Shiphanding with Tugs Slesinger Flashcards
1
Q
What are the three general categories of tugs used in shipwork?
A
- Conventional
- Tractor
- Azimuthing Stern Drive (ASD)
2
Q
What are the five key elements in tug design?
A
- Towing Point
- Propulsion Point
- Propulsion and Steering
- Hull Shape
- Superstructure and Fendering
Note: TP and PP determine Maneuvering Lever (ML)
3
Q
What is the Towing Point?
A
Last physical Point on the tug that fair leads it’s line to the ship. When pushing , Point of contact between the tug and the object being pushed.
4
Q
What is the Propulsion Point?
A
The physical focal point of the tug’s application of horsepower underwater.
5
Q
What is the Maneuvering Lever?
A
The distance between the Towing Point and Propulsion Point. It is what gives the tug leverage to swing its bow or stern around the Towing Point.
6
Q
Describe the length and orientation of the maneuvering lever for:
- conventional tugs
- tractor tugs
- ASD tugs
A
Conventional: Pushing is best lever with TP at bow and PP at stern. Towing with TP midships of aft of midships is shorter lever.
Tractor: TP aft and PP just forward of midships is best lever. TP on bow is shorter lever.
ASD: TP on bow and PP aft makes for lever the length of the boat. TP aft of midships is shorter lever.
7
Q
What causes a tug to capsize while on a towline?
A
Short Maneuvering Lever and the towline pull falling off to the side. If the tug does not have enough power and leverage to push its stern around and Towing Point back in line with the heading of its tow, it will be tripped.
8
Q
What advantage does a single screw tug have over a twin screw tug?
A
Single screw is 20% more efficient than twin screw. Also, helping submarines to/from berth proffered over twin screw due to less likelihood of damage from propeller.
9
Q
What advantage does twin screw tug have?
A
Maneuverability (compared to single screw)
10
Q
What are the disadvantages of a twin screw tug?
A
Less efficient in converting HP to bollard pull (compared to single screw)
Much less power when backing (compared to tractor and ASD)
11
Q
What advantages does tractor tug have over conventional?
A
The orientation of Propulsion Point to Towing Point. The omnidirectional propulsion units are forward of amidships and allow the Towing Point to be moved further aft for a longer and more effective Maneuvering Lever.
12
Q
What advantages does ASD have over a tractor tug?
A
Shallower draft than a tractor, most efficient propulsion units to convert HP to thrust, and initial capital outlay is less than cycloidal units.
13
Q
What are the four components of propulsion and steering systems?
A
- Power source
- Power transfer and control system
- Hydrodynamic driver (thrust)
- Thrust directional control mechanism
14
Q
What is the purpose of the power transfer and control system?
A
To transfer rotative force to the tugs hydrodynamic driver.
15
Q
What effect has the Diesel engine had on the design of tugs?
A
They are powerful and efficient, making it possible to increase both the power and range of tugs. Compact powerful assist designs could not have been built without Diesel engine technology that places more HP in a smaller footprint.
16
Q
Describe the DR system of engine control.
A
The engine is connected directly to the propeller. To reverse the propeller, the engine is stopped and restarted in the desired direction.
17
Q
What made the DR system obsolete?
A
Modern machinery which allows tugs better maneuverability has made DR obsolete.
18
Q
Describe the DE system of engine control.
A
A diesel electric (DE) system consists of main engines turning generators supplying power to electric propulsion motors.
19
Q
What are the advantages of DE drive?
A
Delivers shaft speed from dead slow to full ahead (astern) and responds without delay to changes in speed and direction of rotation. Engine maintenance is minimized, with multiple engines only the engines/generators required need to be run, more flexibility in engine room layout.
20
Q
What are the principal disadvantages of DE drive?
A
Cost and marine electrical vulnerability to dampness in salt in atmosphere.
21
Q
How does an SCR system differ from the conventional DE drive?
A
Main generators produce AC suitable for ship supply and other applications. Current is delivered through rectifiers which convert it to DC for driving the propulsion motors.
22
Q
Are there any particular advantages to the SCR system?
A
AC generators are cheaper than DC and compatible with AC vessel electrical systems.
23
Q
Describe the CPP system.
