57 ROUTE PLANNING AND OCEAN PLANNING PUBLICATIONS

221. OCEAN TRACK-PLANNING DATA

The art of passage planning has been practised from time immemorial. The selection of the best track for an individual voyage demands skilled evaluation of all the factors controlling the voyage and modification of the shortest route accordingly. Most passage planning is done with the aid of statistics on weather, climate and currents in conjunction with the experience of previous voyages, but it should be remembered that such statistic-based or climatic routes, although serving the mariner’s purpose up to a point, do not take into account short-term variations in the statistical pattern which can be detected and even forecast by modern methods, and a prudent mariner will always make provision for diversions on passage according to his individual judgement of prevailing conditions and in the light of radio weather forecast information. There are two major sources of statistical information available to the ocean navigator both of them of vital interest, particularly to the yacht navigator, who is more dependent on the wind than commercial mariners: –

1.   Routing Charts. For each of the world’s oceans, twelve routing charts are published, one for each month of the year. These are all drawn on a scale of 1:13,880,000 at the approximate mid-Lat. and are: –

Admiralty Charts No.     5124 (1) ~ (12) North Atlantic Ocean –   5125 (1) ~ (12)South Atlantic Ocean

                                             5126 (1) ~ (12)Indian Ocean –                   5127 (1) ~ (12)North Pacific Ocean

                                             5128 (1) ~ (12)South Pacific Ocean

Each of these charts shows winds, currents, ice and fog limits, air and sea temperatures dew points, tracks of tropical revolving storms (in any) in recent years, and recommended steamer tracks and distances. Winds are indicated by wind roses placed all over each chart, showing the percentage of winds from various directions and their strengths predicted at the position of each wind rose. The general direction and average rates of ocean currents are shown by green arrows. Ice and fog areas and limits are marked, while small Chartlets show temperature contours of both sea and air, and isobars of mean barometric pressure in millibars. Tracks of tropical revolving storms which have occurred in recent years are also shown.

2.    Ocean Passages for the World is a Hydrographic Office publication (N.P.136) specially written for use in planning deep-sea voyages. It contains notes on the weather and other factors affecting passages, directions for a number of recommended routes, and distance figures designed to help the planner to calculate his voyage time along these routes. It boars much the same relation to the Admiralty charts of the oceans as the Sailing Directions (Pilot Books) bear to the coastal charts. § 1 contains information applicable to all sea areas; the later chapters treat the individual seas and oceans, §s 2 – 8 for power vessels and §s 9 – 11 for sailing vessels. The book is supplied in a wallet, which also contains a card giving a Logarithmic Speed, Time and Distance Scale, and the following Admiralty Charts: –

Chart No. 5301 World Climatic Chart – Jan

5302  World Climatic Chart – Jul

5301 The World – Ocean Routes for Power Vessels

5308 The World – Sailing Ship Routes

5309 The World – Tracks for Sailing and Auxiliary Powered Vessels

5310 The World – General Surface Current Distribution

0.6083 Zone – Areas for the International Load Line Rules.

The above publications contain the basic data for route-planning but should be used in conjunction with the appropriate Admiralty navigational charts of the areas to be covered, both coastal and ocean, and the appropriate volumes of the Admiralty Sailing Directions (Pilot Books). The number of charts bought for a long cruise is likely to be limited by considerations of space, if not also of cost, but it is imperative that all areas likely to be visited are covered, including all those places or ports of refuge to which it may be necessary to make an emergency detour. Where compromises have to be made, it is preferable to have medium-scale charts covering all areas rather than the same number of large-scale charts covering fewer places. A voyage, depending on its length and nature, may require anything between 50 and 250 charts.  These should be arranged in the sequence in which they are most likely to be needed, and divided up into batches of about 20 each. Each such batch or folio should be kept flat in a separate plastic or canvas envelope measuring about 820mm x 560mm. (32” x 22”) with the opening along the long side, and each chart title the same way up at this side. In this way, one can riffle through the edges to find the required chart without withdrawing them all. All charts should be corrected up to date from Admiralty Notices to Mariners immediately prior to departure from each port on a long cruise.  Admiralty Notices to Mariners are available at most ports Abroad, but if in any doubt about this, arrangements can be made to Air Mail them to various ports along the vessel’s intended route.

