log (fig. 4), a taffrail one, which, however, is not as a rule used for
speeds over 18 knots.
Fig. 4.—The Cherub Log.
Owing to the increased friction produced
by a rotator making approximately 900 revolutions per mile,
towed at the end of a line varying from 40 fathoms for a 12-knot
speed to 60 fathoms for 20 knots, the pull of the line and rotator
is borne by coned rollers, having their outlines tapering to a
Fig. 5.—Neptune Pattern for securing Rotator.
common point in their rotation, thus giving a broad rolling
surface. Strong worms and wheels are substituted for the light
clockwork. In fig. 4 the shoe H is secured to the taffrail, and the
rotator in the water is hooked to the eye of the spindle M by the
hook D. The case A contains the registering wheelwork and a
sounding bell. The half
gimbal B pivoting in the
socket of the base C
allows the register to
receive the strain in the
direct line. The bearings
and rollers are lubricated
with castor oil every twelve hours through holes in the sliding
case E, and can be examined by unscrewing the case E and
the eye M. When not in use, the register is removed from the
shoe by lifting a small screw button near C. The tow line is
usually plaited, and to avoid a knot close to the rotator, the
latter is secured to the former by a knot inside an egg-shaped
shell (fig. 5, Neptune pattern).
Fig. 6.—Walker’s Neptune Log.
Walker’s Neptune log (fig. 6) is used for vessels of high speed. Case A contains the wheelwork, and case E the spindle and steel ball bearings; in each case are openings, closed by sliding tubes, for examination and lubrication. In fig. 6 the cases A and E are shown open. Fig. 7 shows the dial plate. In fig. 8 the ball bearings are shown unscrewed from the body of the log, with eye, cap and spindle. They consist of two rows of balls rolling in two pairs of V races or grooves. The outer pair receive the strain of the rotator, and the inner are for adjustment and to prevent lateral movement. The balls and races are enclosed in a skeleton cage (fig. 9) unscrewing from the cap F (fig. 6) for cleaning or renewal; the adjustment of the bearings is made by screwing up the cage cap b, locked by a special washer and the two screws a, a (figs. 8, 9). If the outer races become worn, the complete cage and bearings are reversed; the strain of the line is then transferred to what had previously been the inner with practically unworn balls and races. It is for this purpose that the skeleton cage is screwed internally at both ends, fitting a screwed ring inside the cap F (fig. 6). To enable the indications of the log register on the taffrail to be recorded in the chart room or any other part of the vessel as desired, a chart room electric register has been introduced. By means of an electric installation between the log register aft and the electric register in the chart room, every tenth of a mile indicated by the former is recorded by the latter.
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| Fig. 7.—Dial-plate of Neptune Log. | Fig. 8.—Ball Bearings of Neptune Log. |
Walker’s Rocket log (fig. 10) is a taffrail one, with bearings of hardened steel, and is intended to be slung or secured to the taffrail by a line; the gimbal pattern has a fitting for the deck. In taffrail logs, the movement of the line owing to its length becomes spasmodic and jerky, increasing the vibration and friction; to obviate this a governor or fly-wheel is introduced, the hook of the tow line K (fig. 11) and the eye of the register M being attached to the governor. Fig. 11 represents the arrangement fitted to the Neptune log; with the Cherub log, a small piece of line is introduced between the governor and the eye of the register. The two principal American taffrail logs are the Negus and Bliss (Messrs Norie and Wilson). The former bears a general resemblance to the Cherub log, but the dial plate is horizontal and the faces turn upwards. The main shaft bearings are in two sets and composed of steel balls running in steel cones and cups; the governor is an iron rod about 16 in. long, with 1 in. balls at the extremities. The Bliss resembles the Rocket log in shape, and is secured to the taffrail by a rope or sling. A governor is not employed. The blades of the rotator are adjustable, being fitted into its tube or body by slits and holes and then soldered. The outer ends of the blades are slit (fig. 12) to form two tongues, and with the wrench (fig. 12) the angle of the pitch can be altered.
All patent logs have errors, the amounts of which should be ascertained by shore observations when passing a well surveyed coast in tideless waters on a calm day. Constant use, increased friction (more especially at high speeds), and damage to the rotator will alter an ascertained log error; head or following seas, strong winds, currents and tidal streams also affect the correctness.
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| Fig. 9.—Ball Bearings of Neptune Log in Skeleton Case. | Fig. 10.—Rocket Log. |
Fig. 11.—Neptune Log fitted with Governor.
A Log Book is a marine or sea journal, containing, in the British navy, the speed, course, leeway, direction and force of the wind, state of the weather, and barometric and thermometric observations. Under the heading “Remarks” are noted (for vessels with sail power) making, shortening and trimming sails; and (for all ships) employment of crew, times of passing prominent landmarks, altering of course, and any subject of interest and importance. The deck log book, kept by the officers of the watch, is copied into the ship’s log book by the navigating
