The Tuamotu Archipelago
The Tuamotus were called "The Dangerous Archipelago" by seafarers prior to satellite based navigation because they rise up so suddenly from the deep sea floor and are mostly awash, therefore are very difficult to see until you're dangerously close. Many vessels have been lost on the reefs of the Tuamotus. Even in this age, several vessels a year are washed up and stranded on the reefs.
The Tuamotu Archipelago consists of a group of 78 coral atolls (2 of them are not atolls, but islands) spread over a large amount of ocean between the Marquesas and the Societies. An atoll is a narrow perimeter reef with a lagoon in the center. The reef is awash, and is growing around the cone a long-sunken volcano. The coral growth keeps up with the rate that the volcano is sinking/eroding and the outside wall of the atoll is nearly vertical, rising thousands of feet from the seafloor. Small islets (motus) form, mostly on the windward side, where broken coral and sand has had a chance to pile up (and hasn't been washed away by a storm). The motus are the only habitable space, and are seldom more than a few feet above sea level. The inhabitants subsist by growing copra (coconut meat dried and exported to be made into coconut oil - used to make soaps and cosmetics) and farming black pearl oysters.
The water in the lagoons is exceptionally clear because there is no runoff or silt - only coral. Many of the atolls have "passes" through the reef where water flows in and out of the lagoon due to the tide. Mostly the flow is outward, since water is constantly added to the lagoon by waves washing over the reef on the windward side. Depending on the pass, the currents can run 6kn or more, and entry has to be carefully timed to coincide with slack tide. Some atolls don't have passes, or they aren't deep enough to allow a sailboat to enter. Most atolls have coral heads in the lagoon. A good lookout must be kept while moving a boat around inside. In Kauehi the depth of the lagoon was 100-130 ft, yet coral heads would rise up to the surface... rather disconcerting to be reading 130 ft on the depth sounder and passing an awash coral head 20ft away! Moving the boat around requires good light - the sun high in the sky and behind you. Also getting up high above the water helps, so most of our moving around had Kellie at the helm and me perched on the spreaders (half way up the mast). The water is so clear, they're easy to spot when the conditions are good, but if a cloud obscures the sun, or you have to turn so that the sun is in front of you, they suddenly disappear. Needless to say, timing an arrival from an ocean passage to coincide with slack tide and good light is next to impossible.
When we left Ua Pou in the Marquesas, we were heading for Manihi and Ahe at the northern end of the Tuamotus, a passage of about 450 miles. Part of the way there the wind became a bit light and was too far behind us, so we reconsidered our destination and headed for Kauehi instead. This had the advantage of bringing the apparent wind forward, therefore increasing it and, correspondingly, our boat speed making for better comfort. The passage length increased to around 550 miles, but would take about the same amount of time since we were going faster. We picked Kauehi for several reasons. "Ferdinand", sailing friends of my parents, recommended it as a nice place to hang out, but also because it was one of the few that had an easy pass, a lagoon relatively free of coral heads, and motus on the windward side. Motus on the windward side means that you can anchor in behind them and they give a bit of a wind break. If you anchor on the other side of the lagoon to be near a leeward side motu, the waves build up across the lagoon, making for a rollier anchorage and wet dinghy rides.... and you're on a lee shore, meaning if something goes wrong, you blow onto the beach. Kauehi turned out to be a fabulous place, and fortunately was also picked as a hang-out destination by a bunch of kid-boats that had come through the Panama Canal together. When we arrived there was only one other boat in the lagoon. By the time we left there were 15 boats anchored in a group in the uninhabited south eastern side of the lagoon, at least 8 of them had kids!
We had a marvelous passage with the exception of motoring for about 6 hours somewhere about 2/3 of the way along. We arrived under sail at the pass just before the sun was coming up, which coincided with a 6am low slack tide. We tacked about 1 mile from the pass and were able to sail right in. The current doesn't quite correspond with the tides, and we had 3-4 kt against us in the middle of the pass. We were sailing close-hauled, which means that we were going as much up-wind as possible and the swirling currents made things interesting. At one point we were actually sailing backwards while doing 4kn thru the water! Normally if you need a bit more speed, you'd fall off the wind (turn away from the direction the wind is coming from) slightly, but this was not possible because the pass is only about 200ft wide and we were trying to stay in the middle of it. We anchored for a few hours just inside the pass to wait for the sun to get higher, allowing us to see the coral heads and move across to the other side of the lagoon. Wyndeavor arrived about an hour after us and had 2-3 kt of current WITH them - they came flying thru the pass!
Mantas eat plankton - it took awhile to get used to them swimming towards you with open mouths big enough for one of the kids to fit in! I saw one with a remora in its mouth... when it closed it's mouth to swallow, the remora swam out, then right back in when it re-opened!
Unfortunately, our digital camera went for a swim just before we left Kauehi. I disassembled, rinsed and dried it, but it only beeps now. We've asked Kellie's friend to order us another and bring it with her to Tahiti, along with the waterproof housing she'd already ordered.