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Alaska Pilots > Tug Q&A (old) > Flashcards

Tug Q&A (old) Flashcards

1
Q

What are the 3 general categories of tugs used in shipwork?

A

Conventional, Tractor, and ASD

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2
Q

What are the 6 key elements in tug design?

A
  1. Towing Point
  2. Propulsion Point
  3. Propulsion and Steering
  4. Maneuvering Lever
  5. Hull Shape
  6. Superstructure and Fendering
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3
Q

What is the towing point?

A

The last physical point on the tug that fairleads its line; or the point of contact between the tug and the ship when pushing.

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4
Q

What is the propulsion point?

A

The coal point of the tug’s application of horsepower under the water.

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5
Q

What is the maneuvering lever?

A

The distance between the towing point and the propulsion point.

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6
Q

Describe the orientation of the maneuvering lever for a conventional tug.

A

Towing point forward of the propulsion point - short lever

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7
Q

Describe the length and orientation of the maneuvering lever for a tractor tug

A

Towing point aft of propulsion point - moderate lever

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8
Q

Describe the length and orientation of the maneuvering lever for a ASD

A

Towing point is forward of propulsion point, longest when towing point is the bullnose; shorter when the towing point is the bit.

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9
Q

What causes a tug to capsize when on a tow line?

A

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

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10
Q

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

A

Efficiency, since a single screw will develop 20% more thrust for equal horsepower.

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11
Q

What is the principal disadvantage of a single-screw tug?

A

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

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12
Q

What advantage does a twin-screw tug have?

A

Excellent maneuverability, ability to maintain position when backing, and ability to steer astern.

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13
Q

What are the disadvantages of a twin-screw tug?

A

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

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14
Q

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

A

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

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15
Q

What advantages does an ASD have over a tractor tug?

A

Better sea-keeping ability, shallower draft, more efficient HP to bollard pull, can perform transverse arrest.

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16
Q

What are the 4 components of the steering and propulsion system?

A

Power Source
Power Transfer and Control System
Hydrodynamic Driver
Thrust Directional Conrtol Mechanism

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17
Q

What is the purpose of the power transfer and control system?

A

To transfer and control rotative force to the tug’s hydrodynamic driver (typically a propeller)

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18
Q

What effect has the diesel engine had on the design of tugs?

A

Allowed smaller, more powerful tugs to be built & increased the range and power of tugs.

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19
Q

Describe the DR system of engine control

A

Direct reversible - engine is directly connected to the propeller shaft and rotation is the same as the engine. Engine must be stopped and restarted to reverse directions. May have wheelhouse control.

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20
Q

What made DR systems obsolete?

A

-Limited number of maneuvers
-Engine speed may not allow for most efficient sized propeller
-Vulnerable to failure to start or starting in the wrong direction

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21
Q

Describe the DE system of engine control

A

Diesel Electric - consists of diesel engines running generators that supply current to the electric drive motors connected to the propellers. Direction and control of the propeller is determined by the current supplied from the generators.

22
Q

What are the advantages to DE drive?

A

-A wide range of speeds available from stop to full ahead or astern.
-The fact that the main engine runs at a constant speed.

23
Q

What are the principle disadvantages of DE drive?

A

High cost, vulnerability to salt and dampness.

24
Q

How does an SCR system differ from a conventional DE drive?

A

The SCR system employs an AC generator whose current is then converted by SCRs (Silicon Control Rectifiers) to DC current for driving propulsion motors.

25
Q

Are there any particular advantages to the SCR system?

A

More modern, less expensive, and propulsion generator can provide ships supply power.

26
Q

Describe the CPP system

A

It is not only energy converting but is also a transmission. The amount of thrust is regulated by reducing or increasing pitch.

27
Q

What maneuvering function is housed in the CPP?

A

Amount of thrust and reversing mechanism

28
Q

What are the drawbacks of a CPP system?

A

High cost and susceptibility to damage.
A vessels steering qualities are affected when in neutral pitch.
Wheel is always turning so lines may become fouled.

29
Q

How does a reverse reduction gear change propeller direction?

A

Through sets of additional gears and clutches; one for reverse one for ahead.

