Sunday, May 9, 2010

Destination: Hao, in the Tuamotus

In anticipating our arrival at Hao, the next thing we are sweating about is when we will be able to enter the pass. Most of the Tuamotus are sunken volcanos--atolls with large fringing reefs that totally enclose the lagoon, and one or two passes that a big boat can use to enter. As you might imagine, the small pass on a big atoll can get a pretty good current going, so you have to time your entry/exit for a time of slack current. Fortunately the tidal range here is only about 2 feet.

The challenge we face in calculating the best arrival time is twofold. Exactly when is high/low tide and therefore when to expect a slack current. And what effect will the wind and seas have on things. (and third, how long we feel like hanging out outside waiting...)

We learned that at Hao, when the wind has been blowing hard for a few days, there is NEVER a slack current--the current is always outgoing at 5-10 knots!! This is caused by the waves breaking over the windward side of the reef, filling up the lagoon, and the only outlet is the passes, which are typically on the downwind side of the reef. Our friends on Visions, with a 62' boat that motors comfortably at 8-9 knots, went in without paying too much attention to the slack current issue, and got pooped by a huge standing wave (first time in 5000 miles of voyaging). They have a pretty good blog post about it (, about 10 days ago).

So the reason we left Gambiers in 'boisterous' conditions was to arrive at Hao when the wind was easing. We're hoping there will be a slack current at some point.

We use WXTide32 (freeware downloaded from the web) to figure our tides. Generally it is pretty accurate. But for the Tuamotus, there are only 2 tide points, and there is some confusion about what time zone the tide points are on. For Hao, there are actually 2 tide points--basically the same one but on 2 different timezones. One is labelled '-10', meaning, we think, UTC -10, which is the time zone that Hao is on (one further west than Hawaii). The other is labelled Tz: Pacific/Marquesas. The tide times are 1.5 hours different! (For some reason the Marquesas are on a half a time zone).

But knowing when the tide turns doesn't always tell you when the current will be slack. We have a reference from the Pacific Islands Pilot that says "To avoid a difficult passage through the reef, vessels should wait for the two periods of slack water associated with the flood current, which are short. Slacks usually occur about 4.5 hours and 2 hours before moonrise; and again 5 hours and 3 hours before moonset. When the tidal race slows or stops, the channel may be entered".

So this is what we'll be basing our slack current information on. (Plus the advice of Visions of Johanna, who was at Hao for a few days).
At 5/7/2010 6:39 PM (utc) our position was 20°03.82'S 138°42.00'W

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