Constant calculations

Thu 29 May 2014 23:11
16:03.49S 145:37.28W

We're here! In the magical Tuamotus, the legendary "Dangerous Archipelago", the mysterious atolls that have been the graveyard of many a vessel...

Atolls are islands that  are simply rings of coral, enclosing a lagoon. They vary in size from less than a mile across to more than 30miles. Until Charlie Darwin figured it out they were a real mystery to geologists, they couldn't work out how they had formed, nor how they survived in the middle of a raging ocean. Charles figured it: they used to be the ordinary sort of dramatic volcanic islands, like the ones we saw in the Marquesas. Gradually a fringing coral reef built up on the steep shores. Then, over many centuries, the islands gradually subsided, as they dropped the sea level effectively rose, wearing away at the island and gradually leaving just the mountain peaks exposed as isolated islands. Meanwhile, because the subsidence happened slowly, the coral kept pace with the rising sea level, growing up to the surface each time the land level dropped. This created an encircling ring of coral, marking faithfully the island's original coastline. Eventually the island peaks in the middle of the atoll wear away completely, leaving just the continuously self repairing live wall of coral, the atoll.

We spent the day before we arrived mostly calculating. The atolls have occasional breaks in the reef, echoing where rivers ran down to the sea on the original island (the sediment stopped the coral growing), these passes allow one to sail into the lagoon, into the shelter of the surrounding reef. The problem is, when the tide rises, the sea water pours in through these gaps, often at more than 6 knots, and when the tide drops the water flows back out at a similar rate. When there's been a big swell or high winds the effect is worse, as the surf splashes over the reef filing it even more, so there's extra water to rush out of the pass. So the trick is to try to calculate slack water, when the tide is turning, and go through then. But this varies according to how much wind and swell there's been, and for how long, so takes some calculating! Happily I'd downloaded a spreadsheet model of the tides, made by another cruiser. You put in the atoll and pass, date and estimation for extra outflow due to swell and wind effect and it produces a nice graph showing tide speed over time.

Having figured out when the slack time is likely to be, we then had to try to time our arrival to a little before. So more calculations, based on distance to go, current speed, likely wind conditions etc. All this boiled down to us deciding to slow down as much as possible, skip Kauehi where we were going to go to first, and arrive outside Fakarava (yes, the name makes me giggle too! There's a place called Kuviruviru here too. Try saying that when you're drunk!) around daylight where we'd heave to and wait until the tide was right. We didn't want to wait around too long as it was a 25 knot wind and the low atolls don't give much shelter, and we wanted the end of an outflowing tide, because an inflow would give wind against tide and you can get high standing waves in the entrance.

This meant that we were sailing through islands just a few feet above the surf in the dead of night with no moon, strong winds and unpredictable currents. All the flowing in and out of the atolls, and the swell sweeping around the outside of them, means the archipelago is well known for strong currents in unexpected directions. Not ideal conditions but we were confident that our charts were up to date (unlike in the San Blas!) and I was relieved to find that our radar picked up the low land really well. 

Radar echo of Kauehi as we passed. The radar screen is "heads up", faces in the same direction that the boat is facing, but the plotter is North up, so look along our track line to get the same orientation.

Come morning we could see the land by eye:

Land Ho! See it? On the horizon, left hand side...

Ok, this is taken from an enlargement of the same photo:

See it now? Just a line of surf topped by coconut trees, stretching on and on...

Whilst we waited outside the pass for the right moment another boat came out, with the ebbing tide. We radioed them and got an estimate of the current's speed at that moment, which we could then correlate with our graph to correct our estimate for wind/swell. So, the moment came and through we went.

Approaching the pass. You can see the white caps where the wind and tide is flowing, and the far shore of the lagoon in the distance.

It was just fine gong through, a little bounce from the slight wind-with-tide against us but that's all. Here's a little video clip taken just after the initial steeper waves:

Approaching the line.

And then we were in, all the anticipation and preparation over, keeping a close eye for coral heads, motoring towards the anchorage.  Once anchored we went ashore, and stood on the narrow strip of sandy coral, just a few hundred yards wide, that these people make their home. Look one way and there's the deep blue of the ocean, a thousand feet deep only a mile from shore, with swell breaking white constantly upon it. Look the other way and there's the pale blue of the sheltered lagoon. Magical.