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Oak and triple bronze must have girded the breast of him who first committed his frail bark to the angry sea...
Horace
avigation was an essential skill for every officer in the navy, from midshipman right
through to admiral. More than anything else, this is what separated the officers
from the seamen; navigation required literacy and a high degree of numeracy.
Navigation was the responsibility of the master (essentially the technical head
of the ship, as opposed to the captain, the military head) who was required to
provide all his charts and navigation equipment. To sail anywhere the master had
first to know the exact position of the vessel and out of sight of land this was
calculated by the noon sighting. Latitude was fixed using the sextant. Longitude
was determined by comparing time differences between local noon and Greenwich
using a chronometer. Chronometers were expensive and not supplied by the
Admiralty except to ships on foreign voyages at the captain's request so they
were often purchased by the officers themselves. Every glass (30 minutes) the
ship's course was marked by a peg in the traverse board and at the end of the
watch the course run was worked out and recorded.
Lieutenant Commander John Blake served twelve years as a sea-going officer in the Royal Navy. He writes and lectures on maritime history and is an enthusiastic small boat sailor. This excerpt from “The Sea Chart” published by Conway Maritime is reproduced with their kind permission.
eafaring,until the advent of the meticulous charts of the twentieth century, was an exceptionally dangerous profession. Although warships were usually safer than merchants ships, because they had more men, were bigger and better equipped, most vessels were square rigged and so could only sail up to six points off the wind. With high freeboard they were prone to drift to leeward, and if experiencing a strong current or tide were unable to make any ground to windward at all. Typically the route a sailing ship would sail was determined by the prevailing winds and currents.
The origins of the sea chart
What is a sea chart? Its essence lies in its informational orientation - it differs from a land map in that its accent is on the presentation of maritime information, such as depths, coastline and headlands, tides and the nature of the sea bed for the mariner looking from the sea towards the land. It enables the mariner to set his vessel in the direction he needs to achieve his destination, with a compass course, and then make adjustments to the progressive course when his current position, which will be changed by wind and tide, has been checked.
The chart has to reflect varying information in a fluid situation, whereas a land map is the opposite - it records fixed positional information and the only variable is to record progress across it. Once this fundamental difference is appreciated the need to record information to guide progress and to update position is understood.
Charts are needed for three different environmental situations: crossing oceans or large tracts of sea out of sight of land, sailing along or approaching the coast, and entering or leaving harbour, anchorage or roadstead.
From the European perspective the oldest known map, a moulded clay tablet depicting Babylon at the centre, the world as a flat disk and a huge river referred to by Homer as the Okeanus, dates from the seventh or sixth century BC.
By the fourth century BC Pythagoras and Aristotle demonstrated the world as a globe, and Eratosthenes, who looked after the great library at Alexandria, demonstrated its circumference.
The earliest known form of chart is the portolan by the Venetians and Genoese. The word derives from portolano, an Italian pilot book with sailing directions, notes of headlands and navigational hazards. The earliest known surviving portolan dates to around 1290. Portolans were usually constructed on vellum made from calfskin or on parchment from sheep or goatskins. These had to be soaked in alum and lime to remove the hair, and the damp skin was then stretched on a frame and scraped until it was the requisite thickness, sometimes adding chalk to whiten it. Then they were cut into the shape that would take the charted land outline. Printed portolans started to supersede the handrawn from 1569.
Early charts filled unknown spaces with elaborate but beautiful artwork, even giving credence to fantastic sea monsters and non-existent islands. The necessity for, and availability of, more and more information on a chart, and the need for standardisation, both as to production and interpretation, led to the chart becoming honed to a somewhat more severe style and serious execution. By the 1800s, the fantastic was out, and the straightforward was in.
Navigational tools
Until the advent of the compass, the seaman was reliant on local knowledge and generally kept to the coast. The commonly accepted birthplace of navigation is the Mediterranean. With the system of dead reckoning by estimating the vessel's speed and course through the water an estimation of distance travelled could be assessed. If this was then marked on to a vellum portolan, the navigator could estimate where he had got to, and adjust his course to steer in order to reach his final destination. This was prone to error as any allowance made for drift could only be estimated.
The lead, with a small hollow at its base filled with tallow, and line marked off with fathoms would give the master a good indication of his whereabouts if he knew the nature of the sea bed, and increasing or decreasing depths would give an idea of the profile of the sea bottom.
One of the first tide tables was put together in about 1548 by Guillaume Brouscon. They were very popular - an comparison with The Admiralty Tide Tables of today gives a time of High Water at Dover just 13 minutes different.
Astronomical navigation
The understanding of the way the Earth, Sun, planets and stars interacted was essential to the development of a system of navigating out of sight of land. An important guide at night was the Pole Star - often known by sailors as Stella Maris or star of the sea.
Longitude
Perhaps the biggest breakthrough in navigation was the invention of a time-piece by a Yorkshire carpenter, John Harrison, which could be used to find out longitude.
Out of sight of land, across vast oceans, the means of pinpointing position was haphazard and many ships were lost with their men and valuable cargo.
The longitude problem became so acute that a Board of Longitude was set up in 1714, with a prize of £20,000 offered to anyone who could devise a satisfactory method of finding longitude at sea.
Finding longitude involves the comparison of the local time at two places and converting them to distance. If the time the sun passed directly overhead (its meridian) at say Greenwich could be carried accurately for long distances at sea, and the local time of the sun passing overhead could be noted, then the two times could be compared.
Many methods of finding longitude were put forward, some completely impractical, others were simply crazy.
The key was to design a timepiece that could keep the time at Greenwich accurately at sea. Harrison succeeded with the last of a series of clocks that ran to time, needed no lubrication or cleaning, was impervious to rust, and with moving parts that were perfectly balanced in relation to one another, regardless of ship movements, changes in magnetic field, humidity, temperature or barometric pressure. It was called H4. Alexander Dalrymple coined the term “chronometer” for Harrison’s invention.
The UK Hydrographic Office
Dalrymple was appointed as the first Royal Navy Hydrographer in 1795. He contributed many of his own charts that he had published during his time in the Far East but so huge was his task that he didn't get any charts organized and out into the navy during his tenure. Thomas Hurd succeeded Dalrymple in 1808. After the British victory at Waterloo in 1815 there was an ambitious plan to fill in all the gaps in a world sea survey. The Royal Navy Surveying Service was set up in 1817.
Find out more...
Sea Charts of the British Isles by John Blake, Conway Maritime
Charts of War by John Blake, Conway Maritime
John Harrison and his chronometers
Trinity House a safeguard for all mariners