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These are abstractions that are only approximately met by real ships, which
don't always read the theory books. And it's only very roughly true to
assume that engine and screw efficiencies and slip remain constant over
such a wide speed range. For example, a diesel running at a certain rpm,
even under no-load conditions on the shaft, requires to develop a certain
minimum power just to keep itself turning, because it has to chuff through
itself a constant volume of air per second, compress it and spit it out. So
that results in a diesel's efficiency being near maximum at close to its
designed running speed, and falling at lower speeds and torques.

These are all good points.

It would, then, be a very poor approximation to imagine that power would be
proportional to ship's speed. In answer to the exam question, Doug appears
to be presuming a cube-law, or something very close to it, rather as was
guessed-at above.

I don't know if I'm presuming the cube-law or not.I'll post my proceedure
tomorrow and you guys decide.

It was interesting that Doug's question presumed a propellor slip value of
only 2%. I had always though that ship's propellors slipped much more than
that, but it seems I was wrong, or very out-of-date. Is 2% slip a realistic
value for the prop of a modern merchant vessel? Doug probably knows.

I tried to warn you that these C.G. exam questions sometimes have no real
relationship with the real world.Don't give undue signifacance to any of the
numbers in the question.They're just quantities some gov. employee plugged
into the question and really have no base in reality.The method used to get
the correct answer is what's important.I just used it because I understand
what they want and thought it the easiest example to show fuel consumption.

You guys are doing a better job then I in explaining or examining this
stuff.

Just be careful with what you guys are calling prop slip.There are positive
and negative quantities to slip.Each seperate prop has 1 but not the other.

2 % slip would be simply outstanding.You are correct in your
assumption.Between 4 - 7 % would be more realistic.

   

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