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From: David Hoyte (no email)
Date: Thu Aug 14 2003 - 23:59:40 EDT
Do we have any serving or retired merchant-marine or naval officers
on this List who could comment on this question from their experience?
I find it is usually best to get out of the classroom and look at what
happens
in real-life when simplifying assumptions can produce any answer you please.
The hoary school-book question about the time taken by a man to swim a
mile against a current , then turning round and swimming with it,
compared to the time taken by the man swimming both directions in still
water,
can give the ship a longer time passing through a dip in the ocean, compared
to
travel on a surface of uniform 'g' . . . depending on what assumptions you
care
to make.
Can we hear on this question (repeated below) from someone with extensive
real-life experience of large-ship navigation?
Do large ships in fact ignore the hills and dips in the ocean's surface that
are
due to variations in gravitational force ?
Thank you. David Hoyte
=======================================
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.
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?
David Hoyte, MA Cantab, ()
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