Ship Construction/Electrical Flashcards
Q8
With reference to the stability and construction of ships:
(a) Explain how the loading of wing bunker tanks can be used to aid stability (4)
(b) Explain how the loading of double bottom bunker tanks can be used to aid stability (4)
(c) Define EACH of the following stability terms.
(i) Trim (4)
(ii) Heel (4)
a) Loading wing bunker tanks increases the ship’s metacentric height (GM) by spreading weight outward from the centerline, effectively lowering the centre of gravity and increasing the righting lever (GZ), which enhances stability.
(b) Loading double-bottom tanks places weight lower in the vessel, reducing the centre of gravity and thus increasing the ship’s stability.
(c) (i) Trim refers to the difference in draft between the forward and aft ends of the ship. It indicates how the ship is balanced longitudinally.
(ii) Heel is the tilting or inclination of a ship to one side, typically caused by external forces like wind or uneven loading.
Q9
(a) Sketch and describe EACH of the following ship construction features:
(i) Plate floor (6)
(ii) Watertight bulkhead (6)
(b) Explain how watertight bulkhead integrity may be tested (4)
a) (i) Plate floor:
They are subdivided and strengthen the double bottom.
They have holes cut out of the middle to reduce weight.
(b) Watertight bulkheads can be tested by two ways, either by filling the compartment with water or using a fire hose against the wall. Filling the compartment would only be possible if there was a safe means of emptying the tank. With both methods, you ensure that no water passes through the bulkhead.
Q6
(a) List FIVE protective devices fitted to a main electrical switchboard (10)
(b) Explain the meaning of the term Earth Fault and state why it is undesirable in electrical installations. (6)
(a) FIVE protective devices fitted to a main electrical switchboard are:
Overload relay;
Reverse power trip;
Low/high frequency trip;
Under/over voltage relay;
Fuses.
(b) An earth fault is a defect in an electrical circuit or apparatus that results in a current flow to earth at the point of the defect. Earth faults are undesirable because they can lead to safety hazards such as equipment malfunctions, fire and injury to personnel (If someone were to touch the faulty appliance and make another path to earth they will receive a serious electric shock).
(a) Describe how you would overhaul an electric motor that has been flooded with sea water (8)
(b) Explain how to check the insulation resistance of the motor, stating the minimum acceptable value (8)
a) The motor should be removed from its fixings and dismantled into rotor and stator. Both the rotor and stator should then be bathed in distilled water to clean off the salts and other matter. They should then be allowed to dry before re-varnishing the stator. Once the varnish has dried, electrical tests should be carried out; a Megger test, balance and continuity test. If all tests produce acceptable results the motor can be reassembled with new bearings and commissioned for service.
(b) Before conducting the test it is ensured that the motor has been electrically and mechanically isolated and appropriately rated and certified test instruments are being used. On the terminal block of the motor, each winding’s connections are separated. With the aid of a megger (500V DC for 440V AC motor), two sets of readings are obtained; between phases (U1V1, V1W1, W1U1) and earth (U1E, V1E, W1E). The same test is carried out for the other set of windings. It is also ensured that obtained insulation resistance values are recorded for trending purposes. This allows engineers to predict any maintenance requirements. The minimum acceptable value is 1MΩ.