30
Q

What factors effect propeller efficiency?

A

Quality and velocity of water flow.
Number, pitch, shape, size, and speed of the propeller blades.

31
Q

How do nozzles increase efficiency?

A

The propeller turns within a shroud which increases thrust 15-60 percent more than an open propeller.

32
Q

What are the disadvantages of nozzles?

A

The present more drag and lateral resistance underwater, which hinders maneuverability and light tug running speed.

33
Q

What 3 factors determine a rudder’s efficiency?

A

Velocity of water flow
Quality of water flow
Rudder shape

34
Q

What is the difference between a balanced and unbalanced rudder?

A

Balanced or semi-balanced rudders are constructed so that the leading edge of the rudder extends forward of the rudder post. This is done to provide more effective steering and a mechanical advantage to the steering gear.

35
Q

What is a spade rudder?

A

A spade rudder is not supported at the bottom.

36
Q

What is the difference between a steerable nozzle and a steerable propeller?

A

A steerable nozzle changes the angle of thrust by rotating the angle of the nozzle around a fixed propeller. A steerable propeller changes the direction of thrust by rotating the propeller around a vertical axis.

37
Q

Why are flanking rudders effective when operating astern?

A

Flanking rudders are installed ahead of the propeller, there are usually two of them for each propeller. They are more effective astern as they are on the discharge side of the propeller.

38
Q

Describe the Voith-Schnider (VSP) cyclodial propeller.

A

The VSP functions like a CPP rotating around a vertical axis. The rotor turns while each vertical blade oscillates, changing its angle (pitch) at points along the circumference of the rotor casing track.

39
Q

What are the disadvantages of a VSP?

A

HP to Bollard Pull ratio is less for the VSP than conventional or steerable propeller
VSP will have deeper draft than conventional or ASD
VSP systems are expensive compared to conventional or steerable propellers

40
Q

Describe a steerable propeller system

A

Engine power is transmitted via shaft to the upper casing of the drive unit. A pnuematic or hydraulic clutch regulates engine output and drive unit input. A series of right angle gears and shafts converts the horizontal rotation of the propeller. The lower unit houses the propeller and can be rotated through 360 degrees.

41
Q

What is the disadvantage of a steerable propeller system?

A

Reversing thrust requires rotating the drive units 180 degrees; the unit may be thrusting through the entire rotation.

41
Q

Why are structures set inboard on harbor tugs?

A

To prevent contact with the ship’s side when working under the flare of the ship or when both are rolling.

41
Q

What is bollard pull?

A

The pull is generated by a tug pulling against object. In conventional tugs it is estimated to be 1 long ton per 100 hp, depending on the configuration and whether nozzles are installed.
Bollard pull can range between 22.5 and 38 lbs per HP.

42
Q

Is the tug hull reinforced?

A

Yes, the bow of a conventional and ASD tug is heavily reinforced, the bow should have sufficient radius to distribute force. Heavy rubrails are installed above the water line to reinforce and protect the hull.

42
Q

Is bollard pull a fair criterion for judging a tug’s serviceability for shipwork?

A

Not always, since other factors like “stiffness” and maneuverability are also important.

43
Q

Describe the important aspects of the hull of a tug used for shipwork

A

Heavily constructed hull with low freeboard.
Should be “stiff” enough so that a strain on a line will not capsize it.
Bulwarks have tumble home to prevent damage.

44
Q

Where are the bitts located on conventional American harbor tugs?

A

Fore and aft on the fore deck & athwartships aft of the deck house. Quarter bitts installed augment the forward and aft H bitts.

44
Q

What are the structural requirements for the bitts and deck fittings?

A

They must be large enough to belay the large sized working lines, and strong enough to take the heavy strain. The bullnose, forward cruciform bitt and (end of pdf page)

44
Q

What is a staple?

A

Functions as a tug towing point, a closed chock that is connected to the tug’s internal frames, bulkheads, and plating.

44
Q

What is a bullnose?

A

A large closed chock near the bow of the tug for both the head line and spring to run through.

45
Q
A

Alaska Pilots (35 decks)

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