Chapter - 18

Simple Piloting

Piluting is the science of navigation by physical landmarks—in other words, of making use of recognizable features of the earth's surface and aids to navigation to identify position at any instant.

With clear weather and the coast in sight and identified from a chart, the following methods, given in order of frequency of use, will suffice to fix a vessel's position accurately.

I. Cross Bearings of Two or More Objects II. Bearing and Distance of One Object

a. By bearing and known distance from the object.
b. By two bearings and known distance run between.
c. By bearing and "circle of visibility."

III. By Sextant Angles Between Three Identified Objects IV. By Charted Ranges

V. By Danger Angles and Danger Bearings In thick weather and when no identifiable objects are in sight, the methods of ascertaining position near land are:

A. By Radio Compass Bearings

1. Observed from ship on radio beacons.
2. Observed at stations ashore and communicated to the ship.

B. By Sound Signals

1. Through the air by timing echoes, or sound signals.
2. Through the water by timing sound signals.

C. By Synchronous Sound and Radio Beacon Signals D. By Lines of Soundings

E. By Dead Reckoning, discussed later in this chapter. Actually, under the conditions discussed above, dead reckoning assumes its most important place. In order to know the approximate position during fog or other low visibility conditions, the DR record must be kept in good weather, no matter how useless it might seem to be at the time, in order to know the correct DR position when bad weather sets in.

Laying A Course

After one has become familiar with the different instruments used for piloting, including the dividers, parallel rulers, and course protractors, and if possible the chip log, patent log and the lead, he should provide himself with the best chart of the locality to be traversed, together with the sailing directions and descriptions of the aids to navigation, all of which have been brought up to date as explained, will be to lay his course. This is done by marking one point upon the chart at the boat's position, and another point for which it is desired to steer. A line is then drawn connecting the two points, which will indicate the course to be steered by the boat. The motor boatman should examine carefully along this line on the chart to be sure that it clears all dangers.

After having laid one's course and determined its magnetic (or true) direction by using the course protractor (see Chapter XVII) and then properly applying the deviation of the compass for the particular course to be steered (See Chapter X), the pilot must make frequent checks to determine whether his ship is actually following the course which he believes she is. Winds and currents, compass errors not known or properly applied, poor steering and for many other reasons, the ship may be off her course. Therefore, it is most essential that as many checks on one's position should be taken as possible. Generally speaking, it is better to lay a number of shorter courses which will make it possible to make use of buoys and other fixed aids to navigation at frequent intervals than to lay a long course where frequent "fixes" are not possible. In this way, a frequent check can be kept on one's position and the proper steps taken to correct any errors.

The more common methods of locating one's position or determining a "fix," as it is called, are listed below:

Cross Bearings Of Two Or More Objects

When two or more sharply defined objects can be located on a chart, bearings of these objects can be taken by the pelorus, and these bearings plotted on the chart will give a fix. If three or more objects can be sighted at once, the center of the triangle or "cocked hat" where the various lines of bearing intersect is the most likely position of the ship.

Bearing And Distance Of One Object

If there is only one object available, a single bearing merely gives a line along which the ship must be; but if the distance to the object at the same time can be ascertained by rangefinder or sextant the exact position can be found.

Two Bearings And A Run Between

A more common use of bearings of a single object is to observe one bearing, run a known distance on a straight course, and again observe the bearing. The two bearings are then plotted on the chart, and this point on the second bearing line is found where the distance to the first bearing line, measured parallel to the course, is equal to the distance run. This point is the fix at the time of the second bearing. For simplicity, the method of "doubling the angle on the bow" is most frequently used. If the second bearing is twice the relative angle from the bow that the first bearing was, then the distance from the object at the time of the second bearing is equal to the run between bearings. The familiar "bow and beam" case, when the first bearing is taken broad on the bow (45°) and the second when abeam (90°), gives the distance the object is passed abeam, which is equal to the distance run between the two bearings. Another modification consists of making the first bearing 262 ° on the bow and the second at 45° on the bow, when the object will be passed abeam a distance equal to the run between the two bearings. The same combination, only measuring from the stern around to the beam, after passing an object abeam will give the same data as to distance from the object.

Fig. 3. Circle Of Visibility

At night a rough approximation of position may be made by noting the bearing when a light first comes into view over the horizon.

Fig. 4 Sextant Angles Between Three Known Objects

This gives the most accurate method of securing a fix within visual distance of the coast. For best conditions, neither angle should be less than 30° nor more than 150°. "Revolvers," which will not give a definite fix, occur when the sum of the two observed angles plus the angle formed by lines from the central object to the other two equals 180°, in which case there is a whole circle of possible fixes. To avoid such cases, the central object should, if practicable, be nearer the ship than either of the two objects.

