The courses mentioned in books about Titanic do not always make sense to modern readers. Those used to handling pleasure craft will know that the modern compass dial is divided into 360°, with North at 0°, East at 90° and so on. Many also know that in the days of Cook and Nelson the dial was marked with compass points and fractions thereof. In 1912 British ships used a sort of halfway house. The card was divided into quadrants and each was graduated in degrees. North and South were both marked 0° and East and West were both marked 90°. Headings were specified in terms of their relationship to North or South. For instance, northeast was N 45° E. Southwest was S 45° W. Titanic's final course of S 86° W was roughly west, and corresponds to 266° True in the modern notation. Click here for a large image of a compass of 1912.
This old notation was dropped by the United States Navy in 1909, but lingered on elsewhere until around World War II, when it faded away, due to increasing use of the gyro compass, which had used the 360° notation from the start. The newer method was also easier to teach to the thousands of new recruits joining up.
Another thing that has confused some is that some witnesses gave evidence on Titanic's final course in terms of the compass course steered. Quartermaster Hichens remembered steering N 71° W (289° Compass), but this was the course after allowing for variation and deviation. The real course was S 86° W or 266° True. Most of the difference was due to the variation, which in the Western Atlantic was and is quite large. Hichens had the compass course written on a board convenient to the wheel. He would not have known the true course, which was no use to him.
When in sight of land, Titanic was navigated by various methods, most of which will be familiar to modern yachtsmen, at least to those who are not helpless without GPS.
Much was done by taking compass bearings of landmarks and plotting the results on the chart covering the coastline. Without going into detail, these included such familiar methods as cross bearings, transit bearings and running fixes using a single landmark. These are still found in any basic textbook. The main difference between yacht navigation and navigating Titanic was that on Titanic bearings had to be taken from compasses mounted on stands placed in convenient spots around the upper decks. A hand bearing compass would not work on a steel ship of this size. Each compass was completely independent and had to be individually corrected and its deviation card made out. (Repeater compasses can only be had with a gyro compass or a fluxgate, neither of which were in regular use in 1912). Titanic had a special compass platform on the boat deck. This was made of brass in an attempt to reduce deviation, but the compass mounted there would have been less than perfect.
As well as the compasses, sextants were used inshore to measure the angular height of shore objects of known height, such as lighthouses. Combined with a bearing, this would give a fix. The sextant was also used to measure the angle between landmarks by holding it horizontally.
Soundings were taken by a patent sounding machine, which was able to drop and retrieve a weighted wire much faster than the old hand line. This machine entered the story of the disaster when the piece of timber which held its line clear of the hull had to be chopped away to allow a lifeboat to descend. One hopes this design flaw was fixed on Olympic and Britannic.
High tech was represented by a device called Submarine Signalling. By 1912 the British and US authorities had established a network of over 120 underwater bells near their respective coasts. Many were associated with lightships or lighthouses. Each bell had its own pattern of sounds. These were picked up by two microphones mounted inside the forward part of the hull, one on each side. Each was in a small, sealed, waterfilled container fixed to the inside of the hull. A device on the bridge had two telephone earpieces and through these the sounds from the bells could be heard and identified. The idea was that some idea of the bearing of each bell could be gained from its relative loudness. If necessary, the ship's head was turned until equal volume was obtained in both earpieces and the bearing was read from the steering compass. The range of the device was about 10 to 15 miles. In those days before radar, all kinds of things were tried to overcome the dangers of darkness and fog. This device was an update on the wondrous assembly of coastal buoys which groaned, grunted, whistled and rang bells, powered by wave action, compressed air or gas.
Once clear of land, navigation on Titanic was by a combination of dead reckoning and celestial navigation. By 1912, celestial navigation was highly developed and accurate. Dead reckoning still was far from perfect. No radio aids were available, not even time checks, although the USN was using these on a small scale as early as 1905.
The main tools of trade were the sextant and the chronometer. Sextants of the time were well made, though the mirrors were small by modern standards and the reading was taken from a vernier rather than a micrometer. This required good light and keen sight. Good sextants had a built in magnifying glass. Any errors in the sextant were known and allowed for. Chronometers did not normally keep perfect time, but gained or lost at a known rate and this was allowed for when recording times. Titanic was well provided with sextants and chronometers and all her officers were expert in their use.
Data on the positions of celestial bodies was published in the Nautical Almanac, based on highly accurate observations made at Greenwich and other observatories. Calculations were done with the aid of books like Norie's Nautical Tables and involved a knowledge of spherical trigonometry and much wrestling with logarithms.
It must be understood that a ship's position cannot be determined from a single observation of a single heavenly body. What is obtained is a position line, which is a line on which the ship must lie. For this reason the ship's position was never precisely known during daylight, when only the sun is available. (The sun and moon are seldom used to obtain two position lines during daylight).
