23Speed and Depth bronze to resist the corrosive action of salt water. The rotator is a hollow tube having curved vanes attached to the sides and seized to a hollow frog (often referred to as a bottle) by a short length of sennet laid line (stray length). The opposite end of the frog receives the log line and is secured in the manner shown in Figure 2.2. To ‘Hand’ the Log (heaving the log back aboard) 1. Disconnect the bridge connection to the bridge repeater. 2. Stop the governor from rotating and bring in a little of the log line by hand. 3. Unclip the Englefield clip from the governor. 4. Continue to heave the log line inboard, taking the bare end to the opposite quarter of the vessel. Pay out the bare Englefield clip end as the rotator is heaved in. 5. Allow time for any kinks in the rope caused by the rotator to be ‘turned out’. Heave in the line, coiling down left-handed. 6. A light grease should be applied to the log clock after removal of any salt crust on the casing. All equipment should then be returned to a safe stowage place, except for the line, which should be left to dry naturally. When heaving the log back aboard, mariners should be aware that the rotator when breaking the surface has the tendency to fall back into and under the stern. This could cause damage to the vanes of the rotator and render it useless for future operation. Length of Log Line The length required for reasonable accuracy will vary; it is found by experience when comparing logged distance against observed distance. However, as an approximate guide for vessels with the following speeds, the recommended length is approximately: (a) 75 m to 95 m for speeds of about 12 knots. (b) 100 m to 125 m for speeds of about 15 knots. (c) 130 m to 160 m for speeds of about 20 knots. The length of the log line will effectively change as a vessel changes her draught especially in high freeboard vessels when in ballast. To this end, small adjustments to the real length of line can be made if it is secured to the governor as indicated in Figure 2.1, one of the half hitches being removed to add length to the line or the half hitches spaced out to shorten the real length. In practice, it is normal to check the log against Frog Sennet laid line (stray length) Shaped fin on rotator Rotator Figure 2.5 Frog and rotator. Rotator logs are now limited in use with the advent of various impeller and/or Doppler logs becoming the norm. 24 Seamanship Techniques observed positions and allow for the log reading fast or slow, in preference to continual adjustment of the length of line, although that is a simple process. IMPELLER LOG The impeller log may be considered an electric log, since its operation is all electrical, except for the mechanical rotation of the impeller. There are several designs in general use, but probably the most common is the ‘Chernikeeff’. The principle of operation is based on turning an impeller by a flow of water, the speed of rotation being proportional to the rate of flow past the impeller (turbine principle). As previously stated, designs vary, the two most popular being one with a ring magnet attached to the spindle In the retracted stowed position Check tube Leads to amplifier and electromagnetic counter Log shaft Coil Spindle Water-lubricated bearing sleeve Magnet Impeller Impeller unit Guard ring Sluice valve Log housing Valve wheel Ship’s hull plate Log shaft In the operational position Figure 2.6 Impeller log. 25Speed and Depth and one with the magnet incorporated in the blades of the impeller. In either case a pick-up coil transmits the generated pulses via an amplifier to an electromagnetic counter. This signal is then displayed by a speed indicator and distance recorder. Additional sensors will provide the opportunity for various repeaters to include a direct link to allow speed input into True Motion Radar. Operating power is normally 230/240 volts. It is worth noting that the load on the impeller is negligible; consequently the slip, if any, on the impeller is minimal and can be ignored. The extended log, when in operation, projects approximately 14 in. (35 cm) below the ship’s hull, usually from the engine room position. The log shaft should be housed in the stowed position for shallow water, drydocking etc. The sea valve sluice need only be closed if the log is to be removed for maintenance. However, it must be considered good seamanship practice to close the sluice each time the log is housed. Performance of the log is in general considered to be very good, but obvious problems arise in dirty water areas with a muddy bottom and heavily polluted canals (see Figure 2.6). HAND LEAD The normal length of the hand lead line is about 25 fathoms, and the line used is 9 mm (1 in.) 1 8 untarred cable-laid hemp (left-hand lay). A rawhide becket attached to an eye splice in the end of the line secures the lead, the weight of which is 7–9 lb (3.2–4 kg) when operating from vessels moving at less than 6 knots. From the eye splice, i.e. ‘lead out’, which has the extra safety factor of the length of the lead, or ‘lead in’, measured from the base of the lead, the markings are as follows: At 2 fathoms a piece of leather with two tails. At 3 fathoms a piece of leather with three tails. At 5 fathoms a piece of white linen. At 7 fathoms a piece of red bunting. At 10 fathoms a piece of leather with a hole in it (leather washer). At 13 fathoms a piece of blue serge. At 15 fathoms a piece of white linen. At 17 fathoms a piece of red bunting. At 20 