Charted Ranges

Much used or narrow channels may be marked by "ranges" which guide ships in safe water along the channel. Often, owing to wind or current, the vessel is unable to steer the true course which the range line indicates and is forced to "crab" off the true course in order that her track will follow along the range line; but, regardless of what course she steers, as long as the ranges remain closed (in line) she will be in the safe channel.

Fig. 7

Danger Angle And Danger Bearing

As long as the angle subtended by the distant object at the ship is kept less than the angle subtended at the hidden danger (see Fig. 7), the ship must be farther away than the danger and hence pass clear.

Danger bearings are still valuable when for any reason it is impossible to get a sure fix. Fig. 8 is an example of such use of bearings.

Piloting In Thick Weather Or Out Of Sight Of Land

In fog, mist, rain, or falling snow, most lighthouses or lightships and many buoys sound at regular intervals easily identified sound signals, such as air or steam whistles, diaphones, bells, explosive reports, or the latest development, highly magnified, electrically reproduced audible signals. These various signals give only general warning of the nearness or proximity to danger and also approximate bearings, but neither bearing by ear nor distance by intensity or clarity of signal can be depended upon, as fog often causes acoustics to play strange tricks.

Fig. 8

Sound Signals Under Water. Many lightships and some buoys send out underwater sound signals which may usually be heard to greater distances and more dependably than air-carried signals. At best, they are merely warning signals, supplementing the navigator's estimate of his position but not absolutely confirming it.

Soundings. As a necessary proof of the reliability of other aids to navigation when they are available, the cautious navigator, when on soundings in a fog or reduced visibility and at all times when necessary, should always take steps to ascertain to the best of his ability the depth of water and character of the bottom beneath his vessel.

Physical Aids To Piloting

Lighthouses. All civilized countries provide lighthouses or lightships at intervals along their coasts to warn ships of the coasts and outlying rocks or shoals. The "Light Lists" as published by the United States Coast Guard and the U. S. Hydrographic Office give the characteristics of all lighthouses and lightships of this and foreign countries.

Buoys. Floating but anchored navigational aids except lightships are known as buoys. (See Chapter VI.)

Tides and Tidal Currents. The word tide in its strict sense refers only to the change of elevation of the water; tidal currents refer to the horizontal flow of the water.

Tide. The tide rises until it reaches a maximum height called "high water" or "high tide" and then falls to a minimum level called "low water" or "low tide"; that period at high or low water marking the transition between the tides, during which no vertical change can be detected, is called "stand."

Tidal Currents. Tidal currents which flow in a direction, generally speaking, from the sea toward land are called "flood tides" and those moving in the opposite direction are called "ebb tides"; the intermediate period between flood and ebb and between ebb and flood, during which there is no horizontal motion, is known as "slack water."

Set and Drift. Set and drift are terms used to describe the direction (set) and velocity (drift) of any current, tidal or otherwise.

Range. The range of the tide is the difference in height between low water and high water.

Dead Reckoning

The method of Navigation by Dead Reckoning in the broad sense is the method of determining a position by keeping a careful account or reckoning of the course and distance made good from a previous known position called the point of departure, making due allowance for current when known.
The course made good varies from the compass course of ship's heading by:

1. Compass error.
2. Poor steering.
3. Influence of wind.
4. Influence of current.

The distance made good differs from the product of the speed times the interval of time, according to:

1. Wind.
2. Current.
3. Bottom fouling.
4. Inaccuracies of various speed indicators.

Regardless of the recognized defects of any method of dead reckoning, it is an invaluable aid to the mariner and is used whether or not other methods are in process. It enables the navigator to obtain the following necessary data:

1. The ship's approximate position at any desired time.

2. The approximate latitude and longitude at the time of any celestial observation, which is a great convenience in working lines of position.

3. The set and drift of the current between any two successive fixes (i.e., the discrepancy between the D.R. position and the second fix).

4. During days when celestial or terrestrial observations cannot be taken—that is, in foggy or cloudy weather with visibility poor or nil—dead reckoning affords the only handy method of determining positions, and as such conditions may continue for several days at a time, this is a most indispensable method. Note, however, that the increasing use of radio bearings, due to the vastly greater distances over which they are available at present, is supplementing D.R. and soundings as the navigator's last resort. (See Chapter XXI.)

Dead Reckoning artistry consists in taking advantage of all the aids to navigation that are available and in calculating the effect that the various factors listed above have on the actual track of a ship.

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