The only real fixes were obtained in the morning and evening by observing three or more stars and/or planets in quick succession. This was done when it was dark enough to see the stars, but light enough to see the horizon. The moon was usually avoided, as it involved extra corrections. A position line was obtained from each body and on a ship as fast as Titanic corrections were applied to allow for the ship's movement between sights. The ship's position was at the intersection of the three best lines or, more likely, in the small triangle formed by their near intersection. The observations were taken by at least two officers working as a team. The calculation of each position line took around 15 minutes and required great care in looking up numerous figures from books of astronomical data and mathematical functions. The process was self checking, as an error in the calculations for one star would produce a position line wildly different from the others. The calculation and plotting of the full fix took at least an hour and on ships less professionally run than Titanic there was a constant temptation to skip this precise but tedious work. (Modern cruising yachtsmen usually try to avoid star sights when on the open ocean). In good conditions the fix was accurate to within a mile or so, although the calculations were often done to within a tenth of a mile.
During the night the ship's position was estimated by dead reckoning, based on her speed and the course steered since the evening fix. Dead reckoning depended on accurate steering by a properly corrected compass and speed was estimated by propeller revolutions, based on trials over a known distance. Trailing logs of the type made by Walker were in use, but their accuracy was not much trusted at the speeds reached by fast liners. Winds, currents and fouling of the ship's bottom could lead to major errors. Fourth Officer Boxhall had to resort to dead reckoning when Titanic hit the iceberg and an SOS position was needed.
Daylight navigation began with a sun sight fairly early in the morning, when the sun had risen sufficiently to be clear of serious refraction problems. The ship's longitude was then determined using an old method known as Longitude by Chronometer. Longitude was found to within about a mile.
When the sun neared its zenith at local midday, the sextant was used to measure its maximum altitude and a simple calculation gave the latitude, sometimes to within under a mile. Dead reckoning was then used to carry forward the morning longitude sight and cross it with the noon latitude. This gave the traditional noon position and the day's run was calculated. Although much mystique surrounded the noon position, it was not all that accurate, as it depended on dead reckoning for the longitude. It was really a tradition from the days before chronometers, when the noon latitude was the only reasonably accurate sight possible. The daylight work might conclude with another Longitude by Chronometer in the afternoon. Sometimes extra sights were taken at times when known dangers were near or when a landfall was expected. In cloudy weather, sights were taken whenever possible. Any position line is better than none and a lucky sight in the morning might be carried forwards to an equally lucky one later in the day, to get a reasonable position.
A special sight, unique to the Northern Hemisphere, is the Latitude by Polaris. The Pole Star (Polaris) lies very close to the North Celestial Pole. If it were exactly at the pole its observed altitude would equal the ship's latitude. In practice, a small correction must be applied, based on the time of the observation. The main problem with this simple sight is that Polaris is not particularly bright and a very clear and sharp horizon is needed. The sight comes into the Titanic story because Captain Lord claimed to have established the latitude of Californian by this method.
When working out the ship's position, the work was not done directly on the chart. The scale of an ocean chart is so small that a pencil line can be over a mile wide and accurate plotting is not possible. Position lines found by celestial navigation were drawn on a specially prepared plotting chart which was, in effect, a Mercator chart of the area around the ship's position. When working out a new position by using the course and distance from an established point, the navigator used precomputed Traverse Tables, which gave him the new Latitude and Longitude without any plotting. The new position, however obtained, was finally marked on the ocean chart, to give a general idea of progress.
Celestial navigation was also used to check the ship's compasses. It is not unknown for a ship's magnetic field to change during a passage, especially if she stays on the one heading for many days.
When reading accounts of voyages, it should be remembered that the distance run in a day is a pretty meaningless figure in itself. This is because the time between noon positions was not 24 hours, but varied according to the ship's change in longitude during the "day". At the US Senate enquiry, Third Officer Pitman gave evidence that Titanic's last full day lasted 24.7 hours, or 24 hours and 42 minutes. Her average speed was thus 22.1 knots. It is not clear where Pitman found his information. It may have been from memory, or perhaps a private diary. Olympic, heading east, was experiencing days of under 24 hours. It might be mentioned that in the days of sail, some captains were not above calling minutes of longitude nautical miles, thus inflating the day's run enormously.
It will be seen that there was always a measure of inaccuracy in the ship's position. Even the morning and evening star sights only gave the position at the time they were taken, not at the time the calculations were completed. Good seamen were aware of the limitations of their methods and allowed for them. Joshua Slocum observed that the wreck of Atlantic was due to overconfidence in navigation. "The captain knew too well where he was".