Ship's propulsion problem........

[singhmandy]

Hi all......

I have this question which I am looking for an answer. Plz read the attached word file and try to answer the query.......

[colinstone]

13 knots

[charmc]

13.3 knots. With half the engines not operating (and declutched, presumably) thrust is reduced by half, to approx. 35,000 lbs. On the chart this crosses about 13.3 knots.

I might be oversimplifying the problem; on the other hand, sometimes it's as simple as it looks. Yet I wonder if I missed something. Too easy? Trick question?

[Guest625101138]

I am making the assumption that engine power is halved with half the engines running. Drive prop efficiency remains the same and the non-powered props do not add any more drag - lets say they can spin freely.

Design power at hull is:
70,000 * 18 *6080/3600/550 = 3869HP

Half power is 1935

Try first speed approx of (18^3/2)^(1/3) = 14.3
Check
40,000 * 14.3 * 6080/3600/550 = 1756HP

Try 15kts
45000 * 15 * 6080/3600/550 = 2072HP

Lets say 14.8kts

Rick W.

[Guest625101138]

There is another way that might be simpler to arrive at the same answer.

Just redraw the propeller thrust curve at half values and take the speed where it intersects the hull drag curve. You then do not have to make the assumption about the prop efficiency. The intersection is very close to the 14.8kts using the power calculation as you would expect.
Rick W.

[singhmandy]

I need to thank all of you for answering my query......

[erik818]

Assuming that the propeller slip remains approximately the same around 14 knots with two engines as with four engines and 18 knots, two engines at full throttle will be working at a lower rpm than would four engines at full throttle. Lower rpm usually means less power, so two engines alone would not provide half the power of four engines. The propellers are optimised for four engines, not two.

I don't know how to read the of engine power or propeller thrust as a function of speed from the diagram, so I'm not able to say what the power and hence the speed would be with two engines. Mayby the info is there, but if I really needed to know I would get hold of the dimensioning data (slip as a function of speed) of the propellers and the torque curve of the engines to solve the problem.

Please correct me someone if I'm totally off track.

Erik