fathoms a piece of cord with two knots. Markings of metric hand lead line are as follows: 1 and 11 m – 1 strip of leather 2 and 12 m – 2 strips of leather 3 and 13 m – blue bunting 4 and 14 m – green and white bunting 5 and 15 m – white bunting 6 and 16 m – green bunting 7 and 17 m – red bunting 8 and 18 m – yellow bunting 26 Seamanship Techniques 9 and 19 m – red and white bunting 10 m – leather with a hole in it 20 m – leather with a hole and 2 strips of leather The different materials indicating the various marks are distinctive to allow the leadsman to feel rather than see the difference during the hours of darkness. The intermediate whole fathom values, i.e. 1, 4, 6, 8, 9, 11, 12, 14, 16, 18 and 19 fathoms, are known as deeps. The leadsman used to stand in the ‘chains’, from where he would take the cast and call up the sounding to the officer of the watch. The lead line is rarely used in this manner today, but the soundings are still occasionally called in a traditional manner of stating the actual number of fathoms last. For example, At 7 fathoms . . . ‘by the mark seven’. At 7 1 4 fathoms . . . ‘and a quarter seven’. At 7 1 2 fathoms . . . ‘and a half seven’. At 7 3 4 fathoms . . . ‘a quarter less eight’. At 8 fathoms . . . ‘by the deep eight’. Should the bottom not be reached, then ‘No Bottom’ is reported. Constructing a New Line Splice the eye into one end of the line, then soak and stretch the line, possibly by towing astern. Mark the line off when wet from measured distances marked off on deck, and tuck the fabrics of the marks through the lay of the line. Benefit of the Lead This is the term used to describe the length from the base of the lead to the eye spice. The actual distance is about 12 inches (30 cm) and is always ‘beneficial’ to the soundings, giving more water for the benefit of the ship. Arming the Lead This describes the action of placing tallow into the ‘arming recess’, found at the base of the lead. The purpose of the soft tallow is to act as a glue to obtain the nature of the sea bottom. If tallow is not available, a soft soap will be equally good. The information is passed to the Officer of the Watch with the depth of sounding. It allows an additional comparison with the charted information. ECHO-SOUNDING Principle of the Echo-sounder The echo-sounding depth recorder emits a pulse of sound energy from a transmitter, and the time this pulse takes to reach the sea bed and be reflected back to the vessel is directly related to the distance. Speed of Figure 2.7 Principle of the echo-sounder. Draught Rx Tx Distance to sea bed 27Speed and Depth Sensitivity control Depth indicator or recorder Illumination control Recording paper Amplifier (maybe built as an integral part of the recorder) Reflected sound energy Oscillation generator Transmitted sound vibration Reflected sound energy Range selector Receiving oscillator Transmitting oscillator Figure 2.8 Echo-sounder. Power junction box Watertight gland Rubber seal Tank side Laminated nickel plate pack Reflector plate Weld Sound energy Thin rust-proof steel plate Parabolic reflector Air Fresh water Figure 2.9 Echo-sounder’s transmitting oscillator (magnetostriction type). 28 Seamanship Techniques sound through water being the known value of 1500 metres per second (see Figures 2.7 to 2.9). However, that value will vary with water temperature and salt content (salinity). Let us work out an example: Let the velocity of sound in water = v metres per second. Let the time between transmission and reception of the pulse = t seconds. Let the distance to the sea bed and back = 2s metres. But the distance = speed × time ∴ 2s = v × t ∴ s = vt 2 metres Therefore, s represents the depth of water under the vessel. Possible Errors of Echo-sounding Equipment 1. Differences of the velocity of propagation. Owing to the differences of salinity and temperature encountered in various parts of the world, adjustment tables are available, published by the Admiralty. 2. Transmission line error. This is caused by the misalignment of the reference ‘zero’ on the scale. Reference ‘zero’ sets the timer of the recorder unit, and if it is not set at ‘zero’, then a false time and recording will be obtained. 3. Pythagorean error. This error is encountered with separated trans- ducers rather than with the combined transmit/receive unit. The error is caused by the measuring of the ‘slant distance’ as opposed to the vertical distance under the keel. 4. Aeration. The presence of air in the water will affect the speed at which sound travels through it, since the velocity of sound through air is much less than that in water (330 m/s compared with 1500 m/s). The main causes of aeration are: (a) Turbulence caused by having the rudder hard over. (b) Having a light ship which is pitching heavily. (c) Having sternway on the vessel. (d) Having broken water over shoals. (e) Entering an area where prevalent bad weather has left pockets of air bubbles over comparatively long periods. Possible cures for the above include stopping or reducing the vessel’s speed, and abrupt movement of the rudder either way, to sweep away formed bubbles. False Echoes False bottom echo This may occur if the echo-sounder is incorrectly set in such a manner that in deep water a returning echo is received after the stylus has completed one revolution. 