A
Three components. A reduction gear to reduce to a suitable shaft speed for the propeller, a hydraulic system to control pitch, and a uniquely designed propeller hub to house the mechanism. The CP propeller is always turning and thrust (ahead and astern) is generated by changing pitch of propeller blades instead of changing rotation of propeller shaft. Shaft speed may be constant or combinator (automatic matching of propeller RPM with pitch) control
24
Q
What Maneuvering function is housed in CPP?
A
Pitch and therefore thrust. Also, reversing.
25
What are the drawbacks of a CPP system?
Propeller always turning, neutral pitch disrupts flow to rudders reducing or negating rudder effectiveness, repairs can be expensive and typically require dry-docking.
26
Does a reverse reduction gear change propeller shaft rotation?
Yes
27
What factors determine a propellers efficiency?
Quality and velocity of water flow on the intake or discharge side of the propeller, size and speed of rotating propeller blades, and the number, pitch, and shape of the blades.
28
How do nozzles increase propulsion efficiency?
They improve propeller thrust.
29
What are the disadvantages of nozzles?
Lateral resistance underwater, hindering maneuverability. 50% of ahead thrust when backing (similar to unducted). Subsequent nozzle designs have addressed these deficiencies and can produce 70% of ahead thrust when backing.
30
What three factors determine a rudder’s efficiency?
Velocity and quality of water flow and rudder shape.
31
What is the difference between an unbalanced and balanced rudder?
Unbalanced: all the rudder’s length is on one side of the rudder post.
Balanced or semi-balanced: leading edge of the rudder extends forward of the rudder post to use propeller flow efficiently and produce mechanical advantage to steering gear.
32
What is a spade rudder?
A balanced rudder hung independently of any skegs.
33
What is the difference between a steerable nozzle and a steerable propeller?
Nozzle turns 25-30 deg with fixed propeller vs. nozzle and propeller turn together through 360 deg.
34
Why are flanking rudders effective when operating astern?
The flanking rudder is then operating on discharge side of the propeller.
35
Describe the Voith-Schneider cycloidal propeller.
A controllable pitch propeller rotating around a vertical axis. A rotor casing ends flush with the ship’s bottom. The rotor casing is in a constant circular motion on a horizontal plane. Attached to the rotor casing are vertical blades where each can rotate about its own vertical axis, performing an oscillating motion about their own axis while following the uniform motion of the rotor casing. Gears and mechanical links control the pitch angle of the vertical blades at any desired point along the circumference of the rotor casing’s circular motion. As a result, the same amount of thrust can be generated in any direction. Typically two units mounted forward in tractor configuration.
36
What are the disadvantages of VSP?
HP to bollard pull ratio is lower than conventional or steerable propeller tugs, vulnerability to underwater debris and grounding, high initial cost, high repair cost.
37
Describe a steerable propeller system.
Engine power is transmitted to a drive unit consisting of a propeller (and sometimes nozzle) that can be rotated through 360 degrees by hydraulic motors. Most SPS have the propeller running in only one direction requiring rotation of drive 180 deg to reverse thrust.
38
What are the disadvantages of a SPS?
Vulnerable to underwater debris and grounding, high initial cost (less than VSP), high repair cost
39
What is bollard pull?
Thrust in pounds or kilograms delivered by the engine under static conditions ( pulling against a dock of fixed structure).
40
Is bollard pull a fair criterion for judging a tug’s serviceability for shipwork?
Yes, however other criteria must be considered such as: maneuverability, stability, and backing power - deficiency in these may limit a tugs ability to effectively apply bollard pull.
41
Describe the important aspects of the hull of a tug used for shipwork.
Must be able to withstand heavy contacts without sustaining damage, reinforced in critical areas where propelling power is brought to bear, and have a high amount of stability (stiff) to avoid capsize from heeling forces (line leading off side of vessel, etc.)
42
Is the hull reinforced?
Yes
43
Why are deck structures set so far inboard on Harbor tugs?
To prevent damage to the tug and ship when the tug works under the flare of the bow or overhang of the ship’s side near the stern. Also, to minimize likelihood of damage from rolling of both vessels.
44
What is a bullnose?
A large closed chock
45
What is a staple?
Functions as the Towing Point on tractor and ASD tugs and is subject to extraordinary strain. As such, it is heavily constructed and mounted in a manner that solidly connects it to the tug’s frames, bulkheads, and plating creating a web of strength and structural integrity.
46
Where are the bits located on conventional American harbor tugs?
Shoulder, quarter, and H bitt aft of the deckhouse (and on bow if no tow winch)
47
What are the structural requirements for the bits and deck fittings?
