NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Logs vs Knotmeters
From: Carl Herzog
Date: 2008 Apr 2, 13:01 -0700
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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---