29Speed and Depth Multiple echoes These are caused by the transmitted pulse being reflected several times between the sea bed and the water surface before its energy is dispersed. Such multiple reflection may cause multiple echoes to be recorded on the trace of the sounding machine. They can, however, be reduced in strength by decreasing the sensitivity control on the equipment. Double echo This type of echo is a double reflection of the transmitted pulse. It occurs when the energy is reflected from the sea bed and then reflected back from the surface of the water before being received by the transducer. A double echo is always weaker than the true echo, and can be expected to fade quickly with a reduction in the sensitivity of the equipment. Other causes Side echo may come from objects not directly under the keel of the vessel reflecting the sound energy, e.g. shoals of fish or concentrations of weed or kelp. There may be electrical faults or man-made noise in and around the hull. In addition, turbulence may be caused by the vessel herself, with or without interaction between the shore or other shipping. Deep scattering layer This is a level of several layers believed to consist of fish and plankton which will scatter and reflect sound energy. The layer has a tendency to move from as much as 450 m below the surface during the daylight hours to very near the surface at night. It becomes more noticeable during the day when the cloud cover is sparse than when sky is overcast. 3 MARINE INSTRUMENTS SEXTANT The sextants purpose is to measure angles, either vertical or horizontal to obtain the necessary data to check the vessels position. Latitude and longitude may be determined by a combination of sextant, chronometer and nautical almanac readings. This precision instrument is based on the principle, enunciated by the First Law of Light, that when a ray of light is reflected from a plane mirror, then ‘The angle of incidence of the ray equals the angle of reflection’. In the sextant a ray of light is reflected twice by two mirrors, the index and horizon mirrors, in the same plane. When a ray of light is reflected in this way by two plane mirrors, then the angle between the direction of the original ray and the direction of the final reflected ray is twice the angle between the mirrors (see Figures 3.1 and 3.2 and Plate 7a). Ray of light from observed object Index mirror Telescope (in collar) Observer’s eye Index arm Scale Clamp Micrometer Arc Shades Frame Figure 3.1 Sextant. Shades Horizon mirror 31Marine Instruments Principle of the Sextant The principle of the sextant is based on the fact that twice the angle between the mirrors HAI must equal the angle between the initial and final directions of a ray of light which has undergone two reflections. Proof Let α represent the angle between the mirrors. Let ∅ represent the angle between the initial and final directions of a ray of light. The required proof is: 2α = ∅ Construction Extend the ray of light from the object to intersect the reflected ray from the Horizon Mirror H at point L. Proof of theory (i) The angle between the mirrors α is equal to the angle between the normals to the mirrors. (ii) In triangle HIK β = α + X and 2β = 2α + 2X Ray of light from observed object Norm A α φ K L t t l α H Figure 3.2 Principle of the sextant. 7a. Marine sextant. 32 Seamanship Techniques (iii) In triangle HIL 2β = ∅ + 2X therefore from equation (ii) and (iii) 2α + 2X = ∅ + 2X and 2α = ∅ i.e. twice the angle between the mirrors is equal to the angle between the initial and final directions of a ray of light which has undergone two reflections in the same plane, by two plane mirrors. Errors of the Marine Sextant There are three main errors, which can quite easily be corrected by the mariner. A fourth error, for ‘collimation’, can also be corrected, with care and attention, but only to an older sextant where telescope collars are fitted with adjusting screws. The first error, of Perpendicularity, is caused by the index mirror not being perpendicular to the plane of the instrument. To check if this error is present, clamp the index arm between a third and half way along the arc, remove the telescope, and look obliquely into the index mirror, observing the true and reflected arcs of the sextant. Hold the sextant horizontal, arc away from the body. If the true and reflected arcs are not in line with each other, then an error of perpendicularity must be considered to exist (Figure 3.3). To correct the error, adjust the screw at the rear of the index mirror until the true and reflected arcs are brought together in line. The second error, side error, is caused by the horizon mirror not being perpendicular to the plane of the instrument. There are two ways of checking if this error is present. The first is by observing a star. Hold the sextant in the vertical position with the index arm set at zero, and observe a second magnitude star through the telescope. If the true and reflected stars are side by side, then side error must be considered to exist (Figure 3.5). It is often the case when checking the instrument for side error that the true and reflected stars are coincident. If this is the case, a small amount of side error may exist, but a minor adjustment of the micrometer should cause the true star to appear below the reflected image. Should, however, the reflected image move to one side rather than move in a vertical motion, side error may be considered to exist. The second way is by observing the horizon. Set the index arm at zero and hold the sextant just off the horizontal position. Look through the telescope at the true and reflected horizons. If they are misaligned, as indicated in Figure 3.6, then side error must be considered to exist. To correct for side error, adjust the centre screw furthest from the plane of the instrument at the back of the horizon mirror, to bring either the star and its image into coincidence or the true and reflected horizons into line. The third error, index error, is caused by the index mirror and the First adjustment screw Back of mirror Index glass Index arm Figure 3.3 Adjustment screw on index mirror. Second adjustment screw Clear glass Third adjustment screw (for index error) Frame of sextant Figure 3.4 Adjustment screws on horizon mirror, seen from behind. Figure 3.6 Indication of side error. Side error present True Reflected horizon Horizon No indication of side error Figure 3.5 Images of true and reflected stars, showing side error. [...]