Tugs General - Shiphandling Facts and Rules

Question 1

Tugs General - HP to BP
(MacElrevey)

Answer 1

100 HP = 1 ton bollard pull (MacElrevey)

Question 2

Tugs General - BP to 100BHP by tug
(Henson)

Answer 2

VS: 1.0 tons / 100bhp
ASD: 1.3 tons / 100bhp
Conventional:
- w/ nozzles: 1.5 tons / 100bhp
- w/ open prop: 1.1 / 100bhp
(Henson pg 79)

Question 3

Tugs General - ship speed for conventional tug

Answer 3

maximum 3 kt ship speed

Question 4

Tugs General - negatives for pulling

Answer 4

• may part line
• poor lead for ships in ballast
• wave surge puts shock load on line (Hooyer pg 60)

Question 5

Tugs General - tug position in turn

Answer 5

tug pushing forward on shoulder less effective than on quarter because of pivot point and hydrodynamic forces forward opposing the turn

Question 6

Tugs General - Conventional and Omni positioning

Answer 6

use conventional forward and omni aft

Question 7

Tugs General - VS and ASD positioning

Answer 7

use ASD forward and VS aft (except if more braking force is needed)

Question 8

Tugs General - bow in maneuver tug position - same type

Answer 8

1 or 2 tugs same type - use stronger tug forward

Question 9

Tugs General - stern in maneuver tug position - same type

Answer 9

1 or 2 tugs same type - use stronger tug forward

Question 10

Tug Use in Port - AST / Voith tractor assist
(Henson)

Answer 10

• Voith better at stern indirect towing at higher speeds
• ASD better as forward tug and at stern direct towing at speed (100% thrust any direction)

Question 11

Tug Use in Port - main types of tug assistance
(Henson)

Answer 11

• tugs towing on a line, usually 1.5x length of tug
• tugs operating at the ship’s side made fast with 1, 2, or 3 lines

Question 12

Tug Use in Port - Voith assist towing/pushing switch
(Henson)

Answer 12

can switch from towing on line to pushing, forward or aft, at ship speed of 2 kts or less

Question 13

Tug Use in Port - IMO escort heeling regs
(Henson)

Answer 13

requires tugs engaged in escort operations with the max heeling lever in an equilibrium situation, heeling angle 15º or less

Question 14

Tug Use in Port - line pull relationship to bollard pull
(Henson)

Answer 14

• if prop RPMs double, then line force will increase by a factor of 4 and required engine power will increase by a factor of 8
• applies to most tug ops

Question 15

Tug Use in Port - towline length
(Henson)

Answer 15

• should be as long as possible
• tugs lose 40-60% on short towline
• in heavy wave conditions could lose up to 90%

Question 16

Tug Use in Port - tug assist line strength
(Henson)

Answer 16

• should be 4-6x tug’s BP rating

Question 17

Tug Use in Port - center of pressure relation to tow point and heel
(Henson)

Answer 17

• the smaller the distance from center of pressure vertically to the towing point will reduce heel

Question 18

Tug Use in Port - propulsion point relation to tow point and heel
(Henson)

Answer 18

• longer distance from propulsion point vertically to tow point will counteract heel

Question 19

Tug Use in Port - ASD vs. Voith escort brake / steering forces
(Henson)

Answer 19

• ASD tugs brake better
• Voith tugs steer better

Question 20

Tug Use in Port - minimum tug GM height
(Henson)

Answer 20

• minimum recommended metacentric height is 3m