A Simplified Guide to Small Marine Craft Navigation.

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Status: In Progress  |  Genre: Non-Fiction  |  House: Booksie Classic

Chapter 12 (v.1) - The Marine Log.

Submitted: March 12, 2017

Reads: 154

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Submitted: March 12, 2017



The Marine Log.


The instrument used for measuring the distance and, or speed a craft travels through the water is called a Marine Log, or in the short simply the log. The Marine Log derives its name from two of the earliest methods of estimating speed and distance, the Dutchman's Log and the Common Log. Both of these originated in the fifteenth Century, and are occasionally still used.

The Dutchman’s Log is based on the principle that by observing the time taken to pass a floating and stationary object close alongside, the speed of a vessel can be calculated. Two points on the vessel are selected, and the distance between them measured accurately. A small float is thrown as far ahead of the vessel as possible, and the interval of time it takes for this float to pass between the two measured points on the vessel is taken. Providing the observation and timing are accurate, the Dutchman's Log will give good results, especially for fairly slow speeds. For the best results, the method should be repeated several times and the mean interval of time used to calculate the speed.

In example:  Distance between observing points 50 feet. Mean interval of time for 5 runs 4.6 seconds. Q X 60 seconds. x 60 minutes. Speed of vessel = 4.6 6076 feet. Nautical miles per hour, knots, = 6.44 Knots.

The Common, Hand, or Ship Log consisted of a reel on which was attached a piece of wood shaped like a quadrant or sector of a circle and called the log chip. The chip was thrown over the stern and the line paid out from the reel until the chip was clear of the vessel's wake, at this point a zero mark was usually inserted into the line, and as this passed over the stern timing was commenced with a l4-second sandglass. As soon as the glass ran out the line was held fast and the nearest mark noted, the number of knots on this mark denoting the vessel's speed through the water. This is the derivation of the term knots as maritime speed units.

Small marine craft has a choice of several different types of log equipment, and invariably one or more logs are fitted regardless of what other navigational aids are carried. In fact, an increasing number of navigational aids now require a log signal to operate them. Science has enabled navigators to fix their position with considerable ease and certainty, and has made it possible to see through cloud and fog. However, there are times in bad weather or in poor visibility, and particularly when it comes to small marine craft, that navigation by dead reckoning will depend on skill, and at such times knowledge of the distance run is vitally important.

Many methods of recording Speed and distance are employed on large commercial ships, but advanced technology has only just begun to filter through for small craft. This may be because of the cost barrier; new technology is expensive and often needs, skilled attention for some time in post development.

Nevertheless, the following types of log have been sufficiently proved and developed in small craft applications to be worthy of consideration. Impeller, mechanical cable drive or electromagnetic pickup, electromagnetic, acoustic doppler, and strain gauge rod. In spite of advanced technology, the sales of towed logs are constantly increasing, perhaps because for the serious small craft navigator they are extremely reliable instruments requiring no electric power and which when fouled can be untangled, repaired or replaced without slipping the craft.

The Towed Log, also known as the Patent Log, consists of a streamlined gunmetal rotator having four pitched fins and towed from a craft by means of a patent  logline having a wire heart. The rotator revolves at a speed proportional to the speed of the craft through the water and induces a constant twist into the line. The “inboard” end of the “logline” is attached to the counter mechanism, or register, in which the twist of the line is dissipated and the number of revolutions converted into nautical miles, which are indicated on the dial of the register.

The length of logline to use depends on the average speed of the craft and the height of the register above water level, and the manufacturer’s recommendations in this respect have to be followed to the letter, if accurate results are to be expected.

Towed logs have been in use for close on two centuries and are reasonably accurate provided the correct length. Disadvantages of this type of log are that it will not register accurately at slow speeds; the rotator is liable to damage or fouled by flotsam, weed, and even known to be taken by shark. In addition, it must be hauled in to avoid it sinking or becoming fouled if the vessel stops or goes astern and it is inaccurate in a heavy following sea.

To stream a Towed Log, hook the line on to the governor first, and then stream the rotator gently so that the line does not take up with a jerk at the end. To hand the Log, haul in a few feet of line, unhook from the governor and continue to haul in over one quarter while paying out the hook end over the other; when the rotator is aboard, haul in the line again. Unless this practice is used, the line will become full of turns or snarls owing to the action of the rotator while hauling in.

The Impeller Log, is operated by the flow of water passing along the vessel's hull and rotating a small impeller situated at the base of the log tube that passes through the bottom of the boat's hull. In most makes, this tube is retractable. The registration of impeller logs may be either mechanical or electronic.

