Unique challenges

From Ocean Navigator #104
March/April 2000
Navigation in tropical waters presents some unique challenges, and even skilled sailors from temperate waters will find themselves working harder, taking extra precautions, and acquiring new skills. Reefs are a prominent feature of many tropical environments and account for many of the tropics’ navigational hazards. For sailors, coral reefs are inherently dangerous and unforgiving: stray onto one and it’s almost certain you’ll sustain some damage. Reefs can be hard to see, and the same vertical reef edges that make for wonderful “wall” dives using scuba gear mean your depthsounder may give no warning that you’re about to run aground.

A prominent feature of the tropics is coral reefs. Reefs can be hard to see, and the same vertical reef edges that make for great scuba diving mean your depthsounder may give no warning that you?re about to run aground.
   Image Credit: Tom Zydler

All these factors take on even greater importance in the many areas of the tropics where the charts are old, and islands and reefs are not properly positioned with respect to latitude and longitude. This can render your GPS all but useless when navigating in relatively confined quarters. If you’re sailing in a remote area you can add to this mix a general lack of navigational aids, be it lighthouses, beacons, or buoys.

Dangerous reefs, all but useless electronics, and no markers; do these add up to a navigational recipe for disaster? Not if you prepare ahead. Detailed charts are important, and pilots can be very valuable. More informal information sources can be of equal worth. I’ve benefitted greatly from discussions with local residents and other sailors, and from “mud maps” made on the basis of such talks. You’ll also need to outfit your boat and crew with some simple equipment and practice some basic but effective navigational techniques. The end result is that, while navigation in the tropics may take more time and effort than it typically does back home, it’s simple and within any sailor’s grasp; the effort you expend is your ticket to some fascinating voyaging grounds.Charts

The nautical chart is your most important information source, and you should do what you can to get the largest-scale charts you can for any tropical areas in which you plan to do a lot of gunkholing. Original charts are much to be preferred because of their improved legibility and durability over even the best copies. They’re costly, though, and as a result we and many other voyagers rely heavily on copies (see sidebar).

Whether you’re buying or copying a chart, make sure to check the datenot the date the chart was printed, but the latest survey and update. For localized navigation, a chart based on an old survey may work fine, and indeed some old charts contain a wealth of useful detail that has been omitted from modern editions. This is not surprising, as the concern of most ship captains today lies in avoiding reefs rather than navigating through them. But the older charts may place islands miles out of position; I’ve used charts with major islands plotted as much as seven miles from their actual location. Even where the errors are not as large, the disagreement between chart and GPS datum is often sufficient to render the GPS all but useless. Familiarize yourself with the chart datum settings on your GPS, and check the datum of your charts. The current standard is the World Geodetic System 84 ellipsoid, which thankfully is referred to as simply WGS 84. If your chart is based on WGS 84, agreement should be excellent. If a different datum has been used, but that setting is available, chances are the “fit” between GPS and chart will still be good. If no datum is listed, or your GPS does not include the needed datum, then GPS plots may be well off. In either of the latter cases, check the agreement between chart and GPS before assuming that all is well.

It is possible to establish your own correction for charts that are not in agreement with your GPS datum. To do this, position your boat at a location that you can readily and very accurately identify on the chart. This is most easily done at anchor and may require use of multiple bearings. Note the position as indicated by your GPS, and compare this with the latitude and longitude of your location as indicated on the chart. The difference between the two is the correction that should be applied to any GPS readings before they are plotted on the chart. Note that corrections are reversed if entering waypoints in your GPS whose latitude and longitude were derived from the chart. With some GPS units it may be possible to enter this correction and let the black box do the calculating. Check your manual. Note that corrections are not necessarily constant across a chart, especially if it is a small scaleone that covers a large area.

An increasing number of charts are available in electronic form, accessible with a PC or GPS receiver. Do some thorough checking regarding the availability of electronic charts for remote areas and the accuracy and correction of those charts before deciding to use them. Most electronic charts are simply digitized versions of paper charts, and any inaccuracies will be duplicated. Also, keep in mind that reliance on electronic charts alone is not a wise practice, as electronics fail with disturbing regularity in the tropics; even the poorest of chart copies is better than a blank screen! Pilot books can be a valuable addition to charts, often supplying information your chart lacks. Typically the most useful are those pilots, often referred to as cruising guides, published expressly for use by yachtsmen. The Admiralty series of pilots, published in the United Kingdom, get my vote for the best of the “big ship” pilots; the Sailing Directions published in the U.S. are generally the least useful.Formal aids to navigation

The charts you obtain may or may not show many aids to navigation. Most nations typically do an excellent job of maintaining their markers, but in the remote tropics markers are often absent even when marked on the chart. Many tropical countries are poor and lack the funds to install and maintain aids to navigation; I’ve found this to be true in much of the tropical Pacific. One may be lucky enough to find markers near major passes, especially if they are used by shipping, but more often than not you’re on your own.