... index error is the difference between the two readings divided by two, and would be called ‘on the arc’ if the ‘on the arc’ reading were the greater of the two, and ‘off the arc’ if the ‘off the arc’ reading were the greater Let us consider an example: Adjust the micrometer to bring the true sun into contact with the reflected sun Note the reading, for example RS 0° 36′ off the arc TS Repeat the observation,... athwartships permanent magnets Heeling error magnets are placed in a ‘bucket’ arrangement on the centre-line of the binnacle directly under the central position of the compass bowl (see Figure 3.12) The effect of heeling error magnets can be increased or decreased by respective adjustment of the chain raising or lowering the bucket Binnacle in hardwood finish 42 Seamanship Techniques 9 Modern binnacle manufactured...Marine Instruments horizon mirror not being out of parallel to each other when the index arm is set at zero To check whether index error is present by observing a star, look through the telescope when the sextant is set at zero, and if the reflected image of the star is above or below the true image, then index error must be considered to exist Should the true and reflected images be coincident, then... no error will exist.To check by observing the horizon, set the index arm at zero, hold the sextant in the vertical position, and observe the line of the true and reflected horizons; if they are seen as one continuous line, then no error exists, but if the line between the true and reflected horizons is broken, an adjustment needs to be made to remove the error This adjustment is made by turning the... adjustment screws alternately, by a small amount each time, until concidence of image is achieved Figure 3.7 Dealing with combination of side and index error Collimation error This is an error caused by the axis of the telescope not being parallel to the plane of the instrument To check whether the error is present, insert the inverting telescope, setting the eyepiece so that one pair of the cross wires... used in conjunction with calibrated tables to obtain not only the relative humidity but also the dewpoint.These values are indicated by the difference between the wet and dry bulb temperatures The process of evaporation requires heat, and this heat is drawn from the wet bulb thermometer Evaporation of the distilled water in the reservoir and more directly from the wick takes place, leaving the wet... closer than one and a quarter times the length of the longest needle in the card Heeling error magnets Hard iron magnets 9 inches in length (22.86 cm) by 3 inch 8 (0 .93 cm) diameter, they compensate for heeling error due to field ‘R’ and vertical soft iron They also induce magnetism into the flinders bar and spheres, which helps the heeling error correction Horizontal magnets 3 These are 8 inches in length... screw nearest to the plane of the instrument Index error may also be checked by observing the sun Fit the shaded eye piece to the telescope Clamp the index arm at about 32′ off the arc and observe the true and reflected images to the position of limb upon limb Repeat the observation with index arm set at about 32′ on the arc, and note the two readings of both observations.The numerical value of the... owner The reflector unit was an acceptable advance within the industry, since it obviated the need and the cost of providing a steering compass The idea of achieving a through-deck repeater from the standard compass on the ‘Monkey island’ is based on the development of the submarine periscope.The unit is fitted under the forepart of the compass in order to reflect the lubber’s line and the foremost section... water at the mean level The sample of water should not be taken from the upper surface, as this will probably be polluted in some way and will also be at a different temperature to the water round the submerged hull – and temperature affects the density of fluids By the same reasoning the bucket should be cast clear of overboard discharges After that, float the hydrometer in the bucket of water once . transmitted pulse being reflected several times between the sea bed and the water surface before its energy is dispersed. Such multiple reflection may cause multiple echoes to be recorded on the trace. pulse. It occurs when the energy is reflected from the sea bed and then reflected back from the surface of the water before being received by the transducer. A double echo is always weaker than. echo-sounder is incorrectly set in such a manner that in deep water a returning echo is received after the stylus has completed one revolution. 29Speed and Depth Multiple echoes These are caused by the