In the mechanical type, the impeller is connected to a distance meter located in the cabin, cockpit bulkhead or chart-room by a rotating Bowden-type wire. In the electronic type, electrical impulses generated by the impeller are fed by cable to a computer in a combined speed-and-distance recorder. The impeller is loadless, as it is a screw which has no appreciable work to perform or internal friction to overcome and therefore has no slip and no variable error.

An error can occur when the flow of water past the impeller is not exactly parallel to the impeller shaft, and in some types of Impeller Log, a calibration device is available for use when the log is found to have a constant error in which the impeller blades can be adjusted to give corrected readings. With this exception, Impeller Logs are generally accurate at all speeds, in any weather, at any draught, and whether the vessel is going ahead or astern.

However, the impeller is liable to fouling by weed or damaged by solid objects in the water, although this is less frequent than with the rotator of a Towed Log owing to deeper immersion, and readings will be unsatisfactory if the contacts in the submerged mechanism or in the components of the speed indicator are faulty.

The Electromagnetic Log works on the principle that a conductor moving through a magnetic field has an electric current induced in it. The arrangement is very similar to that for the impeller log with the exception that instead of an impeller unit, a retractable rod meter projects through the bottom of the vessel's hull. The rod meter is simply a sealed tube with an iron-cored coil mounted in a fiberglass shell at its outboard end.

An electric current flowing through the coil produces a magnetic field in the surrounding water near the rod meter. The water, being a conductor, will have a current induced in it provided there is relative movement between the magnetic field and the water, and this current is proportional to the velocity of the rod meter through the water. Two bronze electrodes within the rod meter pick up the induced current, and then the detected signal is transmitted to the Speed and Distance meters. This type of Log is considered extremely accurate.

The Doppler Log works on the principle of the Doppler Effect. Most everyone is familiar with the fact that a locomotives whistle goes higher in frequency as it approaches and lower as it recedes - this is the Doppler Effect. In duller, technical language, the Doppler Effect is the change in the observed frequency of a vibration because of relative motion between the observer and the source of the vibration.

The Doppler Log has no underwater moving mechanical parts, but instead it has a small transducer mounted flush with the under-side of the crafts hull. The transducer transmits a high frequency ultra-sonic signal ahead, and a small part of this transmitted signal then reflects back to the transducer from the boundary layer between hull and water. Whereas the transmitted signal remains at the frequency of the transducer crystal, the received reflected signal increases in frequency as the crafts speed increases.  The difference between the two frequencies is converted into Speed and Distance by the instrument's computer circuitry and displayed simultaneously on the Speed and Distance meters. Considerable expertise is required for the correct siting of the transducer and its aiming at a suitable boundary layer.

The type of small craft Doppler Log described above should not be confused with another type of Doppler Log fitted to large commercial ships. A type that has two pulsed sonar signals transmitting ahead and astern from a small transducer, and averaging the two signals largely cancels errors due to pitch and roll.

From the seabed up to a depth of about 150 meters, 500 feet, a strong return signal is received but beyond 150 meters, the system automatically locks on to the return signal from the water mass, the sonar energy being reflected from small particles present in seawater. The result is a highly accurate and reliable indication of the  true velocity of the ship relative to the seabed up to depths of 150 meters and to the water mass at all other depths.

The Strain Gauge Rod Log derives its information from the bending effect produced in a thin metal rod protruding from the vessel's hull by its movement through the water. The rod is raked aft in order to deflect weed or small sized flotsam, and sensitive strain gauges detect the amount of bending which is proportional to the vessel's speed.

The Regulus Log is of the Strain Gauge type and is a very sophisticated instrument whose display read-out shows not only speed and distance but acceleration as well. This last feature is invaluable in racing craft for optimum performance. Fouling by weed is easily detected by the abnormally high speed reading it produces, and should the metal rod become damaged, it can be easily replaced.

The Pitometer or Pressure Log, operates on the principle that the pressure exerted on it by the water Varies with the speed of the vessel. A bronze tube called a pitot tube is fitted through the bottom of the crafts hull. The pitot tube has two independent openings, one of which faces forward, the impact opening, and one facing downwards, the static opening.

The difference between the impact pressure due to the flow of water past the craft and the static pressure due to the crafts draught is measured and converted into electric current directly proportional to the crafts speed by a controller unit inside the crafts hull, and transmitted  to a recorder unit in the navigation space.

Pressure logs are accurate at all speeds and in all weathers and have the added advantage that there are no moving parts to become foul, and if the pitot tube is damaged it can be withdrawn and changed at sea. However, it is bulky to install and more expensive than other types of log, and in consequence, it is not that often found in small marine craft.


© Copyright 2020 Sergeant Walker. All rights reserved.


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