Even when markers do exist, don’t rely on their being lit or on their placement being as accurate as you might expect. Markers are often placed atop reefs, a practice that simplifies their installation but also means you should not approach too closely, as there may not be deep, clear water up to the maker.

I’ve found “informal” markers in use in many areas. They usually consist of sticks, put in place by fishermen to help guide them through channels when visibility is poor. These can be very helpful in situations when you’ll be transiting the same passage a number of times, but they should never be relied on unless you have already made your way through a passage using your own reconnaissance.Visual clues

Tropical waters provide some unique visual clues that can help in determining one’s position, or in simply making sure you don’t pile up on the reef. We’ve all seen photos of tropical anchorages with beautiful aquamarine water, and water color is certainly a primary indicator of water depth and the presence of reefs. Other less obvious clues can also be found, however; for example, the water surface may show ripples or wavelets in a pass, while none are present above the adjacent reef. Reefs exposed to breaking waves can be seen if there is any swell in the vicinity of a reef, and rough weather will often send a mist into the air, which may be visible long before the reef break.

One quickly becomes attuned to using water color as a visual depth sounder, but keep in mind that it works only in clear, sunny weather. With the sun overhead, a deep blue indicates deep water that is reef free. As the water shallows, the color will gradually lighten, becoming a greenish blue in sandy-bottomed water of 30 feet or so. Isolated coral heads that are well below the surfaceposing no threatwill appear brown, but without distinct form. Coral heads that lie near the surface will also be brown but can be seen much more clearly. Large, shallow patch or fringe reefs will also appear brown in color but will often have a light green area adjacent to them. The latter indicates a sandy bottom and a water depth of seven feet or so; perfect for swimming but too shallow for most boats. These color differences become less distinct in overcast weather, and it’s easy to mistake the shadows of passing clouds for patch reefs.

Regrettably, not all reefs are easy to spot. A reef with a sudden vertical wall can be very hard to see, as you lack the gradual color change that a slowly shoaling sandy bottom produces. Be especially wary sailing in such areas when the visibility is poor. We’ve made our way through a pass off the Fijian island of Kandavuguided by a friend who had negotiated it previouslycompletely unable to see the shallow reefs on either side. Coral can also be very hard to see in areas with muddy bottoms, such as those found near mangroves.

The opposite problem that of exceptionally good visibility can occasionally prove a problem as well. We were once sailing through the pass of a small reef south of New Caledonia’s Isle of Pines when I became convinced that we was about to run aground, solely because the water was so clear that the corals that lay some 16 feet below us looked to be all but breaking the surface! We reasoned this out and pushed ahead, but not without real trepidation.Maximizing visibility

Most often you’ll want to do everything you can to maximize visibility. The sun can be either your ally or enemy, depending on its position in the sky and the bearing to the reef. The best time for navigating reef-strewn waters is when the sun is high; try to time tricky passages for the hours between 1000 and 1400. Even then, it helps to have the sun either at your back or to one side but not between you and the reef. The sun’s position with respect to the reef becomes increasingly important in the early morning and late afternoon.

You can improve your ability to make out the reef at any time of day by increasing your height above the water. Ratlines are the time-honored way to do this, and they still get our vote for the simplest and cheapest method of getting aloft. Mast steps are a modern alternative but may not allow one to see easily around a headsail, depending on the point of sail and position of the reef. (This is one reason to fit ratlines on both port and starboard sides.)

Glare can be a major hindrance to good visibility, and we find polarizing sunglasses to be a worthwhile investment for anyone navigating by eye. We buy inexpensive “fishermen” versions, as they provide protection from the side as well as ahead. The price is such that you can keep a few pairs on board and won’t cry if they’re lost or sat on. Beware of getting these cheap versions wet, though, as a good dousing causes delamination of the thin polaroid film that is sandwiched within the plastic lens.Making a tropical landfall

If you’re approaching an island or pass from offshore, you’ll want a good fix well before you make landfall. Should you not have a GPS (or if yours has packed up, as ours has several times), then you should endeavor to take a star sight on the evening before, and the morning of, your arrival. In either case, make sure to take into account any disagreement between the charted and actual position of the land you’re approaching, and keep in mind that such disagreement may not be noted on the chart. If you’re unsure, navigate conservatively, staying well off any potential hazards. This is particularly important if you’re approaching during the night and wish to heave to safely.

