Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Navigating Around Hills and Dips in the Ocean
    From: David Hoyte
    Date: 2003 Aug 14, 23:59 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, (DavidHoyte---.com)
       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)