Ch. 9 Flashcards
What are the two components of the center of hydrodynamic pressure?
Lift and drag
How should a tugs line be cast off from the ship to the tug?
The ships mates on the bow and stern should direct their deck crews to slowly lower the tugs line and tagline unless directed to drop it by the tugs operator. Dumped lines have potential to cause serious injury to the tugs deck crew as well as foul the ship or tugs propulsion units.
Why is towline work on a conventional tug dangerous?
The tugs towing point is forward of the propulsion point which can make the tug susceptible to tripping or girting.
Explain how a conventional tug trip or girts.
Tripping or girting the tug can be caused by the ship overtaking the tug, or when the tug thrust or maneuvering lever can not counteract the force of the tugs center of hydrodynamic pressure acting on the tugs turning lever.
How can a tug handler best avoid tripping or girting?
By employing a tractor or reverse tractor tug or requiring that the ship go at slow to moderate speeds.
What is the advantage of using a tug alongside?
It takes less time for the ship to respond to the tugs efforts, and working the tug alongside is safer than towline work.
When does a conventional tug use a quarter line?
The conventional tug uses the quarterline to maintain a position approximately at right angles to the ship when backing, otherwise the effect of the tugs propeller, the vessel motion, or wind and current might move the tug out of position.
Explain a wrap line.
A wrap line is a line used by a tug to steer the bow of a ship moving stern first. It is lead from one side of the ships bow to the bullnose on the tug located on the opposite side of the ships bow. This allows the tug to steer the bow in either direction.
Can a tug work a wrap line on all vessels?
No, Bulbous bows, severely raked stems, and ships with flare may prevent a tug from working in this position.
What is meant by the term snapping a tow?
This occurs when a tug with a ship or barge in tow makes a hard turn and at the same time applies more power. Since the tugs rudder will set the stern of the tug in the opposite direction, the bow of the towed vessel will often be set in the opposite direction of the turn at first.
Why should a tug made up for breasted towing have tight lines?
If there is too much slack in the lines, the tugs angle to the ships centerline will vary and detract from the tugs ability to apply effective steering force.
Why is indirect towing so powerful?
Indirect towing uses the tugs hydrodynamic resistance to add force to the towline.
How is indirect towing used to advantage?
Indirect towing can be used to advantage to provide steering and braking forces at ship speeds which are too high for effective direct towing.
Why should a tug back easy on a slack headline?
Because the momentum of a tug moving astern could develop enough force to part the line.
What may happen when a tug crosses a vessels wake?
The propeller may cavitate.
