Ship Handling w/ Tugs Flashcards
What are the 3 general categories of tugs used in shipwork?
Conventional, Tractor, and ASD
What are the 6 key elements in tug design?
- Towing Point 2. Propulsion Point 3. Propulsion and Steering 4. Maneuvering Lever 5. Hull Shape 6. Superstructure and Fendering
What is the towing point?
The last physical point on the tug that fairleads its line; or the point of contact between the tug and the ship when pushing.
What is the propulsion point?
The focal point of the tug’s application of horsepower under the water.
What is the maneuvering lever?
The distance between the towing point and the propulsion point.
Descibe the orientation of the maneuvering lever for a conventional tug.
towing point forward of the propulsion point - short lever
Describe the length and orientation of the maneuvering lever for a tractor tug.
Towing point aft of propulsion point - moderate lever
Describe the length and orientation of the maneuvering lever for an ASD.
Towing Point is forward of propulsion point, longest when towing point is the bullnose; shorter when the towing point is the bit.
What causes a tug to capsize when on a tow line?
The tug does not have enough leverage to counter the pull of the towline.
What advantage does a single-screw tug have over a twin-screw tug?
Efficency, since a single screw will develop 20 more thrust for equal horsepower.
What is the principal disadvantage of a single-screw tug?
Lack of maneuverability, inability to steer astern, and inability to maintain position when backing. Power astern is less than power ahead.
What advantage does a twin-screw tug have?
Excellent maneuverability, ability to maintain position when backing, and ability to steer astern.
What are the disadvantages of a twin screw tug?
May require a stern line when backing, is less efficent than a single-screw tug, backing power is less than ahead
What advantages does a tractor tug have over a conventional tug?
A tractor can steer when going astern, has omni-directional thrust and does not have the same risk of capsizing as a conventional tug when towing.
What advantages does and ASD have over a tractor tug?
Better sea-keeping ability, shallower draft, more efficent hp to bollard pull, can perform transverse arrest.
What are the 4 components of the steering and propulsion system?
Power Source Power Transfer & Control System Hydrodynamic Driver Thrust Directional Control Mechansim
What is the purpose of the power transfer and control system?
To transfer and control rotative force force to the tug’s hydrodynamic driver (typically a propeller).
What effect has the diesel engine had on the design of tugs?
Allowed smaller, more powerful tugs to be built, increased the range and power of tugs.
Describe the DR system of engine control
Direct Reversable - 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.
What made DR systems obsolete?
Limited number of maneuvers, engine speed may not allow for most efficent sized propeller, vulnerable to failure to start or starting in the wrong direction.
Describe the DE system of engine control?
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.
What are the advantages to DE drive?
A wide range of speeds available from stop to full ahead or astern. The fact that the main engine runs at a constant speed.
What are the principle disadvantages of DE drive?
High cost, vulnerability to salt and dampness.
How does and SCR system differ from a conventional DE drive?
The SCR system employs an AC generator whose current is then converted by SCRs (Silicon control rectifiers) to DC current for driving propulsion motors.
Are there any particular advantages to the SCR system?
More modern, less expensive, and propulsion generators can provide ships supply power.
Describe the CPP system.
It is not only energy converting but is also a transmission. The amount of thrust is regulated by reducing or increasing pitich.
What maneuvering funtion is housed in the CPP?
Amount of thrust and reversing mechanisim.
What are the drawbacks of a CPP system?
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.
How does a reverse reduction gear change propeller direction?
Through sets of additional gears and clutches; one for reverse one for ahead.