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My Alaska Pilots > Tug Use in Port - HENSEN > Flashcards

Tug Use in Port - HENSEN Flashcards

1
Q

Which of the following tug types have their propulsion point forward of midships?

A

Voith Schneider/ Tractor

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2
Q

Which of the following tug types have their towing point near midships?

A

Single Screw

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3
Q

Which of the following vessels would generally have the greatest allowable hull
pressure?

A

General Cargo

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4
Q

Where, generally, is the towing point on a conventional tug?

A

.0.55 x LWL from forward

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5
Q

A conventional tug used for towing on a line at speed would be more effective with

A

a towing point further forward

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6
Q

What is the approximate astern thrust of an open fixed pitch propeller?

A

60% of ahead thrust

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7
Q

Which propeller has the greater astern pitch?

A

fixed

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8
Q

Who designed the original propeller nozzle?

A

Ludwig Kort

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9
Q

Nozzles increase thrust in towing and pushing conditions by .

A

15% to 25 %

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10
Q

When fitted with a nozzle, a conventional CPP pulling astern will achieve .

A

45% of ahead bollard pull

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11
Q

What maximum rudder angle is normally used with a steerable nozzle?

A

30º

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12
Q

Flanking rudders are most effective when .

A

moving astern, towing astern with a line on the tug’s bow

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13
Q

A conventional tug used for towing on a line astern is at a disadvantage because .

A

at higher speeds, steering assistance can be given to only one side

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14
Q

Which type of tug is least effective when pulling on a line at speed?

A

Reverse Tractor

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15
Q

Which type of tug is least effective when alongside and push-pulling at speed?

A

Conventional

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16
Q

A combi-tug when towing on line astern may use a ______ to shift the tow point
further aft.

A

Gob Rope

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17
Q

Tractor tugs typically have a large skeg in order to

A

give better course stability

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18
Q

An advantage of a tractor with azimuthal propellers over a VS of the same horsepower is

A

lighter hull draft

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19
Q

Which tug is more effective then a VS tug when direct pulling?

A

ASD

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20
Q

A VS tug of 5000 bhp could have a bollard pull of

A

57.5

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21
Q

When using a tug to turn a ship, a tug on a line is generally more effective than a tug
pushing because

A

of the longer lever developed around the ship’s pivot point

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22
Q

When towing on a line, where is the towing point?

A

the point where the line goes straight line to the ship

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23
Q

For most tugs, the lateral center of pressure when flow is on the beam lies .

A

aft of midships

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24
Q

When a tug’s bow turns into the water flow, the center of pressure

A

moves forward

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25
Q

When a tug’s stern turns into the water flow, the center of pressure

A

moves aft

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26
Q

A conventional tug is more effective than a tractor when working forward on a line
because

A

a turn by the tractor would decrease the towing force

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27
Q

Which tug is more effective when indirect pulling astern?

A

Tractor tug

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28
Q

When towing on a line forward, which tug is best?

A

ASD

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29
Q

When towing on a line aft, which tug is best?

A

ASD

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30
Q

When working alongside, which tug is least suitable?

A

Conventional

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31
Q

The maximum bollard pull required to control a ship due to wind is when the wind is

A

On the beam

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32
Q

Bollard pull required due to current is about 2 1/2 times as much than in deep water when

A

underkeel clearance is 1.5 x ship’s draft

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33
Q

Bollard pull required due to current is about 5 times as much than in deep water when

A

underkeel clearance is 0.1 x ship’s draft

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34
Q

In shallow water, transverse current effects are greatest when

A

The current is on the beam

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35
Q

In shallow water, longitudinal current effects may be greatest when

A

the current is on the bow

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36
Q

Beam waves of 1.0 meter could cause a wave force on a 200m ship of

A

22 tons

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37
Q

Maximum safe approach speeds for a VLCC are in the range of

A

1.4 to 1.9 kts

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38
Q

When calculating the bollard pull for wind, current and waves using tugs or alongside
with short lines, the amount should be increased by

A

20%

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39
Q

When a tug approaches a moving ship near the stern, interaction may cause the tug to

