NAUTICAL KNOW-HOW
	NAVIGATION IN TIME OF COLUMBUS Columbus was arguably the world's first true ocean navigator, being the first navigator in the Atlantic to sail westwards using a compass. In the two decades before he set sail all voyages had been made southwards. A chief navigator's concern had been to find latitude by measuring the height of the Pole star above the horizon with a wooden instrument called a quadrant. In order to sail west, Columbus would have needed to accurately find his longitude and to do this, a navigator needs to know the exact time. The most accurate clock available at the time was a sandglass, turned every half hour by a ship's boy. The best Columbus' men could do was to continuously estimate their ship's speeds and keep a running total of the miles they had travelled. It did not take into account the effects of the currents. TRADITIONS It is a meticulous ruling on all ships that the man at the wheel repeats each and every helm order to confirm that he has heard it correctly, speaking loudly and clearly, as well as verifying the course as soon as he has brought the ship to it. Aside from being a nice, time-honoured touch, the farewell blowing of a ship's siren is a great form of promotion. The Master himself presses the button to sound the air whistle. The tree blasts of about five seconds boom out across the water. In response, a tug replies with tree similar blasts, which sound quite squeaky in comparison to the big ship. At this point, some Masters may give a further, short blast, to which the tug will reply in kind. Many Masters of today have dropped this custom altogether, which is a pity for the passengers and their well-wishers ashore who may regard it as the culminating moment of excitement of a departing cruise ship. STEERING RULES To avoid the risk of collision at sea, all mariners abide by the "International Rules for Prevention of Collision at Sea". These rules set out precisely who must give way to whom in every conceivable situation. For example: If two ships are crossing and risk colliding, the ship which has the other on her own starboard bow must keep out of the way of the other vessel. (Just like entering a roundabout!) An overtaking ship must keep clear of the vessel being overtaken. When two ships are approaching each other on reciprocal courses risk a head-on collision, each must alter course to starboard so that they pass on the port side on the other. WHAT IS A KNOT ? A knot is not the same thing as miles per hour, which is the land measurement of speed. Ships sail many miles of ocean and are in effect sailing across a curved surface, so a linear measurement, such as a statute mile, is of no use in measuring distance travelled and speed. That's why the nautical mile was introduced. It is a curved measurement and is defined as "the length of an arc of meridian that subtends an angle of one minute at the centre of curvature of the place". In other words, one minute of latitude, measured at your latitude on the Earth's surface, equals one Nautical mile. One Nautical Mile has an average length, on the Earth's surface, of 6080 feet (it does in fact vary in length from the Equator to the Pole due to the Earth having a polar compression), while a land, or statute mile has a fixed length of 5280 feet (1760 yards). See also: 'Conversion Tables' ENTERING AND LEAVING PORT The area between the port and the open sea often contains hazards and currents that a visiting Captain could not be expected to know. Special shore-based mariners called pilots are hired for their local knowledge to guide ships through this area. They are licensed by the local port authority, and employed by the visiting ships. Once they have navigated out of the port, they have to be "dropped"; in poor weather, this is a real exercise in ship handling and seamanship. The ship is stopped with the wind at one bow. This creates a lee (patch of smooth weather) on the other side of the ship.
	The pilot's boat can then safely come alongside the foot of the ladder by which the pilot disembarks. In very bad weather it may be impossible to create a good enough lee for the pilot to disembark in which case the poor pilot finds himself sailing on to the next port without any luggage!
	MARKS AND LIGHTS Marks and lights are the guideposts of the sea, helping the Captain navigate unfamiliar coasts and ports. They consist of lighthouses, buoys and other local structures. At night, the most important ones are lit, and each light has individual characteristics of range, colour, duration, type and number of flashes. Visiting ships pay 'light dues' to provide a fund of money for the maintenance of lights, which are looked after by local authorities. With the aid of charts, the navigator can identify which light he is looking at. A chart may describe a lighthouse as: GpFl(3) 15sec 24M. This cryptic message means that the light gives a group of three flashes, every 15 seconds, and can be seen in clear weather for 24 miles. By day, mariners can still take visual bearings from lighthouses. By the way, there are rarely more than 60m high, because their light could be lost in clouds in poor weather. Other navigational aids include buoys of two kinds, lateral marks, which mark each side of the channel, and cardinal marks, which relate to compass direction and indicate hazards. Port-side buoys are red, can-shaped and are topped by a flashing red light; starboard buoys are green, cone-shaped, and bear a flashing green light.  See also: 'Buoyage' BUOYS Buoys act as directional markers for ships' navigators. There are two types of buoys: Lateral marks: These mark each side of a channel and are used when approaching harbours or any narrow strait. There are port and starboard-hand buoys (the hand is determined by the direction of the incoming tide). Port-hand buoys are red, can-shaped and are topped by a flashing red light. Cardinal marks are yellow and black, and are placed according to the compass direction of a danger. Each has a different shaped top-mark and light signal. The light signals all consist of very quick flashing white light, but if you are to the south of a danger, the light signal is six flashes and one long flash, to the north a continuous quick flashing light, to the east three very quick flashes followed by a break, and to the west nine very quick flashes followed by a break. SHIPS' LIGHTS Ships carry lights not as an aid to vision, but to allow them to be noticed by other mariners at night. Steering rules are so precise that it is vital to know what other ships sailing in the vicinity are doing and which direction they are heading in. Ships' lights are therefore arranged at specific angles and are displayed to give the experienced navigator all the information he needs. A ship is required by law to carry five lights: a red light on the port side, and a green light on the starboard side, then identical white lights on the foremast and aftermast (the foremast light is lower in height), the fifth light is a white light on the stern.
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