Publications on board for every ocean voyage should include the Admiralty Sailing Directions (Pilot Books) for all the areas to be visited or passed through by the vessel These volumes supplement the information given on the charts and give invaluable information on safe pilotage into almost every port; the identification of coastlines islands, landmarks lighthouses and lights, leading lines, of flying dangers, shoals and rocks; currents and tidal streams; anchorages and ports of refuge; and port facilities including pilotage regulations, availability of fuel, fresh water, and repair facilities. Further useful information for passage planning will be found in the Hydrographic Office publication The Mariner’s Handbook.

222. CHOOSING THE BEST ROUTE

Planning an ocean passage should begin with a careful study of the routeing charts of the ocean involved for the month(s) when it is proposed to be on passage. The first consideration should be to seek to avoid periods when tropical revolving storms (called various hurricanes, cyclones, typhoons, etc., according to area) may be expected. These should be avoided at all costs; yachts have survived them, but even large commercial vessels take great pains to avoid them since the operative word for any vessel caught within one is survival where winds can reach speeds of 120 knots and over. Full details of Tropical Revolving Storms (TRS) will be found on the routeing charts and in Ocean Passages for the World but briefly the times when TRS may be expected are: –

North Atlantic Ocean, western side       Jun ~ Nov       South Indian Ocean, western side        Dec ~ Apr

Bay of Bengal                                             May ~ Nov      North Pacific Ocean, eastern side        Jun ~ Oct

North Pacific Ocean, western side         Jun ~ Nov       South Pacific Ocean, western side       Dec ~ Apr

N. NW and W coasts of Australia           Dec ~ Mar

Having settled on the months when there is the least likelihood of tropical storms, the next step is to decide on the best route. The speed – and the comfort – of an ocean passage will depend largely on the winds encountered and the currents prevailing. Clearly, a route on which mainly favourable winds of average strength not above Force 6 on the Beaufort Scale can be expected is far preferable to one where mainly headwinds, or winds from forward of the beam, or gales, prevail. Ocean currents too may have a significant effect on the length of the passage. A modest current of only half a knot in the wrong direction means that the yacht will have to sail twelve miles each day before she has made any progress towards her destination, and this can total a considerable amount on, say, a 4,000-mile passages

The final choice of route will be influenced by the relative importance of speed or of pleasure and comfort, and by the windward ability of the yacht. The shortest distance between two places is a straight line, but quite often this will not be the best route even when there are no dangers along this route.

For example, a direct route from the U.K. to the W. Indies passes through large areas of the contrary, or light, winds, while a more southerly route will lead through the belt of the N.E. Trade Winds where for many days a good stiff sailing wind from well abaft the beam can almost be guaranteed.

Where the routeing charts show that there is nothing to be gained by taking a more roundabout routes then the straight-line route may be the best. The choice then has to be made – which straight line, a Rhumb line, a great circle or a composite great circle?  Broadly speaking great circle sailing holds the advantage in distance over the Rhumb line to the greatest extent in high latitudes and on E – W courses. On N – S courses or E – W courses near the Equator the saving in distance is negligible. Where a great circle route is found to be advantageous to a yacht, consideration must be given to the winds and weather to be expected along the route, and whether it should be modified (as shown earlier in these chapters) to a composite great circle track by setting a limiting Lat..

In coral waters, such as the South Pacific and South Indian Oceans and the Bahamas, it must be remembered that coral reefs are frequently steep-to, and depths of more than 200 metres may be found within one cable of the edge of the reef. Soundings are therefore of little value as a warning of their proximity, Navigation among coral reefs is therefore almost entirely dependent upon the eye and in ocean areas where these reefs abound the greatest care is required. Whenever possible passage through the worst parts of such areas should be made in daylight and every precaution should be taken to keep an accurate check on the vessel’s position.  Information on navigation in coral waters is given in the Hydrographic Office publication The Mariner’s Handbook.

Very large areas of the Pacific Ocean are un-surveyed, or imperfectly so. In many areas no sounding at all has been recorded.

The presence of a single sounding on the chart can only prove the non-existence of a shoal or reef within a very limited area, and it may be said, as an approximation, that no shoal is likely within a radius of 7 miles from a sounding of 3660 metres, within 3½ miles of a sounding of 2740m., or within 2 miles of a sounding of 1830m. A danger. may lie within ½. mile or less of a depth of 900 m., so precipitous is the rise of a coral reef or a vigia from the ocean bed. Many reefs, shoals and patches of discoloured water were reported in the years 1943 to 1945, when many vessels were navigating off the usual routes. The routes recommended on the routeing charts and Ocean Passages for the World are considered most likely to lead clear of dangers, but for the reasons stated above, it cannot be stressed too strongly that the only safeguards are a vigilant look- out and careful sounding.