A

increase speed

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40
Q

When a tug is passing a moving ship near the ship’s midship, interaction may cause

A

the tug’s bow to swing outward

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41
Q

An advantage of a short towline is

A

Reduced reaction time

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42
Q

A disadvantage of a long towline is

A

Increased reaction time

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43
Q

A disadvantage of a short towline is

A

increased wear on towing gear

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44
Q

What is false about the actions of an escort tug?

a) a tug towing on a line forward corrects a sheer by steering away from the danger
b) a tug towing on a line aft corrects a sheer by steering towards the danger
c) a tug towing on a line aft should correct a sheer by using the indirect towing method
d) a tug towing on a line forward can brake the ship’s speed

A

Answer: D

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45
Q

What is the minimum recommended GM of an escort tug?

A

3.0 M

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46
Q

The operational requirements that harbor tugs must conform to with respect to ship assistance are mainly determined by the following factors

A
  • The kind of port or harbor and approaches, foreseeable future developments and existing geographical environmental conditions.
  • the type of ships calling at port
  • the services required in and around the port and, if relevant, at offshore locations
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47
Q

At present mainly the following types of tugs are used

A
  • single screw
  • twin screw-
  • tractor type
  • tugs with azimuth propellers aft
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48
Q

Tugs operated at a distance from the ships bow and stern on a towline length normally?

A

at least one and half times the tugs length

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49
Q

Type of tugs with propulsion aft

A
  • conventional type
  • ASD type
  • Reverse tractor
  • combi
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50
Q

Type of tugs with propulsion forward

A
  • voith schneider

- Azimuth propellers

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51
Q

Going full astern an open fixed pitch propeller will develop what percentage of its ahead thrust

A

60%

52
Q

An open CPP going full astern develops what percentage of its ahead thrust

A

40 to 45%

53
Q

Nozzles increase thrust by what percentage in towing and pushing conditions.

A

15 to 25%

54
Q

The large skeg for tractor tugs and in particular VS tractors tugs has what benefit?

A

It gives course stability and brings the centre of hydrodynamic pressure further aft, which is advantageous to both safety and towing performance when towing on a line, especially towing performance when operating as an aft tug at higher speeds.

55
Q

VS tug acting as a forward tug can change to a pushing position at a ships speed approximately at

A

2 knots.

56
Q

Although VS tugs are not the most effective type of tug as a forward tug towing on a line for a ship under speed, due to performance restrictions imposed by the location of the towing point, they are very suitable for?

A

as after tug for course and speed control.

57
Q

Thrusters deliver practically the same amount of thrust in any direction, though astern thrust might be about?

A

5% less than ahead

58
Q

The astern power of reverse tractors is generally about 1.)________ less than ahead power, due to 2.)________

A
  1. ) 10%

2. ) due to the shape of the after hull

59
Q

Conventional tugs are effective as __________ tugs towing on a line, while reverse tractors are effective _________ and are also very suitable for push pull operations.

A
  • Forwards

- Aft

60
Q

True or False: ASD tugs are very effective and suitable for all kinds of shiphandling, owing to their ability to assist like both a reverse tractor tug and a conventional.

A

TRUE

61
Q

The line pull is essentially dependent on the ________ of the propeller revolutions, and the engine power is dependent on the __________ of the revolutions. This means that if propeller revolutions are doubled, the force will increase with a factor of ______, while the required engine power will increase by a facort of ________?

A
  • square
  • cube
  • four
  • eight
62
Q

When considering effective ship handling with tugs there are, apart from the essential issue of bollard pull, two very important aspects to be considered

A
  • correct tug positioning

- the right type of tug

63
Q

In order to assist a ship under speed in a turn, a tug starts pushing at the bow in the direction of the turn, which direction does the pivot point move?

A

AFT

64
Q

The indirect towing method is applied by an after tug at speeds?

A

higher than five to six knots

65
Q

When forward tugs towing on a line give steering assistance, this generally results in a force vector tending to_________ ships speed.

A

INCREASE

66
Q

Generally the maximum speed for effective pushing by conventional tugs is?

