DieselShip's Online Content Library

Q9. A. List the factors that determine the starting torque of the three-phase induction motor. How does this torque generally compare with the value of the rated torque?

B. The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm. When connected to a 220 V, 50 Hz, a.c. supply the current taken is at first 2A, and when the plunger is drawn into the “full-in” position the current falls to 0.7 A. Calculate the inductance of the solenoid for both positions of the plunger, and the maximum value of flux-linkages in weber-turns for the “full-in” position of the plunger.

Q4. A) Calculate the Line and Phase current at full load of a 50 kW, 440 Volt 3 phase deltas connected motor having full load efficiency 90% and power factor O.8 lagging. B) Calculate the starting current if motor is started by star/delta starter and Auto transformer starter which has transformer tapping 65 %

Q9. A. Show how the power that is transferred across the air gap of the three-phase induction motor is represented. Explain the terms. What portion of this is useful power? B. The primary and secondary windings of a 500 Kva transformer have resistance of 0.42 and 0.0019 respectively. The primary and secondary voltages are 11 000 V and 415 V respectively and the core loss is 2.9 Kw, assuming the power factor of the load to be 0.8. Calculate the efficiency on (i) Full load; (ii) Half load;

Q9. The ½ ordinates of a water plane at 15m intervals, commencing from aft, ar. 1, 7, 10.5, 11, 11, 10.5, 8, 4 and 0m. Calculate:

(a). TPC;

(b). Distance of the centre of flotation from midships.

(c). Second moment of area of the water plane about a transverse axis through the centre of flotation.

Q8. A ship 150 m in length, 24 m breadth, displaces 25000 tonne when floating at a draught of 9 m in sea water of density 1025 kg/m³. The ship’s propeller has a diameter of 5.8 m, a pitch ratio of 0.9 and a blade area ratio of 0.45. With the propeller operating at 2 rev/sec, the following results were recorded:

Apparent slip ratio = 0.06

Thrust power = 3800 kW

Propeller efficiency = 64%

The Taylor wake fraction is given by: WT = 0.5CB - 0.05

Calculate EACH of the following for the above condition:

(a) The ship’s speed

(b) The real slip ratio

(c) The thrust per unit area of blade surface

(d) The torque delivered to the propeller.

Q6. A ship 75m long has semi ordinates at the load water plane commencing from forward as follows: 0, 1, 2, 4, 5, 5, 5,4, 3, 2 and 0 metres respectively.

The spacing between the first three semi-ordinates and the last three semi-ordinates is half of that between the other semi-ordinates. Find the position of the Centre of Floatation relative to amidships.

Q7. (a) The residuary resistance of a 1/25 scale model of a ship is 6.35 N when tested at 1.543 m/s in fresh water of density 1000 kg/m3. The frictional resistance of the ship at 12 knots in sea water of density 1025 kg/m3 is 145 kN. Frictional resistance can be assumed to vary with speed to the power 1.825. Calculate the effective power (naked) for the ship at the speed corresponding to the model test.

(b) The following additional data apply to the ship operating in service at the corresponding speed calculated in Q6 (a) with a propeller having a pitch of 4.6 m.

Appendage and weather allowance = 22%

Quasi-propulsive coefficient (QPC) = 0.7

Propeller speed = 1.75 rev/s

Taylor wake fraction = 0.32

Propeller thrust = 565 kN

Calculate EACH of the following:

(i) The torque delivered to the propeller; (ii) The propeller efficiency; (iii) The real slip ratio.

Q6. (a) Explain what is meant by floddable length. B. (i) Construct a graph from the following information:

(ii) From this graph find the TPC's at draft of 3.2m; and 4.3m; (iii) If the ship is floating at a mean draft of 4m, and then loads 50 tonnes of cargo, 10 tonnes of fresh water, and 25 tonnes of bunkers, whilst 45 tonnes of ballast are discharged, find the final mean draft.

Q7. A. Explain how increase of draught and of displacement influence rolling. B. A pontoon has a constant cross-section as shown in Fig. Given below the metacentre height is 2.5m. Find the height of the centre of gravity above the keel.

Q9. Describe the effect of cavitaion on: i. The thrust and ordque; ii. The propeller blades B. A ship 150m long and 19m beam floats at a draught of 8m and last a block coefficient of 0.68 i) if the admicatry coefficient is 600 calculate the shaft power required at 18 knots; ii) If the speed is now increased to 21 knots, and within this speed range resistance varies as speed find the new shaft power.

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.

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

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.