Stout enough to withstand heavy line strain when the tug is backing or coming ahead with full engine RPM.
48
What type of mast should be installed on a harbor tug?
Set up in a tabernacle for easy lowering.
49
With respect to visibility, how should the wheelhouse be arranged?
Operator’s field of view as unobstructed as possible. Horizontal view as close as possible to 360 deg. For working alongside large vessels with high freeboard-additional small windows that face upwards.
50
Why are model hull single screw tugs proffered for assisting submarines?
To avoid sub side shell plating damage from propellers on twin screw tug or chine of v-bottom tug.
51
What structural characteristics of a ship can make it difficult or dangerous for a tug assisting it?
Subs with no ship side for above water tug fenders to lay against, aircraft carriers with overhanging structures and many projections
52
What are pocket or recessed chocks and why are they useful?
Chocks/bits recessed into the ship’s side to which a tug can make fast. Useful on vessels with high freeboard or limited chock locations (car carriers, cruise ships).
53
What are the three functions of fendering?
1. Absorb and dissipate energy
2. Enhance or reduce traction
3. Create space
54
What are the components of a towline system?
Working lines (material and construction), fastening/release mechanisms(bits/cleats, winches, tow hooks), best practices (operational procedures, handling and care)
55
What types of synthetic lines are used in shipwork?
Polyester (Dacron), polypropylene, blends (nylon, polyester, and polypropylene), Aramid, and HMPE (high modulus polyethylene)
56
Are 100% nylon lines used for shipwork?
No
57
What are Dacron lines used for primarily?
Slings, mooring lines, and working lines on tugs.
58
What are polypropylene and polyethylene lines used for?
Polypropylene used in light work applications or as component in blend. Polyethylene is used in blends or alone as working line if HMPE.
59
What are blended lines used for?
Bonds use two complementary types of fiber that compensate for each other’s respective deficiencies. Most have qualities well suited for working lines in shipwork.
60
What are the two common trade names for HMPE fibers?
Spectra and Dyneema
61
What purpose do wire rope pendants serve?
They protect the soft line from chafe and abrasion where the lines pass through the ship’s chocks.
62
What are the five common rope construction patterns?
1. Three strand
2. Six strand
3. Eight strand
4. Twelve strand
5. Double braid
63
What types of wire rope are suitable for pendants and working lines?
6x19 or 6x37 IWRC with fiber core.
64
What are the purposes of a composition line?
Use abrasion resistant components at high friction points (ship’s bitts/chocks), create an easily replaced, disposable component in high areas of wear, accommodate dynamic loading, create an inherent and purposeful weak link in the line, and provide flexibility and softness to working ends handled manually.
65
What are the four important characteristics of a winch?
1. Braking capacity
2. Maximum line-pull capacity
3. Slack line speed
4. Remote release
66
What are the two main operational best practices regarding rigging for shipwork?
Choose the right rope for the job and use 5he rope properly.
67
List the common items of auxiliary equipment.
Heaving lines, tag lines, gob lines
68
Describe the heaving lines commonly used on tugs.
15 fathoms long, 3/8-7/16 dia, made of Manila or other non slippery material.
69
Describe and explain the purpose of a tag line.
Lengths of line secured to the outboard end of the tug’s working lines. 30 to 40 feet long, typically 3/4 to 1 inch dia and made of Manila or other non slippery material. They are attached to the bitter end of a heaving line which is passed to the vessel first. They enable the ship’s crew to pull in 5e heaving line and get a hold on the tag line before the weight of the hawser is felt.
70
Can tag lines be made of Manila?
Yes
71
What is a gob line and what is its purpose?
They act as a preventer to keep the towline from leading over the side and capsizing or tripping the tug.
72
What is a quick-release strap and how is it used?
Used as a substitute for a towing hook. It is a short line (6-8 fathoms) with an eye at one end that is rove through the eye of a ship’s hawser and secured to the after tow bitts, permitting the hawser to be cast off in an emergency.
73
What is a critical decision point?
A point in the maneuvering sequence that requires critical action at a specific time.
74
Describe the effect of propeller torque on conventional single-screw tugs with a right hand propeller.
A “right-hand propeller” will have a tendency to move the stern to starboard and the bow to port when engaged ahead. When reversed, it will move the stern to port and the bow will fall off to starboard.
75
How is a single-screw tug maneuvered when it is necessary to back for some distance?
