Is the lagoon half empty or half full? seasons in French Polynesia, we have observed the frustration many captains feel as they leave their emotion-laden Marquesas landfalls to navigate the sometimes tricky passes of the atolls of the Tuamotus. Predicting the current in the passes is critical to safe navigation. Strong, current-opposing large swells or wind waves can produce standing waves that could stop a vessel’s forward momentum and sweep it sideways onto a reef. For this reason, on those passes with strong current it is critical to predict the timing of slack water.
work when the lagoon is at normal levels, but none of the traditional methods will work exactly when the lagoon has been overfilled. First, a bit of background on atolls and passes: An atoll is lagoon surrounded by an encircling coral
Left, a view of Livia Gilstrap’s Wauguiez Pretorien 35 Estrellita 5.10b from the top of the mast. Above, lagoon depth can be affected by tide and wind.
Some crews will visit several atolls, always correctly predicting the slack, while others will always get it wrong. When comparing their very different experiences on the SSB radio or in person, some of these crews will come to believe that they have (or do not have) an accurate method of estimating slack. However, the truth is that all of the main methods will
1. How easily can water get in and out of the atoll? Water can come in and out through passes — navigable or false — and any broken or submerged areas of the reef forming the atoll. All other things being equal, an atoll with more
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reef that may or may not have one or more navigable passes. All things being equal, with the normal tidal flood, water comes into the atoll through any passes, breaks in the reef or low points, and — with the normal ebb —water exits the lagoon through the same channels. At its most basic, the strength of the current through these passes depends on two factors:
deeper and wider passes and more areas of broken or submerged reef will have less current. 2. How much water is inside of the lagoon? The amount of water in the lagoon is influenced by the range of the tide at that time. While the tidal range is not large in French Polynesia, the larger the range, the stronger the currents will be in the passes. The amount of water in the lagoon is influenced by the size of the atoll. Even if the tidal range is not incredibly large in the Tuamotus, in a lagoon 35 nm long and 20 nm wide (e.g., Rangiroa) those few feet of tide add up to a lot of water. All other things being equal, a larger atoll will have stronger curOCTOBER 2015 OCEAN NAVIGATOR 21