A

Three to Four knots.

67
Q

Most suitable tug towing on a line? List in order

A
  • ASD
  • Combi tugs
  • conventional tugs
  • tractor tugs/ reverse tractor tugs
68
Q

Most suitable as stern tug towing on a line? List in order

A
  • Tractor/ reverse Tractor/ ASD
  • Combi tugs
  • Conventional tugs
69
Q

Most suitable operating at a ships side? List in order

A
  • ASD/ Tractor/ Reverse Tractor
  • Combi Tugs
  • Conventional tugs
70
Q

True of False: This force originally acts almost in line with the ship, thus causes the ship to gather stern way. The fluid forces also contribute to this effect.

A

FALSE ship will gather HEADWAY

71
Q

The following main factors influence tug assistance are?

A
  • port particulars
  • berth construction
  • the ship
  • Environmental conditions
  • Method of tug assistance
72
Q

When operating in the push pull mode towline lengths are short and pulling effectiveness can even be less than _______, depending on how close the tugs propellers are to a ships hull

A

50%

73
Q

Increase of stopping distance due to larger virtual mass in shallow water a ship drags a large amount of water along with her, increasing to as much as 1.________ of her displacement when keel clearance reduces to 2.______ of the draft

A
  1. 40%

2. 20%

74
Q

For ship assistance by harbor tugs, the following aspects of a towing winch are important

A
  • brake holding power
  • maximum pulling capacity
  • rated pull or pull/ speed characteristics
  • slack line speed
75
Q

At a vertical towline angle of 60 degrees the force is already _________ the exerted force of the tug.

A

Twice

76
Q

What are the objectives of escorting?

A
  • to reduce the risk of pollution in port areas and port approaches due to grounding or collisions caused by technical or human failures on board a tanker.
  • to apply steering and braking forces to a disabled vessel by escorting tugs and to keep it afloat, or limit the impact of collision or grounding if they unfortunately happen.
77
Q

When securing near the bow ships speed should not be more than?

A

about six to seven knots

78
Q

The limitations on escorting speed depends mainly on the capabilities of?

A

the forward tug, but also on the size, draft and UKC of the escorted ship and, of course the restrictions of the fairway

79
Q

For speeds between three and seven knots a method used by escort tugs in a growing number of the USA ports for applying steering forces is?

A

Indirect towing

80
Q

Indirect towing in the five to seven knots speed range line pulls of _________ % of the tugs bollard pull have been measured.

A

75-125%

81
Q

DNV assumes the rudder lift forces without propeller turning to be _____ times the forces with the propeller turning.

A

0.53

82
Q

What is the minimum metacentric height recommended for escort tugs

A

3 meters

83
Q

Minimum requirements for escort vessels listed in the CFR’s are?

A
  • tow a tanker at four knots in calm conditions, and hold it in a steady position against a 45 knot headwind.
  • Stop a tanker within the same distance that it could crash-stop itself from a speed of six knots using its own propulsion
  • turn a tanker through 90 degrees, assuming a free-swinging rudder and a speed of six knots, within the same distance (advance and transfer) that it could turn itself with a hard over rudder.
84
Q

A speeds higher than approx 6 knots, towing on a line as bow tug becomes more difficult for a _________

A

Reverse tractor

85
Q

A good position for passive escort is?

A

4 points on the bow, 0.2nm off

86
Q

Braking force is higher when the stern tug is at?

A

a small angle with the ships centre line

87
Q

When ship has small UKC, current force decreases quickly when the angle of attack is less than?

A

90 degrees and may work behind the ship forcing it in the opposite direction

88
Q

Water that is moved by the ships movement is called?

A

hydrodynamic mass

89
Q

When added to the displacement of the ship the combination is?

A

Virtual Mass

90
Q

Does hydrodynamic mass and therefore Virtual mass increase as UKC decreases.

A

TRUE

91
Q

Hydrodynamic mass ranges from ____% to ____%of the ships displacement

A

25% to 100%

92
Q

Virtual Mass =

A

1.3- 2.0 X displacement

93
Q

Yaw moments are generally greater with?

