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    Re: Logs vs Knotmeters
    From: Carl Herzog
    Date: 2008 Apr 2, 13:01 -0700

    I returned last week from a two-month voyage in which we recorded
    approximately 2,500nm at sea on a Walker taffrail log. As part of the
    Sea Education Association's college semester-at-sea program on a
    traditional sailing ship, we rely on a Walker Excelsior log for hourly
    DR positions. (Yes, we also maintain a GPS plot for backup) The DR
    forms the basis of the day's work in celestial that students conduct
    as part of the curriculum
    
    Based on this experience, here are a few thoughts in response to
    questions and comments made in this thread:
    
    Contrary to concerns about wearing gears, a good taffrail log is very
    well suited to long distance use. As a constantly recording odometer,
    it is a generally more reliable source of distance traveled than a
    chip log, and is much easier to use. (Just ask the crew who's hauled
    back a chip log every hour!) The gauge does require regular oiling,
    and the rotor should be hauled back occasionally to ensure that it has
    not fouled with weeds -- if you're sailing in waters where that's
    present. Similarly the line connecting the rotor to the gauge ought to
    be occasionally adjusted at the gauge, as chafing can occur there.
    
    To my knowledge, no patent taffrails logs are still in production.
    Walker ceased production nearly 10 years ago, if memory serves right.
    As a result, taffrail logs are increasingly difficult to make
    practical use of because spare parts (particularly lost or damaged
    rotors) are hard to come by. We keep several on hand and rely on
    White's Instruments in Boston to service them.
    
    A taffrail log doesn't work well at slow speeds, because the rotor
    will cease to spin and simply drag through the water. This is usually
    pretty easily noticed, especially if you're checking the distance
    traveled on an hourly or better basis. With the larger logs we use,
    anything under about 2.5 knots starts to get suspicious. At that
    point, we turn to a Dutchman's log run along a distance of 67 feet (a
    convenient measurement between two conspicuous points on the deck).
    We've found the Dutchman's log to be highly accurate with a little
    practice.
    
    Another key element in the taffrail log's accuracy is the choice of
    line between the rotor and gauge. In our experience, the best line is
    a non-stretching, braided line - which best transfers the twist from
    the rotor to the gauge. We've used a variety of different synthetic
    materials, based on what's available at a reasonable cost.
    
    Many logs can provide speed at a glance. The flywheels are painted
    with a stripe that is only visible through a small hole on the
    covering over the wheel. By using the stripe to count the number of
    revolutions in a given amount of time, speed can be determined with
    the help of a table provided by the manufacturer. This is generally
    accurate, but better if the time is doubled and the count averaged,
    because there can often be a delay while the the twist builds up in
    the line and transfers to the gauge (this is where the quality of the
    line becomes particularly important).
    
    Although the taffrail log's time has admittedly passed, there are a
    wide variety of other knotmeters available and in practical use today
    that rely on an equally wide variety of mechanisms and principles to
    produce accurate results. Among them are knotmeters that rely on
    principles on electromagnetic induction between two coils to measure
    water flow -- though I certainly don't understand the details!
    
    Though we think of DR as no longer being used in the GPS age, it's
    important to keep in mind that accurate measurements of speed through
    the water, as opposed to over the ground, remain critical for vessels
    needing to gauge current and its effect on operations.
    
    Carl Herzog
    
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