Tugs Flashcards
How does a tractor tug towing on a line forward, compare to a conventional tug, in giving steering assistance or creating sideways forces on a ship?
Tractor is LESS effective than conventional
What are the three categories of tugs used in ship handling?
Tractor, ASD, conventional
Jeff, 13
What are the 6 elements that interrelate with different tug designs?
Towing point Propulsion point Propulsion and steering Maneuvering lever Hull shape Superstructure and fendering
Jeff 15
What is the towing point?
The last physical point of the tug that fairleads its line to the ship; OR when pushing, the point of contact between tug and object being pushed
Jeff 16
What is the propulsion point?
The physical point of the tug’s application of horsepower, underwater
Jeff 16
Propulsion point is dictated by what three items?
Number of propulsion units
Type of propulsion
Location of propulsion units
Jeff 16
Define maneuvering lever
The distance between towing point and propulsion point
Jeff 17
What is The most common type of tug used worldwide in ship work?
Conventional
Jeff 18
On a conventional tug, when moving AHEAD, the towing point is ______ of the propulsion point
Forward
Jeff 19
Conventional tugs maneuver best and apply strongest force when ______.
Pushing ahead on its towing point, (and when on a towline, this occurs when PP is pushing on TP in direction ordered by pilot.)
Jeff 19
A conventional tug has a greater maneuvering lever when Pushing ahead than when on a towline, towing. T/F
True.
What are the two inherent weaknesses of conventional tugs?
1) Lack of astern maneuverability
2) Risk of capsizing while on the towline, especially when ship’s speed is excessive
Jeff 20
Conventional tugs, when backing on a headline, produce between ____% and ____ % of its ahead bollard pull.
45 to 60 percent
Jeff 20
When a twin screw tug is backing at a 90 alongside a ship that is moving ahead or astern (or with wind/current) it must twin-screw to stay in position. This results in backing power being less than _____ of its full astern power with both engines
Half
Jeff 22
If a conventional tug must back alongside at a 90 for extended periods or with full backing power, what must be done to maintain position?
Rig a stern line, aka quarter line
Jeff 22
Single screw conventional tugs are preferred for assisting what type of vessel?
Submarines
Jeff 23
When comparing horsepower of single vs twin screw conventional tugs, single screws are ____% more efficient.
20 percent
Jeff 25
The key difference between a tractor tug and a conventional tug is:
The orientation of the propulsion point and the towing point. The tractor tug has a longer and more effective maneuvering lever.
Jeff 25
A steerable propeller (ie: Z-Drive) can produce as much as _____ more bollard pull than a cycloidal drive, for a given HP.
One third
Jeff 28
Which type of tug has the deepest draft?
A) conventional
B) tractor
C) ASD
B) tractor
Jeff 30
When lying alongside a ship, delivering full thrust, A tractor tug may heel up to _____ degrees
21 degrees
Jeff p. 30
Where is an ASD tug’s primary towing point, as used for ship work?
On the fore deck
Jeff 33
What is the term used to describe distance between the towing point (TP) and the propulsion point (PP)?
The maneuvering lever (ML)
Jeff 187
The distance between the towing point (TP) and the center of hydrodynamic distance (CHP) is know as _______.
Turning lever (TL)
Jeff 187
What is the result of a longer maneuvering lever in relation to the turning lever?
Less thrust is required to keep the tug in position, therefore more thrust is available for pushing
Jeff 187
A tug pushing alongside, as ship begins to move forward has to compensate for the increased leverage of water flow, resulting in a lengthening of the turning lever, due to a shift of which point, in which direction?
Center of hydrodynamic pressure,
Shifting aft
Jeff 188
What ship speed is generally the maximum for a conventional tug pushing alongside?
3-4 kts
Jeff 190
When a tug is breasted or on-the-hip, it is important to have propulsion units ______, in order to maximize leverage
Aft
Jeff 194
A gob rope is used to alter which point?
Towing point
Jeff 197
Direct towing for tractor and ASD tugs is effective at ship speeds _______
Five to six kts
Jeff 202
Powered indirect towing, working a tug at 90 degrees to a ship’s centerline in relation to the towline, allows a tug to apply ____ to ____ percent of the tug’s rated bollard pull
75 to 120 percent
Jeff 202
Describe the jackknife maneuver
When a tractor or ASD tug quickly repositions from indirect to direct towing (typical for continuous steering forces applied while a ship reduces speed)
Jeff 205
Tug thrust is equal to pulling force at any hawser angle. __________, however increases dramatically with increases of hawser angle.
Line tension
Jeff 209
What are two components of the Center of Hydrodynamic Pressure (CHP)?
Drag: parallels the water flow,
Lift: transverse force
Jeff 175-176
What are the four basic positions for tugs assisting ships?
1) leading tug, towing on a line
2) tug alongside, pulling on a line or pushing
3) breasted or on-the-hip
4) trailing tug, pulling on a line or pushing
Jeff 174
What is the Center of Lateral Resistance of a ship?
The point on a ship where, if a lateral force is applied, will move the ship sideways with no rotation.
Jeff 215
What are the three basic ship handling principles that must be taken into account in order to have a clear vision of the ship’s behavior?
1) Center of lateral resistance
2) pivot point
3) maneuvering lever
Jeff 215
What is the best tug position for steering a vessel, (moving ahead)?
Trailing tug
Jeff 247
Conventional tugs can provide steering assistance as a trailing tug at what speed?
1-3 kts
*At slightly higher speeds, conventional tugs may assist steering, to ONE side only.
Jeff 249
When tractor or ASD tugs are acting as trailing tug, indirect towing is used as speeds ______, and direct towing at speeds_______.
5 kts or less;
Over 5 kts
Jeff 249
What is the Assist Lever (AL)?
Distance between tug and ship’s pivot point.
Jeff 222
A ship is stopped. When a tug pushes on one end, the ship is turned in approximately _____ ship’s length (s).
Two
Jeff 254
When tug forces are applied to both ends of a stopped vessel, the ship is rotated in approximately _____ ship’s length.
One
Jeff 255
A ship with no headway uses steering and propulsion to turn, with a tug on the bow (pushing or pulling to assist turn) To counter forward motion generated by ship’ propulsion, what must the tug do?
Adjust angle to counter ships ahead motion.
Jeff 255
Transverse arrest can be performed by which type of tug(s)?
ASD or tractor (only with steerable nozzles)
Jeff 274
At what speed does reverse arrest start to lose braking force effectiveness, and transverse arrest becomes a better option?
7 knots
Jeff 360
In ship escort, what is the most effective tug position?
Tethered ASD or tractor, to ship’s stern
Jeff 357-358
What is the basic signal to instruct a tug to “come ahead on your engines,” if stopped?
One short blast
Jeff 366
What is the signal given to a tug to instruct “stop” if they are going ahead or astern?
One short blast
Jeff 366
What is the whistle signal to a tug, instructing to “back your engine?”
Two short blasts
Jeff 366
What is the ship to tug whistle signal for “more speed.”
Three very short blasts
Jeff 366
What is the whistle signal for a tug, for “slow speed?”
One prolonged blast
Jeff 366
A ship signaling to a tug to “let go or shift,” is:
One long blast, followed by two short blasts
Jeff 366
Inward turning twin screws are better for _______, whereas outward turning is better for ________.
Inward walks better
Outward turns better
Livingstone 5
Tugs tend to creep which way while walking
Ahead
Livingstone p.5
