SHIPHANDLING WITH TUGS Flashcards by Joe Semler

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

Conventional
Tractor
ASD

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What are the 5 key elements in tug design?

Towing point
Propultion point
Propultion and steering
Maneuvering lever
Super structure+underwater structure

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What is a towing point?

Point at which the line connects from tug to ship
-Bullnose on the bow, staple on stern, tow bit aft, gob line, towing hook

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What is propulsion point?

Where the propultion from the tugs come from. ASD aft, Tractor Fwd, Conventional Aft

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What is the maneuvering lever?

Distance between tow point and propultion point- longer the better

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Describe the length and orientation of the maneuvering lever for
- conventional tugs
-tractor tugs
-ASD tugs

Conventional - Tow point aft, propulsion aft (Less Efficient)
Tractor Tug - Tow Point Aft, Propultion Fwd Amidships (More Efficient)
ASD - Tow Point Fwd, Propulsion Aft ( Most Efficient)

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What causes tugs to capsize while on towline (GIRT, In Irons or Tripping

Speed
SHort Maneuvering lever

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What advantages does a single screw tug have on twin screw?

20 percent more efficient due to propellor size, Hull set up and propellor placement for subs,

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What is the principle disadvantage of single screw tugs

Maneuverability

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What advantages does a twin screw tug have

Maneuverability compared to single screw tugs

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What disadvantage does a twin screw tug have

Horsepower compared to single screw tugs, power at speed

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Advantage of tractor tugs over conventional

Maneuvering lever, maneuvering, quick speed

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What advantages does ASD tug have over tractor tugs?

Efficiency, maneuvering lever, shallower draft, most effective design to carry horsepower into thrust.

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What should pilots question about where to put tugs on the jobs

Type, Horsepower, superstructure above and below the hull

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What are the tugs four components of steering and propulsion systems?

Power source
Power transfer and control system
Hydrodynamic driver(Thrust)
Thrust directional control mechanism

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What is the purpose of power transfer and control system?

How the ship powers its rudder/movement

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What effect has the diesel engine had on tugs?

Powerful and efficient, providing ability to make more complacent tugs

Describe the (DR) Direct reversing system in tugs engine control?

Transfers power to propellor(Thrust) using reduction gears. Problems occur when going reverse because you need to stop and restart the engine in reverse using up air from air banks.

WHat made the (DR) Obsolete

Effectively maneuvering the tug for and aft in a timely manner.

What is the (DE) Diesel Electric system of engine control

3 basic elements powered by the diesel engine
- Main engines
- Generators
- electric motors (get power (DC) from the generators)

Advantages of the Diesel Electric (DE)?

Engine maintenance is minimal since diesel engines run at constant speed
Engines and generators required for propulsion need to be on when running
Allow for more flexibility in engine room because engines do not directly connect to shaft.

Disadvantage of Diesel electric engines?

Cost, vulnerable to damp and salt conditions

Difference in an (SCR) Silicon-controlled Rectifier and Diesel Electric system?

Main generators create (AC) Power which then gets converted to DC power by rectifiers to power the propulsion motors.

What are the advantages of the (SCR) silicon-controlled rectifier system?

Costs due to AC generators are cheaper than DC generators and the AC units is compatible with he ships electrical equipment.

Describe the CPP System?

Controllable Pitch Propellor - Shaft rotates at constant speed while the propellor pitch can adjust from neutral pitch (No Thrust) to Full Thrust in ahead or astern direction.

WHat maneuvering function is housed inside the CPP system?

A solid shaft located inside the hollow drive shaft, the control shaft actuated by electrical or hydraulic servomotors engages cams on the base of the propellor blades altering pitch.

Drawbacks of the CPP system?

Propellor is always turning - even at neutral the ship will not glide through the water straight. Cost for repairs is also high.

How does the Reverse reduction gear change propellor shaft rotation?

Connection from the gears input shaft to the ahead and reversing gears is made by hydraulic or pneumatic clutches.

What factors determine the propellor efficiency?

1. Quality and velocity of water flow on the intake or discharge side of the propellor 2. the size/speed of the rotating propellor blades and the number, pitch and shape of the blades.

How do nozzles increase the propulsion efficiency?

Directs more water through the propulsion unit increasing efficiency 15-60 percent over open wheel

What are disadvantages of the nozzle?

Backing power is only 50 percent of ahead power, they create drag at high speeds and created lateral resistance underwater reducing maneuverability

Three factors the determine the rudders efficiency?

1. Rudder Wash 2. Rudder Shape 3.

What is the difference between a balanced and unbalanced rudder?

Balanced - Stock of rudder mounted back from the front of the rudder making it easier for rudder to stay at angles. Unbalanced - Stock of rudder mounted at front end of rudder creating a need for larger steering gear to hold angles.

What is a spade rudder?

A balanced rudder hung independently of any skeg. Cons - must be stoutly constructed because it has no structural support on the bottom or protection from grounding. Pros- It has a good foil shape and is placed in optimal section for propellor wash

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

Steerable Nozzle directs the flow from the propellor Steerable Propellor directs the flow via propellor

Why are flanking rudders effective going astern?

They are mounted ahead of the rudder and help direct the propellor wash while going astern.

Describe a Voith Schneider system

Controllable pitch propellor acting around a verticals axis - Verticals blades continuously rotate in a circular motion with engine rpm determining the rotor speed, pitch levers control longitudinal thrust and a wheel that controls transverse thrust.

What are the disadvantages of Voith Tugs?

Efficiency of horespower to bollard pull is lower than conventional or steerable propellor tugs

Describe a steerable propellor system-

Propellor can be operated 360 degrees by using Engine power is transmitted through a shaft to the upper casing of the drive unit housed inside the tug, a clutch regulates the connection between engine output shaft and drive unit input shaft. A series of right angled gears and shafts converts the horizontal rotation of the propellor shaft.

What are the disadvantages of a steerable propellor system?

The time it takes for the units to spin 360 degrees

What is Bollard Pull?

Thrust in pounds delivered by the engine under static conditions (pulling against a dock or other fixed structure)

Is bollard pull a fair criterion for judging a tugs serviceability for ship-work?

Yes, however other factors must bee looked at -maneuverability -stability -backing power