It is steered when moving astern by giving an occasional “kick” ahead on the engine to correct the heading. This is done briefly enough so that it does not destroy sternway.
76
How is a single-screw tug turned in a short distance?
It is turned by “backing and filling”, i.e. alternately going ahead and astern on the engine with the helm turned toward the direction of the turn.
77
Will a single screw tug turn more readily in one direction than the other?
Usually it will since the stern will be cast to one side or the other when the tug backs its engine. This will be to port with the standard right-hand propeller. Tugs with nozzles may not react the same way.
78
What is the principle advantage of flanking rudders or a steerable nozzle rudder on a single-screw tug?
They enable the single-screw tug to steer astern and maintain position when backing during shipwork.
79
Are steerable nozzles as effective as flanking rudders?
No, because when the tug is backing and filling to make a tight turn, the nozzle must be reversed between the ahead and astern engine maneuvers.
80
How does a twin-screw tug steer with engines?
Both propellers thrusting in the same direction with the same RPM will balance each other’s torque and off-center position. By manipulating the balance between the two engines, a twin-screw tug can steer with its engines.
81
What is meant by the term twin screwing?
The tug is being maneuvered with one engine ahead and the other astern. The tug can be turned completely about without moving ahead or astern.
82
What is meant by the term “flanking” and how done?
Flanking means moving a tug laterally by using its engines and rudders in opposition. For example, if the tug’s port engine is maneuvered ahead with the rudder to port and the starboard engine is backed, the tug will move sideways to starboard.
83
How do conventional tugs usually undock?
By coming ahead on a springline until the stern is open and then backing clear of the dock. However, a twin-screw tug may “flank” off the dock.
84
How does a conventional tug normally dock?
It will approach the dock at an angle of 15 degrees to 20 degrees, stop it’s engine, then bear off, and back its engine when close alongside. It can then be worked ahead on a springline until it is in position.
85
What do the the wheel and pitch levers control on a VSP tractor tug?
The wheel controls the bow and the pitch levers control the stern. The pitch levers also control fore and aft movement of the tug.
86
A tractor tug will steer _________ a turn.
into
87
What two methods can be used to bring a VSP tractor tug to a stop?
One is by using pitch levers alone (reversing pitch); the other is a combination of pitch levers and wheel (turning the tug 90 degrees to its initial heading).
88
How can you flank or walk a VSP tractor tug?
Flanking a VSP tractor tug is done by turning the wheel in the desired direction of lateral movement; setting the outside lever (the one closest to the direction of desired movement) forward and the inside lever aft. Fine adjustments between the three controls will propel the tug directly sideways.
89
Why is it sometimes necessary to have a higher approach speed when docking a VSP or tractor tug?
When holding the tug’s bow off the dock the wheel-induced transverse thrust may prematurely retard the tug’s way.
90
Do control handles on an ASD tug always point in th direction of the tug’s motion?
No, they always point in the resultant direction of the azimuthing drives thrust. When moving bow first, the drive units are rotated in the direction opposite of the desired turn. This is because the stern is steering the bow. When steering while moving stern first the drive units are rotated in the same direction as that of the desired turn.
91
How can you flank or walk an ASD tug?
The outboard drive unit (one closest towards the direction of lateral movement) is in a “backing” alignment while the inboard drive unit is angled ahead and in. The azimuth and RPM of each drive unit are adjusted to produce the desired effect.
92
Why must you think ahead on an ASD tug?
The tug operator must anticipate the path of the tug and the drive unit configurations required during the sequence of the maneuver.
93
What three factors must a tug operator balance when maneuvering around ships?
Tug hull resistance, opposing forces, and application of force to ship.
94
Hydrodynamic interaction between ship and tug are a function of which three factors?
Ship speed, displacement, and hull shape.
95
What hydrodynamic pressure zones are caused by the ship’s passage through the water?
High pressure at the bow, lower pressure alongside the ship, and suction at the stern.
96
What are the hydraulic effects on the tug as it makes an approach close to a ship underway?
How should the tug handler compensate for these effects?
It draws the tug toward the ship by the suction aft and it pushes the tug away from the ship forward.
By pacing the ship long enough to see how the tug is affected and then gently easing the tug alongside. An experienced operator lets the tug be drawn in slowly by the suction aft with the helm turned slightly away from the ship and works the tug in against the hydraulic forces forward.
97
What is stemming?
