Tug Bollard Pull - Shiphandling Facts and Rules

Question 1

Tug Bollard Pull - Three phases of tug assist and ship approach

Answer 1

1 - Reasonable speed phase
2 - Intermediate phase - tug assist may be necessary
3 - Final arrival maneuver - Bollard Pull Most Important

Question 2

Tug Bollard Pull - bollard pull required…
(Henson)

Answer 2

should compensate for wind, waves, and current PLUS a 20% safety factor

Question 3

Tug Bollard Pull - tugs working at ship’s side OR on short towlines safety effect

Answer 3

• increase the safety margin by 50%

Question 4

Tug Bollard Pull - drift rate / UKC / BP

Answer 4

• ship drift rate decreases as UKC decreases
• increased BP required to overcome small UKC

Question 5

Tug Bollard Pull - current velocity on beam that creates 10 Tons of force in deep water (5x) increases as UKC decreases; namely…
(Henson)

Answer 5

10T @ 5x draft
20T @ 1.6x draft
30T @ 0.5x draft
40T @ 0.2x draft
(Henson pg 139)

Question 6

Tug Bollard Pull - UKC / angle of attack

Answer 6

• when ship has small UKC, current force decreases quickly when angle of attack is less than 90º and may work behind the ship forcing it in the opposite direction

Question 7

Tug Bollard Pull - hydrodynamic mass is…

Answer 7

• mass of the water moved by the ship’s movement
• ranges from 25% to 100% of the ship’s displacement
• greatest with sideways ship motion
  (Henson pg 141)

Question 8

Tug Bollard Pull -
virtual mass is…
(Henson)

Answer 8

• hydrodynamic mass + ship displacement
• Virtual Mass = 1.3x to 2.0x ship displacement
  (Henson pg 141)

Question 9

Tug Bollard Pull - hydrodynamic mass / UKC

Answer 9

• hydrodynamic mass and therefore virtual mass increases as UKC decreases
  (Henson pg 141)

Question 10

Tug Bollard Pull - VS tug / wash

Answer 10

• VS tug has least negative effect of wash against ship’s hull
• due to propulsion design and placement

Question 11

Tug Bollard Pull - conventional tug propulsion point / wash

Answer 11

• conventional tugs have propulsion points closest to the ship
• propeller wash hitting ship’s side reduces pulling effectiveness

Question 12

Tug Bollard Pull - Yaw Moments - BP required

Answer 12

• require higher BP
• generally greater with wind on the quarter

Question 13

Tug Bollard Pull - bow thruster - HP to Tons

Answer 13

100HP = 1.1 Tons of force

Question 14

Tug Bollard Pull - tug wash mitigation / UKC

Answer 14

• proper towline length and towing angle
• the less UKC the more power needed = towline should be longer
  (Henson pg 141)