Tuesday, 30 May 2017
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Nautical ChartsNAUTICAL CHARTS have been in use since the end of the 13th century, the invention of the magnetic compass in the 12th century seems to have been the catalyst for the development of charts. It is only in the last 150 or so years that they have become accurate and generally available.

Medieval charts were just artistic impressions of what people thought a coastline was like, often with colourful pictures suggesting the presence of dragons and other awful sea monsters lurking just over the distant horizon.

Planning a passage using a Mercator chart in the Caribbean on an Oyster 56When Portuguese navigators under the leadership of Prince Henry ‘The Navigator’ started to explore south down the west coast of Africa, they found that by merely extending the type of chart that they had used for European coasts, they experienced enormous problems because their ‘rhum lines’ could no longer be used as straight lines, and the compass courses and distances shown on their charts could no longer be used for ‘dead reckoning’ over the longer distances that they were now travelling. It was therefore necessary to devise a chart where positions could be expressed as latitudes and longitudes rather than bearings and distances.

In 1569, Gerardus Mercator devised a mathematical way of projecting a globe onto a flat piece of paper. This system is still in use today for most of our charts, but it does involve certain substantial inaccuracies. This is because with this projection the meridians are shown as parallel lines, whereas in fact they meet at the poles. Therefore Greenland for instance appears to be a very large continent; whereas its actual area is in fact much smaller.

In addition parallels of latitude are actually shown as further apart as a chart covers areas further to the north and south of the equator. Whilst Mercators projection was achieved mathematically, the reason for this is the same as putting a light bulb into the centre of a globe and then projecting the image of the surface of the globe onto a flat piece of paper held a few feet away.

Whilst on long passages across the Atlantic using a Mercator chart, a straight line from your departure point to your destination, which is known as a ‘rhum line’, will get you to your destination, it will have involved your sailing in a gradual curve and therefore sailing further than is actually necessary. For long ocean passages the direct route is known as a ‘great circle’, a great circle is the shortest distance between two points on a globe. To sail a ‘great circle’ involves having to alter course by a few degrees every other day or so. To make this calculation easier for the navigator the Gnomonic projection was devised. This projection involves curved parallels of latitude and straight meridians of longitude that become closer together as they move away from the equator. A straight line drawn on this type of chart from for instance Lands End to New York shows clearly that to stay on this ‘great circle’ track it will be necessary to alter course every few days.

CHART PUBLISHERS

‘British Admiralty’ charts are excellent, and are my preference over ‘copies’ produced by companies such as ‘Imray’ and ‘Stanford’, which are designed specifically for yachtsmen and women in that they often show harbour plans and give information that is specifically of interest to the ‘recreational yacht-person’, but all this information will also be in the pilot books that you should have on board. The disadvantage of this type of chart is that they are folded like a road map and when laid out on the chart table they present ridges from the folds which can make moving your navigation plotter smoothly around the chart more difficult.

Both Admiralty charts and charts which are produced by the Portuguese Hydrographical Office (which are equally good) are produced in various scales. When undertaking a passage you will require firstly a chart that covers the entire passage and then all the larger scale charts of the coastline en-route. It is my idea of hell having to approach a coast without a large scale chart to make me aware of any dangers, I have often heard people say that they do not intend stopping along a particular coast and so therefore they are reluctant to pay the money for a large scale chart, fine but what if, and it is the ‘what ifs’ that make a good seaman, you have to approach a coast and go into a harbour at 3 am because a crew member needs hospital attention or perhaps because of rig failure or an engine problem. THEN you are going to wish that you had bought that larger scale chart at the chandlery instead of that totally useless gadget that you did buy.

KEEPING YOUR CHARTS UP TO DATE

Charts are not cheap and I don’t know of anyone who can afford to buy new ones every year, unless your are just cruising in one small area and only need one or two charts and even then replacing each one will cost as much as a good meal in a restaurant for two people. The alternative is to keep them up to date; on a naval or merchant vessel this is traditionally carried out by the most junior officer on board, for us recreational sailors if we want our charts to be up to date this is probably a task that we can’t delegate, we have to do it ourselves.

British Admiralty charts are updated weekly in ‘Notices to Mariners’ and for a fee ‘The United Kingdom Hydrographic Office’ will send you the notices four times a year, alternatively you can download them to your computer when ever you like from: http://www.nmwebsearch.com/

Charts produced by the Portuguese Hydrographical Office (O Instituto Hidrográfico) can be updated in exactly the same way; their ‘Notices to Mariners’ (Avisos aos Navigantes) which are published every two weeks can be obtained by post and can also be downloaded from www.hidrografico.pt

 

Martin Northey - Yachtmaster Examiner/Instructor for Sail and Power

The Iberian Sea School - RYA Sailing, Motor Cruising and Powerboat School
Apartado 1039 - Vilamoura, 
8126 - 912 Quarteira,
Portugal

Tel:00 351 965 800702
Email:martin.northey@mail.telepac.pt
Web Site:www.theiberianseaschool.com 
Skype:martinnorthey

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