“Stemming” can occur when a tug takes a towline from the bow of a moving ship. If the tug comes in contact with the ship’s side it may loose steerage, in which case the tug may be caught by the ship’s bow and rolled over.
98
What are two common cases of stemming?
Misjudgment of the tug’s set in towards the ship’s bow or oversteering as the tug transits through the ship’s pressure zones at its bow.
99
Should a tug ever back it’s engine when coming alongside a ship?
The tug operator should avoid backing when the tug is overtaking the ship to get in position to push or pull. When the tug backs it’s engine, the tug may not respond and can cause damage to itself and to the ship.
100
What are the two components of the Center of Hydrodynamic Pressure?
Lift and drag
101
How should a tug’s line be cast off from the ship to the tug?
Slowly lower the tug’s line and tag line unless directed to drop it by the tug’s operator. Dumped lines have the potential to cause serious injury to the tug’s deck crew as well as foul the ship or tug’s propulsion units.
102
Why is towline work on a conventional tug dangerous?
The tug’s Towing Point is forward of the Propulsion Point which can make the tug susceptible to tripping or girting.
103
Explain how a conventional tug trips or girts.
Tripping or girting can be caused by the ship overtaking the tug, or when the tug’s thrust and maneuvering lever cannot counteract the force of the tug’s center of hydrodynamic pressure acting on the tug’s turning lever.
104
How can the tug handler best avoid tripping or girting?
By employing tractor or reverse tractor tugs or requiring that the ship go at slow-to-moderate speeds.
105
What is the advantage of using a tug alongside?
It takes less time for the ship to respond to the tug’s efforts and working the tug alongside is safer than towline work.
106
When does a conventional tug use a quarter line?
To maintain a position approximately at right angle to the ship when backing, otherwise the effect of the torque of the tug’s own propeller, the vessel’s motion, or wind and current might move the tug out of position.
107
Explain a wrap line.
A wrap line is used by a tug to steer the bow of a ship moving stern first. It is led from one side of the ship’s bow to the bullnose on the tug located on the opposite side of the ship’s bow. This allows the tug to steer the bow in either direction.
108
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.
109
What is meant by the term snapping a tow?
When a tug with a ship or barge in tow astern makes a hard turn and at the same time applies more power. Since the tug’s rudder will set the stern of the tug in the opposite direction, the bow of the towed vessel will often also be set in the opposite direction of the turn at first.
110
Why should a tug made up for breasted towing have tight lines?
If there is too much slack in the lines, the tug’s angle to the ship’s centerline will vary and detract from the tug’s ability to effectively apply steering forces.
111
Why is indirect towing so powerful?
It uses the tug’s hydrodynamic resistance to add force to the towline.
112
How is indirect towing used to advantage?
To provide steering and braking forces at ship speeds that are too high for effective direct towing.
113
Why should a tug back easily on a slack headline?
Because the momentum of the tug moving astern could develop enough force to part the line.
114
What may happen when a tug crosses a vessel’s wake?
The propeller may cavitate.
115
How can cavitation endanger a tug crossing a ship’s wake?
The tug may not respond and can cause damage to itself or the ship.
116
What are the principle causes of accidents involving tugs doing shipwork?
Of the causes cited, which is the most common and why?
Human error, mechanical failure, and weather.
Human error in regards to excess speed, because if it is on the part of the ship, it can overpower a tug on a towline and trip or girt it. Excess speed can also capsize a tug using a quarter line and even endanger a tug fast alongside with only a head line out. It will also destroy the tug’s ability to assist the ship. When the tug uses excess speed it can part lines (injuring personnel) and lead to collisions with the ship it is assisting.
117
What three fundamental principles of shiphandling are essential to a shiphandler’s eye?
1. Center of Lateral Resistance
2. Pivot Point
3. Maneuvering Levers
118
The Center of Lateral Resistance determines the resultant proportion of __________ vs. __________ force of an applied lateral force to the ship.
lateral vs. rotational
119
Describe pivot point location for a ship that is:
- stopped
- moving laterally
- moving ahead
- moving astern
- turning
- Stopped: amidships at the CLR depending on draft and trim
- Moving laterally: amidships at the CLR depending on draft and trim
- Moving ahead: approximately 1/4 ship length aft of the bow
- Moving astern: approximately 1/4 ship length forward of the stern
- Turning: approximately 1/3 ship length aft of the bow
120
The pivot point functions as the __________ of the ship’s maneuvering lever.
fulcrum
121
Define a tug’s assist lever.
