Ch. 3

Question 1

What advantages does an ASD tug have over at tractor tug?

Answer 1

Better sea keeping ability, shallower draft, more efficient in converting horsepower to bollard pull and can perform a transverse arrest.

Question 2

What advantages does a tractor tug have over a conventional tug?

Answer 2

A tractor tug can steer when going astern, has omni-directional thrust and does not have the same risk of capsizing as a conventional tug when towing

Question 3

What are the disadvantages of a twin screw tug?

Answer 3

May require a stern line when backing. Is less efficient than a single screw tug and backing power is less than power ahead.

Question 4

What advantages does a twin screw tug have?

Answer 4

Excellent maneuverability, ability to maintain position when backing ( if ship is Stopped), and ability to steer astern.

Question 5

What is the principle disadvantage of a single screw tug?

Answer 5

Lack of maneuverability, inability to steer astern, and inability to maintain position when backing. Power astern is less than power ahead.

Question 6

What advantage does a single screw tug have over a twin screw tug?

Answer 6

Efficiency, since on a horsepower basis, a single screw tug will develop about 20% more thrust.

Question 7

What causes a tug to capsize while on a towline?

Answer 7

When the tug does not have enough leverage to counter the pull of the towline

Question 8

Describe the length and orientation of the maneuvering leer for ASD tugs.

Answer 8

Towing point FWD of propulsion point, longest when towing point is the bullnose shorter when towing point is the tow bitt.

Question 9

Describe the length and orientation of the maneuvering leer for tractor tugs.

Answer 9

Towing point aft of propulsion point moderate maneuvering lever.

Question 10

Describe the length and orientation of the maneuvering lever for conventional tugs.

Answer 10

Towing point fwd of propulsion point maneuvering lever is short

Question 11

What is the maneuvering lever?

Answer 11

The distance between the towing point and the propulsion point

Question 12

What is the propulsion point?

Answer 12

The focal point of the tugs application of horsepower under water. Location and type of propulsion.

Question 13

What is the towing point?

Answer 13

The last physical point on the tug that fairleads its line, or the contact point between tug and ship when the tug is pushing

Question 14

What are the five key elements in tug design?

Answer 14

• Towing Point
• Propulsion Point
• Propulsion and steering
• Maneuvering lever
• Hull Shape
• Superstructure and fendering

Question 15

What are the three general categories of tugs used in shipwork?

Answer 15

Conventional
Tractor
Azimuthing stern drive