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: Traditional navigation by slide rule
    From: Bob Goethe
    Date: 2016 Oct 28, 10:39 -0700


    The idea of being able to do rhumb line calculations is enormously appealing. 

    I have known, of course, that if you are sailing straight north or south, your rhumb line course IS your great circle route.  Intuitively you know that if you are sailing mostly north, or mostly south, or easterly/westerly in lower latitudes, it is not worthwhile goofing around with great circles.  You also know, intuitively, that if you are sailing mostly east or west in higher latitudes, that doing great circle calculations is enormously worthwhile. It follows that there is a break point somewhere in between worth-it and not-worth-it.

    What your equations do is to provide an easy way to give mathematical definition to one's intuitions.  They scratch an intellectual itch that I have had for several years now.  They give me the same sort of "eureka" feel that discovering Arvel Gentry's mathematical approach did when I first discovered them, as he unpacked how to give mathematical definition to the intuition that there was an optimal angle to use in tacking downwind.  Being able to give some precision to navigational intuitions like this is hugely satisfying.

    Action Steps for Bob

    I will work through some problems with these equations to ensure that I can use them properly. 

    Following that, I will see if I can get a natural logarithm scale outputting graphically in my software.  Assuming I succeed here, then it will be a small thing to modify the design of the slide rule to add another scale.

    The issues I will face then will be interrelated ones of usability and cursor design.  I experimented with spacing between scales and font sizes quite a bit before settling on the current design.  I have sought, with the current dimensions of the Mark 1, to combine legibility with a cursor design that incorporates a 75 mm microscope slide.

    If I make the rule much bigger, using a standard microscope slide will become infeasible. I will be compelled to look more closely into fabricating a transparent cursor of some plastic substance.

    So between correcting the errors from the manual that you, Paul, and Stan have been so helpful in pointing out, revising the time/speed/distance approach per Paul's suggestion, and working on rhumb-line-related issues (with the concomitant growth in physical dimensions of the rule), I have my work cut out for me for the next several weekends.

    Last weekend was a a very family-oriented time, so no slide rule work then.  So also with the weekend commencing seven days hence.  But as we head into an Alberta winter, this is a perfect set of projects to take on.  All I really need to move ahead dramatically is to be snowed in for a couple of days with a blizzard.  Indirectly, you will be able to estimate the rate of my progress by going to https://www.theweathernetwork.com/ca/weather/alberta/edmonton .

    Thank you very much for your continued help in this!!


    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