The distance between the location of the tug’s applied force and the ship’s pivot point.
122
Why is it easy for a ship with headway to overwhelm a lead tug on a towline?
The ship has greater power and can apply that power on a longer and more efficient maneuvering lever. This can easily overpower the tug.
123
What are the three means a tug has to apply force when working alongside?
Push
Act as drogue
Pull on a line
124
What are the four factors that affect a tug’s effectiveness when working alongside?
Angle of the tug
Offset position of the tug
Drag force
Tug’s applied HP
125
Why is a conventional tug backing on the stern quarter, on the outboard side of a turn, ineffective in turning the ship?
At all but the lowest speeds the conventional tug allocates almost all its horsepower to maintain position at, or close to, 90 degrees. There is minimal lateral pull applied to the tug’s headline. The tug’s angle, offset position, drag, and HP are all working on a long assist lever to counter the turn.
126
In breasted towing, a tug made up on the ship’s stern creates, in effect, a __________ ship.
twin-screw
127
In breasted towing, a tug made up on the bow functions as a __________.
steerable bow thruster
128
What is negative water flow?
Negative water flow is when the tug’s wash opposes the general direction of water flowing by its hull.
129
How is negative water flow a factor in conventional tug performance?
Negative flow considerably increases torque loadings on the propeller and engine. High negative water flow may increase the load enough to stall, or damage, the tug’s engine, clutch, or couplings.
130
Why is a trailing tug the most effective position for steering a ship?
A trailing tug creates the longest assist lever and allows the tug t apply equal forces to both starboard and port. The tug can enhance the effect of the ship’s rudder on a ship with headway and can also control the bow and steer a ship with sternway.
131
What precautions should the shiphandler exercise when employing a conventional tug as a trailing tug on a towline?
The shiphandler must execute careful management of ship speed and propulsion wash.
132
What two methods can tractor and ASD tugs use to steer a ship as trailing tugs?
A tractor or ASD tug can utilize direct towing at ship speeds of 5 knots or less and indirect towing at higher speeds.
133
What is the essence of effectively using tugs to steer and propel ships?
Steering and propelling ships with tugs is a matter of creating and using the tug force as an effective lever in conjunction with the ship’s maneuvering lever.
134
Is the pivot point always near the ship’s center of lateral resistance?
No, the pivot point may coincide with the CLR when the ship is stopped, but once a lateral force is applied the pivot point moves in response to the applied force.
135
What is the effect on the ship’s pivot point of one tug pushing at the bow?
The pivot point moves away from the tug end of the ship toward the ship’s stern.
136
How many ship lengths will it take to turn a ship with one tug pushing at the bow?
Approximately two ship lengths.
137
What is the effect on the ship’s pivot point of two tugs pushing with equal force, one on the port bow and one on the port stern of a stopped ship?
The pivot point will remain approximately amidships.
138
In order to move a stopped ship laterally the external forces applied must have __________ on either side of the ship’s pivot point.
Equal and opposing leverage
139
What is drift angle?
The angle between the ship’s centerline and the direction in which the ship’s bridge is actually traveling in a turn.
140
Why is knowledge of drift angle useful to the pilot?
Drift angle serves as a visual cue for the pilot to estimate the degree of lateral motion, or slide, that a ship has as it executes a turn.
141
What are the three principal techniques used by tugs to apply a braking force to the ship?
Drag of the tug
Direct bollard pull
Transverse arrest (steerable propellers only)
142
What are the three phases of ship handling task management in maneuvering ships with tugs?
Initial assessment
Create a maneuvering plan
Adjust the plan
143
What are the factors in choosing the placement of tugs in shipwork?
Sequence of maneuvering events, safety of tug, limits of tug design, skill level of tug operator, need and location of highest HP tug, required allocation of tug’s HP to maintain or achieve assist position.
144
What is the difference between handling a dead ship and assisting a powered vessel?
With a “dead” ship the tug is the prime mover, with a powered vessel the tug is merely assisting.
145
Is one tug ever used to handle a small ship by itself?
Yes, just as tugs frequently handle fairly large barges unassisted.
146
If only one tug is used for a dead ship movement and a hard turn must be made, where should the tug be?
The tug should be hipped up on the inboard side of the turn, since even if the tug backs the ship will continue to swing in the direction of the turn. If the tug (especially a single-screw tug) is on the outboard side of a turn, it may not be able to back as this would kill the swing.
