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: Propulsion power: was [Nav-L] Barrels
    From: Doug Royer
    Date: 2005 Jan 19, 15:43 -0800

    These are abstractions that are only approximately met by real ships, which
    don't always read the theory books. And it's only very roughly true to
    assume that engine and screw efficiencies and slip remain constant over
    such a wide speed range. For example, a diesel running at a certain rpm,
    even under no-load conditions on the shaft, requires to develop a certain
    minimum power just to keep itself turning, because it has to chuff through
    itself a constant volume of air per second, compress it and spit it out. So
    that results in a diesel's efficiency being near maximum at close to its
    designed running speed, and falling at lower speeds and torques.
    These are all good points.
    It would, then, be a very poor approximation to imagine that power would be
    proportional to ship's speed. In answer to the exam question, Doug appears
    to be presuming a cube-law, or something very close to it, rather as was
    guessed-at above.
    I don't know if I'm presuming the cube-law or not.I'll post my proceedure
    tomorrow and you guys decide.
    It was interesting that Doug's question presumed a propellor slip value of
    only 2%. I had always though that ship's propellors slipped much more than
    that, but it seems I was wrong, or very out-of-date. Is 2% slip a realistic
    value for the prop of a modern merchant vessel? Doug probably knows.
    I tried to warn you that these C.G. exam questions sometimes have no real
    relationship with the real world.Don't give undue signifacance to any of the
    numbers in the question.They're just quantities some gov. employee plugged
    into the question and really have no base in reality.The method used to get
    the correct answer is what's important.I just used it because I understand
    what they want and thought it the easiest example to show fuel consumption.
    You guys are doing a better job then I in explaining or examining this
    Just be careful with what you guys are calling prop slip.There are positive
    and negative quantities to slip.Each seperate prop has 1 but not the other.
    2 % slip would be simply outstanding.You are correct in your
    assumption.Between 4 - 7 % would be more realistic.

    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