|
|
From: Trevor J. Kenchington (no email)
Date: Sat Aug 16 2003 - 12:19:01 EDT
Michael Dorl wrote:
> Another thought.....
>
> Even if the water surface were somehow distorted from an equi-potential
> surface, it's still a zero sum game what goes up must come down.
I doubt it, at least where the ship owner is concerned.
Lifting a ship up a hill (relative to the gravitational equipotential
surface) needs the conversion of chemical energy, in the molecular
structure of the fuel, into potential energy. It is not possible to
reverse that process. When the ship comes down the other side, the
potential energy is lost as heat. To be a zero sum game from the ship
owner's point of view, the amount of fuel burned in turning chemical
energy into potential energy, on the way up, would have to be equal to
the amount of fuel saved as the potential energy helped drive the ship
on the way down.
But if the captain maintains constant revolutions, the ship would slow
on the way up (some of the power going into lifting the ship, instead of
driving her forwards) and speed up on the way down. Yet wave-making
resistance does not increase linearly with speed. That non-linearity
means that what you lose on one side is greater than what you gain on
the other. The ship burns fuel at a constant rate but is slower by more
than she speeds up, thus lengthening the voyage, burning fuel for longer
-- and perhaps encouraging the route planners to set higher revolutions,
in order to arrive on schedule, which would mean more fuel burned.
If the captain were clever enough to maintain constant speed, backing
off the revolutions on the down slope and increasing them on the up
hill, he would still face non-linearities in engine efficiency,
propeller slip and so forth. Assuming that the constant speed was
selected for optimum performance in the absence of hills and dips, then
facing those slopes at the same speed is very likely to be sub-optimal
-- increasing costs.
I strongly suspect that the deviations from a zero sum are very much
less than negligible, when considered relative to all of the other
variables facing ship operations. (The very obvious hills and valleys
created on the sea's surface by wind likely have a much greater effect
on fuel use!) But I still doubt that it is literally a zero sum game.
Trevor Kenchington
--
Trevor J. Kenchington PhD
Gadus Associates, Office(902) 889-9250
R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251
Nova Scotia B0J 2L0, CANADA Home (902) 889-3555
Science Serving the Fisheries
http://home.istar.ca/~gadus
|