Q8. A model propeller of 0.3 m diameter and 0.25 m pitch is tested in fresh water of density 1000 kg/m3. At a speed of advance of 1.8 m/s and a rotational speed of 10.0 revs/s, the shaft torque is 12 Nm and the thrust developed is 260 N.
A geometrically similar ship’s propeller 5.4 m in diameter, is operating in sea water of density 1025 kg/m3 at corresponding linear and rotational speeds.
(a) For the ship’s propeller, calculate EACH of the following:
(i) Revolution per second. (1)
(ii) Speed of advance. (1)
(iii) Real slip. (3)
(iv) Delivery power. (3)
(v) Efficiency; (4)
(vi) Calculate the hull efficiency when the propeller is operating on a vessel at a Taylor wake fraction of 0.26 and a thrust deduction fraction of 0.22. (4)
Note: For geometrically similar propellers of corresponding speeds it can be assumed:
Linear speed is proportional to (diameter)1/2
Rotational speed is proportional to (diameter)-1/2
Thrust is proportional to (diameter)3
Torque is proportional to (diameter)4.
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