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Tackling coral atoll passes

Nov 4, 2014
An impromptu “slide rule” used for lining  up moonrise and moonset to determine slack tide.

An impromptu “slide rule” used for lining up moonrise and moonset to determine slack tide.

Michael K. Hawkins

To the editor: Though we had cruised around many reef-surrounded islands, our first true experience passing into an atoll was in the Tuamotu Group of French Polynesia. This group was known as the “Dangerous Archipelago” for good reason. Many helpful cruising guides by fellow boaters and organizations offer lots to think about. They can, frankly, scare you to death about the entries through these atoll’s passes.

On our passage to the Tuamotus, we both just re-read Thor Heyerdahl’s Kon-Tiki. This story is about the balsa log raft that safely negotiated 4300 miles of open ocean to ultimately crash onto the eastern reef of Raroia. That was the atoll we would also be entering as our virgin attempt into a protected lagoon surrounded by reef. Reading a book about a vessel crashing into a reef was like watching Jaws just before going on your first open-water dive! Plus, as we passed other atolls, we saw several remnants of crashed vessels on the reefs.

So what is the secret of safely getting into the atoll and enjoying the calmness of the inner lagoons? The charts today are quite accurate. We used Navionics charts on our Raymarine chartplotter and had a back-up of the Garmin Blue Chart G2 Vision on our Garmin GPS. We had paper charts of the area and used the Tuamotu compendium put out by Soggy Paws. And today, the advantage of the Google Earth charts is not to be underestimated — the satellite images over the Open CPN charts are fabulous. Information and charts were not the problem — we could find the passes. And now, many are marked with ranges and navigation buoys.

There are many other things you have to consider before making your grand entrance through a cut. The number of openings into each lagoon is a major consideration. The fewer the openings, the stronger the current pushing water into and out of that lagoon. The width of each opening is also critical — narrow openings create a bigger push than wider openings. Winds over the last several days factor into the equation. Contrary winds can stack up water against the opening and fight the current to create standing waves. There is a lot to calculate and consider before making a run at a pass entrance.

Tides play a big role in atoll pass decision-making. The largest challenge for us was finding out when the slack tide really occurred. The tide tables from NOAA, the French Meteorological Society and various computer programs, all provided quite different information, often hours apart. The tides were usually for other atolls, not necessarily the one we would be entering.

Advice was plentiful. There are even a few tools designed by other boaters to help. A computer spreadsheet helped calculate the various factors: how many openings in the reef; recent weather conditions; wind speed and direction; wave height; and tidal difference for that particular day. Another boater also created a “slide rule” that allows you to line up moonrise and moonset to determine slack tide. And, of course, you have the local information — local dive shops and fishermen who watch the currents daily in order to earn a living. There was lots of information and on some occasions when we tried all methods, the information regarding slack water still varied by as much as four hours.

An atoll  pass in French Polynesia.

After an invigorating four-day sail from the Marquesas Islands, we prepared for our first entry into Raroia. We tried to determine the slack for an ebb tide and began gathering our information. We did the “slide rule” using moonrise and moonset; we looked at several tide tables from various sources; we calculated the factors of weather, winds and seas for the last few days. And we knew there was only one pass into this lagoon and it was quite narrow. Knowing a few boats were already on the “inside,” we called them on the SSB radio. With loads of information from all sources, we took everything into consideration and then made our best guess. That’s what we decided — it is ultimately a best guess. Or perhaps it’s even a bit of voodoo, witch doctoring and a lot of luck.

We estimated the slack would be around 1015. At 0900 we started to really watch the reef by edging close to the cut to take a good eyeball look. Visibility is also a critical factor and we had a wonderfully clear day with the sun getting higher in the sky with each passing hour. We saw a lot of breaking waves over the reef on either side of the cut, and the current was very obvious on the water outside of the pass. So we circled. In another 30 minutes we took another look. We continued this process for a few hours and finally headed into the pass. It was a good move and in the company of five large, playful dolphins, we made it through. Our first atoll safely conquered!