If it’s an atoll you’re headed for, bear in mind that they’re rarely visible more than 10 miles out. Mid-ocean reefs will typically be picked up only two or three miles away, when the breakers, or the mist they throw up, become visible. That distance will be further reduced in calm weather with no appreciable swell. Your ratlines or mast steps come into play here, as increasing your height above the water pays real dividends in visibility.

We’re all eager to make landfall after spending time at sea, but don’t push it. Night landfalls are potentially hazardous in the best of circumstances and particularly so in the tropics, with a crew that may not be well rested. If it’s rough, keep in mind that conditions near islands may worsen instead of improving. Islands will disturb, and often enhance, the prevailing swell. Likewise, both wind and current will often increase in the vicinity of an archipelago. This phenomenon is well known to sailors in Hawaii, where moderate 15- to 20-knot northeast trades are often accelerated to become 20 to 30 knots between the islands. When such a wind is blowing contrary to the current, steep seas often result.Entering an unmarked reef pass

Unmarked reef passes have struck fear in the hearts of sailors for generations. Consider the comments of the English navigator Matthew Flinders, who was writing about sailing through passes in Australia’s Great Barrier Reef some 185 years ago:

“The commander who proposes to make the experiment, must not, however, be one who throws his ship’s head round in a hurry, so soon as breakers are announced from aloft; if he do not feel his nerves strong enough to thread the needle, as it is called, amongst the reefs, whilst he directs steerage from the masthead, I would strongly recommend him not to approach this part of [the coast].”

From A Voyage to Terra Australis, published 1814.

“Threading the needle” is an apt description of what it’s like to negotiate some passes, but it’s Flinders’ reference to directing steerage from the masthead that is particularly important. Safe negotiation of reef passes relies more on good visibility than any other factor; as we’ve discussed, visibility in coral waters improves dramatically with height. It may be possible to safely transit some passes with only limited visibility, but these tend to be the exception.

Just how dangerous coral passes can be was brought home to me during a visit to Palmyra Atoll, located in the central Pacific just north of the equator. We negotiated the long pass in midday, with good visibility. We experienced no troubles and found ample depth and width through the pass. In the next few weeks, however, a sailboat, a small cruise ship, and a large Coast Guard inflatable all went aground. The Coast Guard and the cruise ship managed to get themselves off, but it took six of us working for two days to extricate the sailboat from the maze of coral heads into which she had strayed. The island’s caretaker, who has spent years at the island, estimates that 90% of the boats that enter the lagoon run afoul of the reef; some have never gotten off.

The specifics of how a pass should be transited will of course vary, but there are some common features. The first rule is to never commit to entering the pass unless you are sure you have correctly identified the entrance. Some passes are wide and distinct, while others may be narrow and broken up by patches and coral heads; the latter can be hard to spot, particularly if the pass is on the island’s leeward side. Passes on the windward side of islands may be navigable only in moderate weather, when the wave break is not excessive, but are generally easier to locate than passes that lack a break.

The best strategy is to proceed slowly. We’ve often been forced to probe slowly and carefully at the edge of a reef, staying in safe, deep water but close enough to the reef to be able to make out gaps and breaks. When you think you’ve found the pass, check the bearing with what is shown on the chart. If they agree, proceed cautiously, while maintaining a lookout aloft and an eye on the compass.

The slow and careful strategy needs to be modified in a rough windward pass with a large swell. In such situations you’ll want your reconnaissance to be done slowly, but from a greater distance than would be necessary in a calm, swell free area. Your lookout should try not only to ascertain the safe route but also to judge the hazards posed by the swell. Whenand if everything checks out, head into the pass, and do so at speed, as you won’t want to be a sitting duck for the waves.

Negotiating passes under power alone is a common but dangerous strategy. Many sailors seem to think they’ll be better off with sails lowered, as they’ve got enough to do without worrying about the sails in the bargain. What they may not have considered is how quickly they can be driven onto the reef should their engine die. By all means drop the jib if your course is to windward or if the jib is impeding visibility, but keep the mainsail up. It’ll give you steerage if the engine quits, and will steady the boat as well.Navigating reef-strewn waters

Having found a tropical island and safely negotiated any pass that may be present, the tropical sailor frequently faces the challenge of piloting in a lagoon or coastal waters that are strewn with reefs. While we’ve discussed some aspects of reef navigationsuch as the importance of large-scale charts and good visibilitythose are only part of the picture. Successful reef pilotage demands attention to some navigational basics, skills that all sailors should have long before they reach the tropics but that few routinely practice.

The skills in question include taking bearings of distant objects, using steering marks to determine safe and unsafe zones, and making use of ranges or transits. To illustrate how these work, we’ll take a sail into the lagoon of Papua New Guinea’s remote Hermit Island (chart at right and on next page).

