Semi-Technical Spiel on Anchoring in the Tuamotus

Sarah would say that I have an unhealthy obsession with anchors and anchoring. She’s probably right, but I am nevertheless going to indulge myself by mentioning a little trick that we learned while we were in the Tuamotus and which I have become an advocate of. I apologise if I get carried away; if you’re not a boater (and even if you are) you might just want to skip this post!

The problem with anchoring in the Tuamotus is that although there are an abundance of beautiful places to visit, there aren’t really any proper anchorages. Furthermore, things can get pretty nasty if you find yourself on the wrong side of a lagoon when the wind shifts around to leave you with your stern to the reef and a few miles of fetch ahead, as we discovered last December in Amanu. The anchorages are generally between 15 and 25 meters deep (45 to 75 feet) and there are numerous coral heads that stick up every 15 feet or so. It’s simply not possible to find a nice, sandy, unobstructed bottom to anchor on, so even if you do manage to lay your chain out along the seabed without snagging anything you’re sure to wrap it around at least one coral head as soon as the wind shifts a few degrees. This is decidedly not great for the coral and also very dangerous for your boat. You can very quickly find yourself unable to raise your anchor and with very short scope such that the chain snatches at the bow and threatens to rip your bow cleats or windlass out of the deck, if it doesn’t snap first. You can always let out more scope, but how much do you have?

Back when we were in Hao last year we had the good fortune to meet some strong advocates of a technique that I had previously read about and mused on. We gave it a go. The idea is to suspend a portion of your anchor chain in the water column such that it passes over the tops of the coral heads as the boat swings, rather than wrapping around the bases. You need solid buoys ideally because soft buoys (such as fenders) will compress and shrink with increasing depth due to water pressure, whereas solid buoys maintain a constant internal volume and therefore consistent buoyancy. Fortunately such buoys are very easy to come by in French Polynesia because the pearl farmers use them and they frequently come adrift in large numbers. Just take a walk down any windward beach (East-facing) and you’ll be able to pick some up.

We started out by letting out the normal amount of scope for whatever depth we were in and then buoying an extra portion at the end so that if (when!) the chain on the bottom got wrapped we’d still have a good bit of slack to play with to stop the chain from snatching. This was fine but because it would still get tangled at the bottom I didn’t like the damage that was being done to the reef from the chain moving about all over the place. We played around with it a bit and have now figured out where on the chain to place buoys so that only the very last bit of chain stays on the bottom. Below is a series of pictures which illustrate the principle quite well (taken by Sarah of course 🙂 )

First section of chain (above).

 

 

 

Bottom section of chain. You can also see the abundance of coral heads scattered about. In fact, this was the best anchorage we found in the entirety of our time in the Tuamotus, at ‘7 reef’, Tahanea.

 

 

One drawback to this technique is that the boat moves about a bit more in light winds because there’s no weight of chain sitting on the bottom that needs to be dragged about, and which stops you from drifting about willy-nilly with every slight puff of breeze. Perhaps a more important drawback is that the effect of catenary (the sagging of the chain due to it’s weight, which helps to keep the pull on the anchor as close to horizontal as possible) is lost, but we feel that the advantages are far more numerous than these two slight disadvantages. I would argue that the effect of catenary is minimal in high winds anyway. If the buoys are arranged correctly then the angle of pull with buoys on the chain should be the same as it would be in high winds, when the chain would be pulled out straight anyway and it’s weight would only serve to increase the load on the deck gear, which normally has to support the vertical weight of the chain as well as the horizontal force holding the boat in place. With buoys, the anchor has to work a little harder than it might under normal conditions but this becomes less true as wind speed increases (and the chain loses it’s sag) until it virtually disappears at very high wind speeds (incidentally, tests have shown that a 7:1 scope is sufficient to maintain a sufficiently-shallow angle of pull on the anchor with a rope (near-zero-catenary) rode and that increasing scope further yields negligible benefit). Since the bow gear no longer has to support the weight of the chain (because the buoys now support this weight), the forces on your boat are considerably reduced in a deep anchorage. A large part of the vertical component of the force acting on your bow is removed; what remains is the horizontal component that is necessary to keep your boat in place. Thus, even in 25 knots sustained wind I can take up on the snubber line by hand, inspect the chafing gear for wear, make fine adjustments etc. and I sleep better because I don’t have to listen to the snubber line creaking over my head whenever a swell puts a sudden load on it.

We now use buoys in any deep anchorage regardless of whether there are obstructions on the bottom or not.

 

Finally, here’s one more picture that nicely illustrates what a moderate squall looks like. This boat was anchored astern of us as one came through. We were sheltered from chop behind a spit of reef ahead so there’s no sea running, but you can get an idea of the wind strength by the fact that the surface of the water appears to be smoking as the wind lifts the water into the air. This one was maybe 35 knots max. Not too bad.

 

 

 

 

 

Disclaimer: All of the above spiel is pure conjecture on my part. My practical Physics may well be lacking. If you think it is, I’m all ears for rebuttals!

The most beautiful place in the world?

Having visited two of the atolls in the Tuamotus at the end of last year I already had an idea that they are a sight to behold, but nothing could prepare me for the splendour of Tahanea. This deserted atoll is breathtakingly beautiful with unspoilt sand bars, pioneering palm trees, turquoise waters and white sandy beaches carpeted with shells. I always thought that those photos you see in travel magazines advertising idyllic paradise getaways to secluded tropical islands were a bit of a fabrication. Not so. I’m actually in one of them, it’s the real deal and it’s just absolutely beautiful. But don’t just take my word for it…. have a look at these:

Tahanea after a spot of rain

 

Our anchorage near the main pass

I think these are Spinner dolphins. They came to say hello during our sail to Seven Reef at the south of the atoll

Anchored on our own at Seven Reef

Sand bar at Seven Reef

What’s that strange new species?

As you might expect in any deserted island, we spend our time snorkelling, spear fishing and coconut gathering. Tahanea is uninhabited apart from a small settlement that’s occupied for only four months of the year for the copra business. It might be uninhabited at the moment, but Tahanea is not much of a secret with the cruising community as there were already three sailboats here when we arrived. Not that this hindered us in any way, the presence of other sailboats inevitably led to new friendships being formed. Also, we managed to find a private place to anchor completely by ourselves for a number of days in a very protected area in the south of the atoll known as seven reef (as it looks like the number ‘7’ from the satellite imagery). This is the most stunning place I think Bob has ever anchored and the snorkelling around here is just fantastic.

The bird life here is also very special. Alex noticed a sign written in French which might have said that Tahanea is part of a wildlife reserve. Then again, it might have said that all people are free to hunt, kill and eat all the birds – we wouldn’t really have been able to tell the difference! Having said that, we were lucky enough to see what I think is the endangered Tuamotu sandpiper. I noticed a small brown bird that looked like it was an imitating a sand piper but with a shorter beak and seemingly more interested in the scrub habitat near the beach rather than the beach itself – more like a typical insect feeder. The bird also happened to have some rings on it’s legs. I had to do a double take as I thought it was very strange that bird ringing would be going on in this incredibly remote location, and this species certainly didn’t look like any of the usual sea birds I was used to seeing migrate across the oceans. Anyway, I read later on in the Tuamotus compendium that this bird is resident on Tahanea and is a highly endangered species endemic to the Tuamotus. Unfortunately I didn’t get a photo, but I did get some of these crested terns instead. Not in the slightest bit rare but their lives of long distance migration across the oceans is impressive nonetheless.

Crested Terns

Crested Terns