147
How may a tug on the outboard side of a turn moving a dead ship reach a point of no return?
If the ship’s turn rate is too slow and there is neither sufficient room ahead for the ship to advance or to the outboard side for the ship’s stern to rotate about its pivot point.
148
Does a ship propelled by a tug on the hip have the tendency to skid sideways?
Yes, this is caused by the offset location of the propelling and steering forces.
149
When a larger ship is shifted, are more tugs required?
Yes, the number of tugs required (unless stipulated by port regulations) will depend on the size of the ship and the HP of the tugs.
150
Are dead ship vessels ever handled stern first?
Yes, but extra care should be taken to avoid damaging the rudder and propeller.
151
Describe several hookups for handling dead ships.
2 tugs: A powerful tug aft on the hip to propel and a smaller tug forward for steering placed on either bow (opposite bow of propelling tug preferred)
3 tugs: A powerful tug aft on the hip with two smaller tugs forward on each side for steering, or two tugs aft with one tug forward for steering.
4 tugs: Two tugs forward and two tugs aft on the hip for very large ships
152
Why is the use of a tug on a towline forward and a tug hipped up aft an effective way to use tugs in a dead ship movement?
Because two moderately powered tugs can propel a sizeable vessel since both tugs are moving the ship. The forward tug can steer the vessel, and the after tug can back to check its way. This is an efficient use of power especially for longer shifts.
153
How are tugs fast aft on beamy vessels used used most effectively for steering?
By using the tug’s as if they were components of a twin-screw ship and by using their engines in opposition for steering effect.
154
Why is an anchor called a poor man’s tugboat?
Because when a vessel uses its anchors, it can dispense with the services of a tug. There are also occasions when the use of anchors is preferred.
155
How does using an anchor differ from using a tug?
The anchor is essentially a “passive” device that cannot push nor pull like a tug unless it has been “spotted” beforehand and is used as a “deadman” to heave against, or is used as a “brake” to check a vessel’s way.
156
What are the three functions of an anchor when a ship is maneuvering?
Restrain but not stop the ship
Fix or hold the anchored end of the ship
Provide a deadman to heave against
157
How is the anchor used when tugs are also employed?
The anchor is used to provide directional stability when the ship is towed by the tug and to fix an end for the tug to turn the vessel.
158
How are stern anchors used?
Stern anchors may be used to keep ships from, swinging with the tide, and are often used on passenger ships to hold up the stern of the vessel and provide a lee for the launches. When used with tugs they are often used in docking in fair tide situations.
159
What is the danger when heaving a stern anchor?
The possibility of the ship overriding the anchor and as a result doing damage to the rudder and propeller.
160
What additional considerations must be given to a tug assisting ships in exposed waters?
The tug should be heavily rendered and it’s working lines should be stronger than those required for normal harbor service.
161
What is a roller fender and is it suitable for exposed waters shipwork?
Large pneumatic fenders (like tires) that are set on axles in a well in the bow of a tug. They are useful for shipwork in exposed areas since they will roll if the tug is pitching heavily as it pushes up against the ship.
162
In addition to using roller fenders, what other precautions should be taken in exposed waters shipwork?
Work he tugs on the lee side when possible and give the lines a longer lead to prevent parting them in the rise and fall of the swell.
163
Does a tug handle the same in a seaway as it does in protected waters?
No, it takes the tug’s longer to respond to helm and engine in a seaway than it does in a bay or harbor.
164
How does a tug assist a vessel mooring to a SPM?
It usually puts up a line on the lee bow clear of the chocks where the mooring lines are passed and assists the vessel to hold its position while the gear is being run. The tug can also assist the steering while the vessel is making its final approach and backing to stop.
165
When a vessel goes aground what forces must be overcome to refloat it?
The resistance of ground effect (the weight of the vessel resting on the bottom) times the coefficient of friction of the bottom and the effect of wind, current, and heavy seas.
166
What procedures are used to refloat a vessel?
- wait for high tide to float a vessel
- discharge ballast or fuel
- discharge cargo into lighters
- pull vessel off with tugs
- heave vessel off with beach gear
- jettison some of the cargo
167
What is the least desirable method to refloat a vessel?
Jettisoning the cargo is considered to be the last resort since the vessel’s cargo is sacrificed to save the ship.