A

wind on the quarter

94
Q

Ships drift rate reduces as?

A

UKC reduces

95
Q

Which tug type has the least negative effect of wash against a ships hull of all the tug types?

A

Voith Schneider

96
Q

Wave conditions will reduce?

A

tugs effectiveness

97
Q

Tethered at high speed is easier for?

A

ASD than tractor

98
Q

In all cases it is safer to fasten tugs?

A

As soon as possible

99
Q

ASD tugs uses transverse arrest at what speed?

A

Above 8 kts

100
Q

ASD tug uses reverse arrest at what speed

A

below 8 kts

101
Q

Where is the towing point located on a conventional?

A

0.45 LWL from aft

102
Q

Where is the towing point located on a VS?

A

0.1 to 0.2 LWL from aft but may differ due to operational requirements

103
Q

True or False: In most cases it is better to oppose a turn than to assist it

A

FALSE it is better to assist it

104
Q

Rudder lift forces approximately _____ when propeller stopped

A

half

105
Q

Which tug heels more when using the indirect method, VS Tractor or ASD

A

ASD

106
Q

Where is the propulsion point located on a VS?

A

0.25-0.30 from forward

107
Q

when water flow towards a tug comes from abeam, caused either by crosswise movement of a tug through water or by a current at right angles, where is the center of pressure?

A

conventional - 0.3 LWL from aft
tractor - 0.4 LWL from aft

for any tug, it should not ever move forward more than 0.5 LWL

108
Q

the consequence of a further aft towing point on a tractor tug makes it less effective as a ___________ _________ because it requires more sideways steering.

A

forward tug

109
Q

Generally speaking, modern harbor tug length to width ratios range from:

A

2.8:1 to 3:1

110
Q

according to the book, below 8 knots an ASD tug can use “reverse arrest mode”?

A

TRUE

111
Q

_________% added bollard pull to any calculations to compensate for unexpected wind, current, etc…..

A

20 percent

20-25 when working with VLCC’s

112
Q

the drift speed of a ship __________________ with reduced UKC

A

decreases, because the forces created by the opposing water increases when the UKC gets smaller

113
Q

when tugs are in a push-pull configuration, towline forces can be reduced by ____________ depending on how close the tugs propellers are to the hull

A

50 percent

114
Q

at a speed of 4 knots, attraction towards a bank and yawing moment away from it are ______________ times as high as at 2 knots

A

4 times

115
Q

Daniel Bernoulli established a relationship between waterspeed and water pressure. He showed that when water speed doubles, pressure is reduced to_______________

A

one quarter

116
Q

does a steep bank create more effect than a sloping bank?

A

steep bank does because sloping bank some sideways water inflow is possible causing a smaller reduction in pressure

117
Q

Braking capacity of a harbor tug tow winch is?

A

2 to 3 times the bollard pull of tug

118
Q

Polypropylene always fuses first…. T/F

A

TRUE

119
Q

The book says when a tug doubles the length of the towline, the reaction time tripples as compared to a tug on a short towline. T/F

A

False, the reaction time only doubles

120
Q

the Non-Pre-rigged emergency towing arrangment should be rigged in the harbor within _______________ (time). The Pre-rigged should be assembled by one person within how many minutes?

A

NON PRE RIGGED-not more than 1 hour

PRE-RIGGED- 15 minutes

121
Q

the most effective position for an rudder tug during an escort to oppose sheer would be on the?

A

quarters

122
Q

A voith turbo fin on the end of the skeg can add up to ________________ percent lift

A

18%

123
Q

a breaking strength test is performed on tug towlines. It is found that breaking strenghts are _______________% higher when no splice is used

A

15%

124
Q

Shaft horsepower is ________ percent of brake horsepower

A

97%

125
Q

Becker/Jastram rudders can create sideways thrust up to ______ percent of ahead thrust

A

50%

126
Q

A cow hitch in a line can reduce its strength by _________ %

A

15%

127
Q

Splices reduce strength by ____________ %

A

10%

My Alaska Pilots (10 decks)

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