NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Navigating Around Hills and Dips in the Ocean
From: David Hoyte
Date: 2003 Aug 13, 22:29 EDT
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, (DavidHoyte@aol.com)
From: David Hoyte
Date: 2003 Aug 13, 22:29 EDT
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, (DavidHoyte@aol.com)