168
What are the first steps to be taken after a vessel goes aground?
The vessel’s draft should be checked to see how much buoyancy it has lost; soundings should be taken around the perimeter of the ship’s side to determine what portion of the hull is aground; and the bilges should be sounded to determine if the vessel is holed.
169
If a vessel is holed, should it be refloated?
Only if there is enough pumps or compressors to keep it afloat.
170
What is one of the effects of current on a grounded vessel?
The buildup of sand or mud around a grounded vessel that will make refloating it difficult.
171
What effects can heavy seas have on a vessel aground?
They can force a vessel farther aground and hamper salvage operations. Conversely, they can also help refloat a vessel by breaking the bottom suction or grinding down soft coral or limestone bottoms.
172
What should a vessel do to avoid being washed further aground?
Lead out anchors to restrain it, or ballast it down until the seas subside or assistance arrives.
173
If the vessel can refloat itself, why is it sometimes prudent to use a tug?
The vessel may require assistance steering to avoid broaching when it is refloated.
174
How may a small tug assist a ship that is aground?
A small tug can take a strain on the ship that will prevent it from broaching or “climbing the beach” until more assistance arrives.
175
What is beach gear?
“Beach gear” consists of heavy wire rope falls that are set up by a winch. They are attached to a shot or two of heavy chain and an anchor (usually an Ell anchor) which is spotted offshore of the stranded vessel. This gear was often set up on the ship’s deck or on a barge attached to the ship, but modern salvage tugs sometimes are equipped to place the anchors and are attached to the ship by their own tow cable
176
Are tugs sometimes more effective than beach gear?
Yes, since they can apply their force from several different directions and “swing” the ship back and forth while beach gear can on,y pull in one static direction
177
To what type of emergencies might a tug have to respond?
To assist in beaching a vessel that has been holed, remove a burning tanker from a dock, breast a listing ship to the dock, and assist a vessel that has lost power or steering.
178
How should a tug take a ship in tow that is on fire?
First, the tug should have its own crew in firefighting gear or oilskins and have hoses led out and ready to provide protection to the tug’s crew. Then, the tug should back down and connect its towing gear to the vessel’s fire warps and take the vessel in tow after streaming about 250 feet of towline or cable.
179
Can one tug control the ship?
Usually, but the vessel will have a tendency to sheer away from the tug. It will be more easily controlled if there is another tug on the towline or pushing the other end of the ship.
180
How can a tug assist a ship that has a power failure?
By taking the vessel in tow astern or on the hip.
181
How can a tug assist a vessel that has lost its steering?
By handling the ship with a towline to the vessel’s bow; making fast alongside of the vessel forward; and by steering the stern of the ship.
182
If a tank vessel is holed and the area is gassy, what should the tug do?
Stay well to the weather of it; or clear out!
183
List a tug captain’s responsibilities in order of importance in an emergency.
The safety of his crew, the safety of his vessel, and the safety of the vessel being assisted. His crew’s safety should not be risked unless lives are at stake.
184
What are the two primary purposes of tugs in ship escort?
Emergency stopping
Emergency steering
185
What are the three tug response modes?
Retard
Assist
Oppose
186
What is the least effective tug response position?
Forward shoulder of ship
187
What is the most effective tug response position?
Rudder tug tethered to ship’s stern
188
What are the three maneuvering positions for a tethered ASD or tractor tug?
Direct
Indirect
In line arrest
189
Why is ship speed critical to ship a ship escort response?
Because speed:
Determines advance and off track carry
Compounds effect of time delay
Determines effectiveness of response positions
190
What is the danger of new line and winch technology used in ship escort?
The line may not break or release if the tug is in peril.
191
What are the three qualities of good verbal communications?
Simple, clear, easily understood/not misunderstood
192
Why did the verbal radio communication replace whistle signals?
More complex tugs and maneuvers required a bigger vocabulary than whistle signals could provide.
193
What terminology must be clear for effective command and response communication?
Identification of tugs
Indication of the direction of force to be applied
Indication of the amount of force to be applied
Indication of the desired push or pull angle
Verification of the receipt, understanding, and implementation of the command by the tug
194
Why should the terms “full”, “on”, and “off” be avoided?
They can be mistaken for other words
195
How does the tug acknowledge an order?
The tug operators response includes the tug’s name and repeating the order back to the pilot
196
What is a means of non-verbal communication in shipwork?
The action of the tug
