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
    Re: Navigating Around Hills and Dips in the Ocean
    From: Trevor Kenchington
    Date: 2003 Aug 16, 13:19 -0300

    Michael Dorl wrote:
    > Another thought.....
    > Even if the water surface were somehow distorted from an equi-potential
    > surface, it's still a zero sum game what goes up must come down.
    I doubt it, at least where the ship owner is concerned.
    Lifting a ship up a hill (relative to the gravitational equipotential
    surface) needs the conversion of chemical energy, in the molecular
    structure of the fuel, into potential energy. It is not possible to
    reverse that process. When the ship comes down the other side, the
    potential energy is lost as heat. To be a zero sum game from the ship
    owner's point of view, the amount of fuel burned in turning chemical
    energy into potential energy, on the way up, would have to be equal to
    the amount of fuel saved as the potential energy helped drive the ship
    on the way down.
    But if the captain maintains constant revolutions, the ship would slow
    on the way up (some of the power going into lifting the ship, instead of
    driving her forwards) and speed up on the way down. Yet wave-making
    resistance does not increase linearly with speed. That non-linearity
    means that what you lose on one side is greater than what you gain on
    the other. The ship burns fuel at a constant rate but is slower by more
    than she speeds up, thus lengthening the voyage, burning fuel for longer
    -- and perhaps encouraging the route planners to set higher revolutions,
    in order to arrive on schedule, which would mean more fuel burned.
    If the captain were clever enough to maintain constant speed, backing
    off the revolutions on the down slope and increasing them on the up
    hill, he would still face non-linearities in engine efficiency,
    propeller slip and so forth. Assuming that the constant speed was
    selected for optimum performance in the absence of hills and dips, then
    facing those slopes at the same speed is very likely to be sub-optimal
    -- increasing costs.
    I strongly suspect that the deviations from a zero sum are very much
    less than negligible, when considered relative to all of the other
    variables facing ship operations. (The very obvious hills and valleys
    created on the sea's surface by wind likely have a much greater effect
    on fuel use!) But I still doubt that it is literally a zero sum game.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

    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)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site