After a week enjoying various motus (islands) of Raroia, we determined we had a good weather opportunity to exit the pass and proceeded with the same ritual. We watched the tides for several days and on our departure date we estimated that it would be an early morning slack. We weighed anchor and did several drive-bys before transiting the pass. This time, we also kept our eye on a local fishing boat and when they headed for the pass to dive and fish, we also knew it was a good time to make our exit.

The next stop would be the atoll of Fakarava. There are two passes on this larger atoll — a north and south pass. The north pass is a wide one and all the guide books said that if you have a good engine, this pass could be negotiated in a wider time frame with tide playing a less critical role. We arrived at the pass at daybreak and had determined that it would be an outgoing tide early. However, the tide had not turned and we “flew” into the pass in relatively calm water, with the help of an incoming current sending us over the bottom at 6.9 knots. The good news was we still had steerage (or we would have turned around) and we made it through the pass quite quickly. You can’t do this type of entry in many passes, but the North Pass of Fakarava was one in which it was possible, as it is wide and not many reefs surround the edges.

After spending a wonderful 10 days exploring several anchorages in the Fakarava Atoll, we ended up near the South Pass for our exit. We spent several days while at anchor, watching the water. A dive in the pass to see the thousands of sharks that “hang out” there also helped us see the cut from a different perspective. We talked a lot with the local dive masters who dive the pass at slack water daily so they really understand the water flow. They also told us that there are days that the tides aren’t what they expect and they have to sit out on their dive mooring often waiting hours for the right conditions. This was a much narrower pass and wasn’t a straight run. It was well-marked with navigational aids. We were successful negotiating this pass, too.

Since our first atoll entrances in French Polynesia, we have visited three of the four “all atoll” countries: Tuvalu, Kiribati and The Marshall Islands. We have entered and exited many coral atoll passes — some very narrow. We have sat off an entry for hours waiting for squalls to pass and give us the visibility that is critical. Not that we are pros, but we are more confident in timing the entrances and exits to these ancient sunk volcanoes. The lagoons are worth the price of entry — but every time we tackle another atoll entrance, it puts a few more grey hairs on our heads and gets the heart pumping more quickly. The best tools we have found for entering these atolls: good visibility, common sense and the willingness to not enter if conditions aren’t right! 

—Barbara Sobocinski and Michael C. Hawkins have been voyaging for five years aboard their 1987 Moody 422 Astarte. Prior to becoming full-time cruisers they both worked in the television industry.

Edit Module

Dec 11, 2016 10:41 am
 Posted by  Anders

There are several complications to consider.
Firstly "slack water" (no current) in strongly flowing passes almost never happens at the same time as the turn of the tide (low water and high water). Consider, for simplicity, a big atoll with only one narrow, shallow pass. It will empty much more slowly than a small atoll with the same sized pass.
And so the falling water level in the lagoon of the big atoll will lag further and further behind the falling level in the sea as the tide continues to drop. The bigger the lag, the greater the current.
And the more narrow the pass in relation to the displaced volume (surface area of the lagoon on the chart multiplied by the tidal fluctuation inside the lagoon), the longer will be the time lag between low water, and the atoll finishing its emptying. Emptying finishes when the water level inside matches sealevel outside, which will happen because of the sea rising enough to make up for the time lag.
This may take hours. And in such cases it is likely that the maximum flow through a pass will happen at LOW tide, rather than mid tide as you might expect. Low tide is when the difference in levels, which will have been increasing as the tide has been dropping, will be a maximum. So the time of maximum flow will be much more predictable then the time of zero flow.

Second complicating factor: there is something else which can fill, but not empty, a lagoon. This is ocean swell. If large enough, swells will wash over the reef and into the lagoon, but the reverse cannot happen to any great extent, because a lagoon's smaller size means it can only develop a chop, and not a swell.

So on days with a big swell, it can easily happen that the level inside the atoll never drops as low as sealevel, even at high tide. And on days like this the current in the pass(es) will constantly flow outwards. Once again, it will be strongest when the ocean tide is lowest, and weakest when it is highest.

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