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 🙂 )

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!