Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    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:

            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:

            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

            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)
    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

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