The island is mis-charted, and although the GPS can help in finding the island, it’s of no use within the lagoon. To enter the lagoon we made use of a range, lining up the northern edge of Jalun Island with a distinctive peak on Luf Island. Here is a good illustration of the fact that ranges are only as good as your identification of the features you’re sighting. Had I sighted the wrong peakusing the northern peak on Akib Islandwe could have easily missed the pass and gone on the reef. To ensure this doesn’t happen, take a bearing; in this case the false range, using Akib, bears 120°, while the proper range, using Luf, lines up at 109°. If we are sailing on the latter course and have the island and peak lined up, we know we’re heading for the pass.

Once through the pass, which we can determine from the depth and by looking for any wave break, it’s time to turn and head south. We want to set a course that avoids the small patch reefs in the lagoon but keeps us clear of the reefs that fringe the central islands as well. If the visibility is good we could sail close to the west coast of Jalun, making our turn to the south when we see the reef. But all it takes is a squall to make one wish for a better strategy. Here’s where the use of bearings and steering marks comes into play.

On the chart we can easily determine courses that will keep us clear of both patch and fringing reefs; the difficulty lies in ensuring that we stay on those course lines, particularly if there is a current setting us in one direction or another. All it would take is a weak west-setting current to have us heading directly for the patch reefs. We could take bearings to various islands to fix our position as we go along, thus checking for current set, but in reality there is rarely time for that when maneuvering in confined waters.

A better solution is to make use of a steering mark: something fixed, toward which we can steer while on a safe course. In this instance Leabon Islet is ideally placed to guide us safely south; what’s more, because it’s the last of a number of islets, it is simple to pick out. It’s easy to lose one’s reference or become confused when steering for an islet that lies in the midst of a group. If we remain on our initial course of 109° until Leabon islet bears 154°, we’ll be in the midst of a safe route south. If we now steer for Leabon, any change in our compass heading indicates we’re being set by current. If the course increases, we’re being set east; a decrease in the course indicates a westerly set. If we establish the outer edges of our safe route (151° and 157°), we’ll know simply from a glance at the boat’s compass if we’re approaching the edge of the safe area. To correct, simply alter your heading slightly to place Leabon on the port or starboard bow; when it is once again possible to head for Leabon on a course of 154°, then you’re back in the midst of the safe zone.

Because they make use of your steering compass and boat’s heading, steering marks and safe routes are very simple to use, and the chart work can be done before you enter a lagoon or other dicey area. If you set up safe routes that overlap, you’ll always know if you’re in safe water. Look back at the chart of Hermit Island. As we head south toward Leabon, there is no guess work about when it is time to turn east: it is safe as long as our heading to Luf’s southern tip is between 79° and 65°. Once on that course we can make use of a range (between Luf and Tset) to make our turn toward the anchorage.

Navigating through Hermit Lagoon is vastly simplified by the presence of a number of islands and islets, many with distinctive peaks. In some places, though, there are no islets, and the coast is low and rather featureless, offering few opportunities to establish convenient ranges. In those instances it may be necessary to rely, at least in part, on visual navigation, though you shouldn’t pass up any chances to take a bearing or grab a fix.Navigating at night

As for sailing at night in coral waters, in a word: don’t. That’s not to say we never do, but in general we can’t recommend it. The only reasonably safe exception is well-charted waters where navigational aids are in place and are lit. Even then, beware of putting yourself in a position where you are entirely dependent on your GPS. For example, the Australian coast is well charted, using an up-to-date datum, which makes it possible to negotiate even unmarked passes at night with the aid of a GPS. One would be in a pickle, however, should the black box fail in the midst of the exercise. As a result, we always choose passes that are marked and lit.

Even then, be wary! We entered Bali’s marked and supposedly lit pass at night under sail after our engine choked on the high-sulfur Indonesian diesel (the problem proved to be a clogged injector). We had transited the pass several times before during daylight, so we knew the route and were initially impressed by the fact that the lights on the markers were working. We made short tacks as we headed up the narrow pass, wanting to stay well clear of the reef to either side, one that had claimed several fishing boats, a tug, and a barge in the preceding weeks. All seemed fine until a dark shape appeared just to leeward; we shone the torch and were confronted by a coral outcrop not more than 10 feet away. We tacked immediately, just missing the reef. We carefully scanned the channel and found the problem: a marker whose light was not working and that we had not seen as we sailed by. Another 10 feet and we would have joined the fleet already on the reef!

Editors Note: Mark Smaalders, who has sailed in the tropics for the past 12 years, logging some 30,000 miles through the Pacific and Indonesia, is currently in Australia, preparing his 35-foot wooden sloop Nomad for a trip to New Caledonia, where he’ll be based for the next few years.

By Ocean Navigator