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From: George Huxtable (no email)
Date: Thu Aug 14 2003 - 05:10:07 EDT
David Hoyte <> wrote-
> The joint NASA-German GRACE project has released the most
>accurate map yet of Earth's gravity field. It shows Gravity Anomaly,
>(mGal), on a global map at the URL:
>http://photojournal.jpl.nasa.gov/catalog/PIA04652
>
> These gravity anomalies cause the geodic heigh of the ocean's
>surface to vary around the world by up to 200 meters, 650 feet. Ref:
>http://www.csr.utexas.edu/grace/publications/press/03-07-21-ggm01-nasa.html
>
> In the Atlantic ocean, for example, there is a hill South of
>Greenland of +200 feet, and a dip in the Caribbean of -250 feet, approx.
================
This is interesting information. Then he went on-
>
> I heard as far back as 1975, at the IBM Maritime Center in
>Italy, that a large ship will use significantly more fuel if it passes
>down into a gravitational dip and climbs the other side, rather than
>following a longer path around the dip which will keep it more "on the
>level".
>
> Is there a published algorithm that relates the parameters
>such as ship's tonnage, the size of the hill or dip, the path followed
>and fuel savings?
>
>
> Is there perhaps a simple "rule of thumb" for the courses to
>steer, for use at sea?
=============
Response from George Huxtable.
I am a natural sceptic about most matters, and here's another to be
sceptical about.
What is being mapped, presumably, is a gravitational equipotential surface,
with all its ups and downs with respect to the Earth's centre. If the Earth
was spherical and non-rotating and of uniform density, then that surface
would be a sphere. None of those applies, so it's not a sphere.
What that surface maps out is an equal gravitational potential, so that
moving from anywhere on its surface to anywhere else, there is no change of
energy due to gravitational differences. It maps out the water surface,
because it it didn't, any drop of water on the surface, being free to flow
"downhill" to minimise its energy, would immediately do so, until the water
surface conformed to the equipotential. (For the purists, I will mention
that small "geostrophic" forces caused by ocean currents on a rotating
Earth are here ignored, as are the oscillatory gravitational forces of the
Moon and Sun that give rise to tides).
What applies to that drop of surface water applies equally to any object
floating in it, including a ship. It that were not so, such objects would
congregate at certain locations, which would become floating graveyards of
lost vessels!
Gravitationally speaking, then, when a ship takes a path from A to B, both
being on that equipotential we call the ocean surface, it matters not at
all which path she follows, there's no loss or gain of gravitational
energy, even though one path would take her nearer to the centre of the
Earth than another. The fuel burned would depend mostly on the work done in
displacing water to make way for her path, and so depends largely on the
length of that path, but has to allow for likely winds and currents too.
So where David Hoyte says-
>I heard as far back as 1975, at the IBM Maritime Center in
>Italy, that a large ship will use significantly more fuel if it passes
>down into a gravitational dip and climbs the other side, rather than
>following a longer path around the dip which will keep it more "on the
>level
I just don't believe it. On the ocean surface there are no gravitational
dips. I suggest that the IBM Maritime Centre in Italy got it wrong, or else
perhaps David has misremembered. Presumably someone was working on ship
routeing algorithms at that time. If ship routeings have ever been adjusted
to take account of the ocean's hills and valleys, then I suggest someone
has got things badly wrong.
Perhaps I should make it clear that my arguments are based on general
physical principles, and I know little about the complex topic of
ship-routeing. If anyone on this list can correct my conclusions, I hope
that they will.
George.
================================================================
contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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