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Nelson and His Navy - Navigation
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When sailing on long voyages or between continents only the general direction can be determined. Sailing ships by their nature are reliant upon the wind for power and this cannot be relied upon to always blow in the direction required. So how where ships navigated? The main system used in the Age of Sail was one where you deduced where you were from knowledge of where you have been. For example consider a journey commencing at Hartlepool which sails due east at 10 miles per hour for ten hours. At the end of the period we can deduce that we are now 100miles due east of Hartlepool. This system is know as deduced reckoning or 'dead reckoning' for (mis-spelled) short.
Every half-hour or 'bell' of the ships watch system details are entered in the ships records of speed, sailing course, wind direction and strength. Course or where the ships bow is pointed is determined from the mariners compass that is marked in 32 'points of sailing' not degrees. Each point has a specific name (e.g. East North east, East by North, South south west) and sailors learned to recite (or box) each point in any direction starting from any point. The wind direction and strength are necessary in order to determine the actual direction the ship moves in. Only when the wind is directly on the stern of the ship will she sail exactly where the bow points. If the wind is coming from any other direction there will be a slight sideways movement of the ship as well called 'leeway'. The more the ship sails towards the wind the more leeway. A square rig sailing ship like HMS Trincomalee can sail (or bear up) 2 'points' or 22.5 degrees into the wind.
Speed
of the ship is measured using the 'log' system. This derives its name from
the old practice of casting a log or piece of firewood off the bow of the
ship. As the ship moved forwards the navigator would follow the log to the
stern as the ship passed it by. The ships speed could be determined from
the pace he had to follow the log. By the beginning of the nineteenth century
and more accurate system had been derived. This used a 'log ship', a triangular
weighted piece of wood to which was attached a long rope. The log was cast
off the stern of the ship and the line allowed to pull out as the log floated
stationary in the water. The rope had a series of marks or knots every 48
feet. When the first mark was reached a small sand timer was turned which
ran for 28 seconds. The number of marks in the rope that passed in this
period was then counted. If exactly one mark passed then the ship was doing
one nautical mile per hour or one knot! Knots are still used today for ship
and aircraft speed.Using a set of 'traverse tables' for each half-hour the ships net change in latitude and longitude could be calculated. These were then summarised and entered into the ships main record book to determine the net change over the day. The record book of course contains information about the ships speed and is therefore known as a 'log book'.
The
vagaries of wind, sea and course did give many errors into the calculations
over a long voyage. It was therefore necessary to correct the dead reckoning
measures as often as possible by direct observation of latitude and longitude.
This usually took place at noon. The nearer you are to the equator the higher
the sun is at noon. By observing the height of the sun at noon, correcting
the observation for effects of season, observation point, and the earth's
atmosphere the navigator could determine his latitude to an accuracy of
5 miles. To do this he used an accurate angle measuring device know as a
sextant. By comparing the local noon time with that on an accurate time
piece (chronometer) set to noon at a known point (e.g. Greenwich) the navigator
could also determine his longitude to a similar accuracy. The navigator
could also observe the position of the moon relevant to the other stars
and by use of reference tables also determine time at a known point.The combination of these methods coupled with years of experience allowed the navigators of Nelsons Navy to sail the seven seas to a degree of accuracy only now bettered by sophisticated satellite based systems - until the battery goes flat!
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