DieselShip's Online Content Library

Q5. In some circumstances electrical current may be induced into the shafting of rotating machinery.

a) State the problem that may be caused by this current. (6)

b) Explain with aid of sketches, how currents may be avoided or reduced in the following instances:

(i) D.C machines

(ii) Main shafting fitted with a bronze propeller (10)

Q6.a) Define centre of buoyancy and show with the aid of sketches how a vessel which is Stable will return to the upright after being heeled by an external force (6)

b. An oil tanker of 17000 tonne displacement has its centre of gravity 1 m aft of midships and has 250 tonne of oil fuel in its forward deep tank 75 m from midships. This fuel is transferred to the after oil fuel bunker whose centre is 50 m from midships. 200 tonne of fuel from the after bunker is now burned. (10)

Calculate the new position of the centre of gravity:

(a) after the oil has been transferred

(b) after the oil has been used.

Q10. A. What are the factors which determine the synchronous speed of a motor? (6)

B. A twelve-pole, three-phase, delta-connected alternator runs at 600 rev/min and supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm and inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine (a) frequency of supply (b) current in each coil (c) current in each phase of the alternator (d) total power supplied to the load. (10)

Q10. A. Describe how protection against short circuit is provided in a 3 phase induction motor circuit.
b) An eight – pole alternator running at a speed of 720rev/min supplies current to synchronous and induction motors with forty – eight poles. Calculate the frequency and speed of rotation of the motors if thr induction motor runs with 2 percent slip.

Q8. A. (i) What is direct-connected alternator? (3)

(ii)  How is a direct-connected exciter arranged in an alternator? (3)

B. A twelve-pole, three-phase, delta-connected alternator runs at 600 rev/min and supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm and inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine (a) frequency of supply, (b) current in each coil, (c) current in each phase of the alternator, (d) total power supplied to the load. (10)

Q9. A three-phase transformer has 560 turns on the primary and 42 turns on the secondary. the primary winding are connected to the line voltage of 6.6KV. Calculate the secondary line voltage when the transformer is connected a) Star-Detta b) Detta-Star

Q9. (a) With aid of a simple sketch, show the normal positions of centre of gravity of a stable ship relative to keel, centre of buoyancy and meta centre. (6)

(b) A ship has 300 t of cargo in the hold, 24 m forward of the midships. The displacement of the vessel is 6000 t and its centre of gravity is 1.2 m forward of midships. Find the new position of the centre of gravity if this cargo is moved to an after hold, 40 m from midships. (10)

Q8. A bilge holding tank of 5 m length, 3 m width and 1.8 m depth has water of density 1.020 t/m3 upto a sounding of 1.3 m and oil of density 0.86 t/m3 is floating on top of water upto an ullage of 0.3 m. Calculate the mass of water and oil in the tank (16)

Q8. a) Explain clearly the meaning of the term "reserve buoyancy". (6)

b) The centre of gravity of a ship of 5000 tonne displacement is 6 m above the keel and 1.5 m forward of midships. Calculate the new position of the centre of gravity if 500 tonne of cargo are placed in the t’ween decks 10 m above the keel and 36 m aft of midships. (10)

Q7. A moving-coil instrument has a resistance of 10 Ω and requires a current of 15 mA to give full- scale deflection. Calculate the resistance value of the resistor necessary to enable it to be used to measure

a) currents up 25A

b) voltages up to 500V. (16)

Q5. A 4-pole lap wound DC shunt generator has an open e.m.f of 250V when the flux per pole is 0.08 Wb and the speed is 10 rev/sec. The speed of the generator is reduced to 10 per cent and the flux per pole is increased by 5% when the generator supplies a load of 100A. Determine the terminal voltage, if the armature resistance is 0.06 ohms and the new total field circuit resistance is 200 ohm. (16)

Q7. (a) Describe the function of the stern frame. (6)

(b) A ship displacing 10000 tonne and travelling at 16 knots has a fuel consumption of 41 tonne per day. Calculate the consumption per day if the displacement is increased to 13750 tonne and the speed is increased to 17 knots. Within this speed range, fuel consumption per day varies as (speed)^3.8 (10)