Chapter 1 Arrival Flashcards
Shallow water effects
- Become significant at 1.5 times draft
- Full shallow water effects felt at 1.2 times draft
Master’s tests
- Should be done in water 1.5x draft
- Include Hard turns both directions
- Backing and filling from dead stop
- Half astern times stop from 6kts
- Backing to stop while maintaining heading Backing with rudder midship
- Turning ship with thruster at 3kt and 1kt
Engine speed during a turn
- Accelerating engine speed while turning will create smallest diameter turn
- Same speed throughout turn creates medium diameter
- Stopping engine in turn creates largest dia.
Backing and filling
When done properly vessel can turn in 1.5 x length
- Steps
- Half ahead, hard right
- Half astern, hard right
- Half astern, hard left
- Half ahead, hard right
Half astern to dead stop
- Ship will end 90degrees to original heading
- Stopping distance in deep or shallow water not much difference
- Ship will want to head into wind when rudder control is lost, and will back to wind when sternway develops
Stopping and maintaining heading
- Apply left rudder to start swing to port
- Begin backing (prop walk will stop swing)
- As ship begins swinging right apply left rudder and short burst ahead to swing left again
- Repeat until vessel is stopped
Thruster advantages
- At extreme ends of vessel making them very effective
- Available at all times unlike tugs
- Good lateral control without effecting headway
- Reduces need for tugs
Thruster disadvantages
- Ineffective as speed increases
- less powerful than modern tug,
- will not slow a ship,
- maintenance
- unreliable at light drafts,
- no ability to control headway or sternway
- * most useful at less than 2kts. innefective over 3kts
In shallow water directional stability
- Becomes more positive
- directionally unstable ship will become more stable
- This is true until vessel squats by the bow and direction stability is lost again
In shallow water Rate of turn
remains essentially the same
In shallow water diameter of turn
Increases
PG18
Large course changes in shallow water
Speed will decrease but to a lesser extent than deep water
When engine is stopped in shallow water
headway will decrease slower than in deep water. (vessel carries way longer)
When backing in shallow water
heading will fall off to starboard (stern walks to port) at a greater rate as depth decreases
Positive directional stabilty
- Ship tends to steady up when rudder is put midship
- Trimmed by the stern will result in positive directional stability
pg18
Negative directional stability
- ship will swing at an increasing rate of turn when rudder is put midship
- Timmed by the head will result in negative directional stability
pg19
Neutral directional stability
- continues swinging at the present rateor continues along current heading until external forces take over
- no tendancy to increase or decrease ROT when put to midship
pg19
As UKC decreases directional stability
Increases
pg 20
Directional stability becomes more positive
- as length increases
- as drag increases
pg 20
Directional stability decreases
- As block coefficent increases
- As beam increases for a given length (length/beam ratio decreases)
- As area of forward sections increases relative to after sections (pivot point shifts ahead)
Bank cushion
- The ships bow moves away from the shallow water because of increased pressure between bow and shallow water
In shallow water the vessel’s midbody
- will move bodily sideways toward the the shallows
- caused by increased velocity of water passing between the ship and shallow water
pg 21
Bank Suction
- Stern of the ship will move toward the shallow water
- caused by reduced flow of water to area behind ship, and the ships propeller being drawn to shore
- bank suction is stronger than bank cushion
pg 21
3 Methods to reduce headway
- Using engine astern- easiest, most common and least effective
- slewing vessel on base course
- large heading change including a complete round turn
- * ship going 12 knots can lose 2-3 knots at the completion of a round turn without reducing rpm
pg 24
Loss of speed during heading change rule of thumb
ships can lose 25-20% of headway for every 90 degrees of heading change
Speed through the water with prop wash
*When engine is astern*
- Wash is astern of vessel = Greater than 2kts
- Wash begins making its way up the vessel (at stern) = 2kts
- Wash at midship = dead in water
pg 28
High sided ships feel effect of wind
at 3 times the vessel’s speed
pg 32
Loaded tankers will feel wind effect
at 5 times the vessel’s speed
pg 24
