Ch. 4 Flashcards
What are the four components of steering and propulsion systems?
Power source, Power transfer and control system, hydrodynamic driver, Thrust directional control mechanism
What is the purpose of the power transfer and control system?
To transfer and control rotative force to the tugs 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
Engine directly connected to the propeller shaft. The engines must be stopped and reversed in order to back. Engines can be fitted with wheelhouse controls
What made the DR system obsolete?
Limited number of maneuvers, engine speed may not permit a propeller of the most efficient size, vulnerability to mishap.
Describe the DE system of engine control
Engine driven generators that supply current to the electrical drive motors connected to the propeller shaft. The direction and speed of the drive shaft is driven by the current supplying the motors.
What are the advantages of DE drive?
A wide range of speeds available from stop to full speed. 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 the SCR system differ from the conventional DE system
The SCR system employs an AC generator who’s current is then converted by silicon control rectifiers to DC current for driving the propulsion motors.
Are there any particular advantages to the SCR system?
Less costly and M.E, can supply current for ship supply
Describe the CPP system
Energy converter and transmission since it changes the pitch of the propeller blades, stopping flattening the pitch so it is neutral
What maneuvering function is housed in CPP?
Amount of thrust and reversing mechanism
What are the drawbacks of the CPP System?
High cost, susceptibilty to damage. A vessel’s steering qualities are effected in neutral pitch. Propeller is always turning
How does a reverse reduction gear change propeller shaft rotation?
Through sets of additional gears and clutches, one for reverse and one for ahead
What factors determine propellers efficiency?
Quality and velocity of water flow, number, pitch, shape, size, and speed of rotating propeller blades.
How do nozzles increase propulsion efficiency?
The propeller turns within a shroud which increases propeller thrust by 15% to 60% more than an open propeller.
What are the disadvantages of nozzles?
They present more drag underwater hindering maneuverability and light tug running speed.
What three factors determine a rudders efficiency?
Velocity of water flow, quality of water flow and rudder shape
What is the difference between an unbalanced and a balanced rudder?
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 mechanical advantage to the steering gear.
What is a spade rudder?
A “spade” rudder is one that is not supported at the bottom by an extension of the vessels keel or skegs.
What is the difference between a steerable nozzle and a steerable propeller?
A steerable nozzle changes the angle of thrust by rotating the angle of the nozzle around a fixed propeller. A steerable propeller changes angle of thrust by rotating propeller around a vertical axis.
Why are flanking rudders effective when operating astern?
Flanking rudders are installed ahead of the propellers and there are usually two of them for each propeller. They are on the discharge side of the propeller when operating astern
Describe the voith schneider cycloidal propeller.
Egg Beaters
What are the disadvantages of a VSP?
HP to bollard pull ratio is lower than conventional tugs. Deeper draft. Expensive.
Describe the steerable propeller system.
Lower drive unit houses the propeller and can rotate 360 degrees with either mechanically or electronically driven motors.
What are the disadvantages of an SPS?
Reversing thrust requires rotating the drive 180 degrees the SPS could be constantly thrusting while shifting thrust direction.
What is bollard pull?
The pull in pounds generated by a tug against a fixed object
Is bollard pull a fair criterion for judging a tug’s ability for shipwork?
Not always, stiffness and maneuverability are important too
