicks ahead and use of helm will bring the bow in.
How do you approach a berth starboard side to on a single screw ship?
Approach parallel. Transverse thrust will bring the bow closer to the berth. Kicks ahead and helm will bring the stern closer.
How does lateral motion of the ship act when manoeuvring?
Lateral motion is always at right angles to the direction of motion and away from the direction of turn.
