Chapter 2 Ship Handling in channel

Question 1

Balanced spade rudder

Answer 1

• Will not steer well when large angles of rudder is used
• turbulent flow across rudder causes a loss of lifting effect
• They will act as though no rudder is applied even as more is added
• beware of turns using more than 5-10 degrees

Question 2

Variable pitch propellers advantages

Answer 2

• many speed choices
• quickly go from ahead to astern
• can go astern indefinitley

Question 3

Variable pitch propellers disadvantages

Answer 3

• when reducing speed water flow is seriously disrupted resulting in adverse affects on steering. speed reductions must be slow
• Pitch cannot be set to zero as it cuts off all water flow to rudders
• less effective when going atsern than conventional props

Question 4

Directional Propulsion At sea mode

Answer 4

• rotation limited to 35 degrees
• full power available
• pods are synchronized
• can be steered with wheel
• power is about double that of harbor mode
• power cut to zero if pods are turned to more than 35 degrees

pg 53

Question 5

Harbor (manuevering) mode

Answer 5

• power about 50% of sea mode
• pods can rotate 360 degrees
• can be rotated independantly and used at different angles
• See pg 61 for illustrations

Question 6

Pod Commands

Answer 6

• engine commands are percentages in all modes
• Normal rudder angle commands are used in at sea mode
• position of pod controls given in reference to clock postions in harbor mode

Question 7

One ship length rule

Answer 7

• Pods should be taken out of auto mode when within one ship length of dock or obstructions

Pg 66

Question 8

As trim to the stern increases

Answer 8

• Ship becomes more directionally stable
• increase in ships turning diameter as bow comes out of the water
• Ship steers better with drag

PG 70

Question 9

An even keel ship

Answer 9

• Large block coeffiecent handles poorly, directionally unstable
• Finer lined ship tends to be directionally stable or neutral

Question 10

Handling a ship with poor directional stability

Answer 10

• More rudder required to start the turn,
• more rudder required to check swing
• Rudder should be returned to midship as soon as swing starts
• Directionally unstable ships will turn in small diameter

Question 11

Using aids during a turn

Answer 11

• Relative bearing of an aid changes toward the bow, the vessel is closing on the aid.
• Relative bearing constant means the ship is turning at a fixed rate and will maintain distance on aid
• Relative bearing of aid changing toward the stern means distance to aid is increasing and vessel will end turn further from the aid

Pg75

Question 12

Sinkage

Answer 12

• Bodily increase in mean draft due to moving through a restricted channel

Pg 84

Question 13

Trim

Answer 13

• Roatation around the transverse axis due to change in pressure and resulting in a change of draft along the ship with greatest change at the bow or stern

Pg 85

Question 14

Squat

Answer 14

• The combination of sinkage and trim
• Squat in shallow water often twice as much as deep water
• Caused by water displaced by ship moving out and around the hull creating pressure differences causing the hull to sink at differnt rates (bernoulis principal

PG 85

Question 15

Blockage Factor

Answer 15

• Restriction of the flow of water trying to be displaced by a ship
• Less area for water to flow out means it must flow out faster which creates greater pressure drop
• As blockage factor increases speed decreaeses as it is harder to drive ship ahead.
• Steering becomes difficult (more directionally stable usally when UKC is .5 draft)
• Vibration and wake size increase

Pg 85

Question 16

Variables of blockage factor

Answer 16

• Speed of the ship
• ratio of draft to depth of water
• ratio of ship cross section to cross section of channel
• block coefficent
• ship displacement
• speed of acceleration

Pg 86

Question 17

Squat and speed relationship

Answer 17

• Squat increases in proportion of the square of the speed
• If speed is doubled squat increases by 4
• Speed through the water is what must be considered

Pg 86

Question 18

Rule of thumb for where squat will occur

Answer 18

• Vessel with block coefficent greater than .75 will squat by head
• Block coefficent less than .7 will squat by the stern

Pg 90

Question 19

Open water squat formulas

(Barrass formula)

Answer 19

• Squat (meters) = Cb x V^2/100
• Squat (feet) = Cb x V^2/30
• S= Squat (or total sinkage)
• Cb= Block coefficient
• V= speed in knots

Question 20

UKC (general)

Answer 20

• Speed through the water is most critical factor in determining UKC
• squat increases with speed with this increases being greater on high block coefficent ships
• 6kts is generally safe speed when UKC is less than 5kts

Pg 95

Question 21

Stability and UKC

Answer 21

• High block coefficent ships will have more predictable UKC based on squat rather than rolling
• Lower Block coeffecient ships will squat by the stern accelerate quickly and roll in turns making their UKC less predictable

Question 22

UKC while accelerating

Answer 22

• Initial squat while accelerating is approx. double calculated squat
• Speed should be increasesed incrementally if UKC is not double squat

Pg 97

Question 23

UKC while meeting and crossing

Answer 23

• Squat is additive when meeting and passing. If 2 ships are meeting and one is squatting 3 feet while the other is squatting 4 feet, both vessels will squat 7 as they pass
• Squat will generally increase by 50% when meeting and passing but may increase up to 100% where channel is narrow or shallow
• Similar rules and assumptions as above should be made while overtaking. Also assume these affect would last longer due to the longer time of the passing