Anchoring Around Coral

The Tuamotus Diaries #13, Day 27 – June 24^th 2016

The Tuamotus are a quite special place.

The Atolls are (relatively) safe heavens in the middle of the vast ocean.

The water is often super clear and transparent.

The rims of the atolls are composed of reef and sometimes islands. These islands, called “Motus”, are white sandy and lined with Palm Trees.

And the sea bottom is covered by a clear white sand. And coral heads.

The coral heads are everywhere. They are formations of coral that come out of the bottom in varying sizes. Some are small – a few feet wide and tall – while others are so big that even constitute navigational hazards.

When choosing a place to anchor, it is often nearly impossible to find an area where the sea bottom is completely free of coral heads. And when they are around, chances are that the anchor’s chain will get tangled around them.

As the boat swings on an anchorage to the direction of the wind, the chain swipes the bottom of the bay, and gets tangled on the coral heads in the process. At one occasion, we had our anchor wrap a full 360 degrees around a tall coral head.

There are a few problems associated to wrapping the chain on coral. First, the chaffing of the chain against the coral grinds off the galvanizing of the chain, thus accelerating its decay to corrosion. A more short-term problem is that when the chain gets wrapped, the anchor becomes ineffective and it is now the coral head that is holding your boat. Wouldn’t be much of a problem, but now the chain is substantially shorter as well – and thus more prone to breaking – the chain or the bow fittings – as a consequence of shockloads. Moreover, if the chain wraps too tight around the coral, it may be difficult to take it out. In our case, our chain is too heavy to be handled, and there is very little we can do to manually untangle it from a coral head. Finally, and not the least important, having the chain rubbing against the coral damages it badly. Not nice at all.

One solution I’d read about and didn’t think we would be employing is to set the anchor hanging ABOVE the coral at anchorages. Essentially what you do is to position flotation devices at set points along the chain’s length, and hang the chain off them. Instead of resting at the bottom, the chain now floats somewhere between the bottom and the surface. If done properly, it will work free of rubbing any coral as the boat rotates around the anchor.

It’s somewhat of a handful, but after using it several times, and in even rough weather, we are quite satisfied with the results. Here’s what we have been doing:

• try to arrive at the anchorage in conditions of good visibility
• scout the anchorage for a place with fewer – or at least lower – coral heads
• whenever possible, drop anchor in places no deeper than 30 ft (that’s the depth I can comfortably reach the chain at the bottom and tie a line to it).
• Drop the anchor and keep it approximately 5 feet ABOVE the bottom, hanging by the chain
• Let the boat move slowly by the wind until the anchor is hanging over white bottom – sand (by doing so, we avoid having the anchor land on coral at the first place, which would require resetting it altogether)
• Set our desired length of chain – usually a 4:1 ratio has been working fine.
• Try to dive as quickly as possible. Scout the area around the chain, using the anchor as the center of rotation, and by doing so, chose which coral heads pose the greatest probability of grabbing the chain. This will tell me which points of the chain need to be lifted, and by how much
• Tie fenders (floaters) to the chain at the points defined above – in our case, we’ve found that 4 fenders work well in keeping the chain away from coral. This is an iterative process, as each new fender set will alter how the previous one was working. After having done it a few times, it takes me approximately 1:30hrs to set four fenders along the chain.
• I prefer to set the fenders in a way that they are still visible on the surface. This way, other boaters won’t sail on top of them. Moreover, it is then easy to check if the chain is unwrapped (if all fenders are aligned, the chain is free. If it wraps, the fenders will form a zig-zag on the surface)

When it is time to raise anchor, I bring the chain in slowly, and untie each fender as they come up on deck.

Again, it is a bit of a handful, but I prefer to do it just after anchoring than running the risk of having to untangle our heavy current when in the need to leave. Or to have it seriously wrapped around coral during bad weather risking serious damage to the coral and to the boat gear.

 

 

Now, workload apart, does it work?

I will start by saying it does work in keeping the chain above the coral heads, if set properly.

But what about holding power? Does it get affected?

It is true that the chain is now pulling at the anchor at an angle – not just horizontally when on normal circumstances. So, I believe the anchor has to work harder. However, I have observed that the chain still provides substantial shock absorption while hanging from the fenders. There is quite a bit of catenary forming in between the fenders, acting like a “spring” absorbing the shockloads that would be transferred to the anchor. Moreover, in our case – and now it is a specific one – our anchor is well oversized for Pesto’s dimension and has plenty of holding power left to hold us.

The fact is that this set up has held us in place for over two months being anchored this way, on anchorages that have become wavy in winds of up to 40 knots on gusts.

One drawback of this system is that the boat tends to “sail” at anchor much more. With the chain floating free from the bottom, the boat has much less resistance to move laterally. And it does! Over these two months of anchoring with a “floating chain” Pesto has zigzagged behind its anchor, like being impatient to be sitting still.

 

POST SCRIPTUM: After having written this post I tried to deploy the anchor, chain AND the floaters WITHOUT having to swim and dive, and found it is actually possible. Using a bit of geometry, and assuming a given depth and chain length, I calculated the length of line that would be required for each floater to hang the chain just above the imaginary straight-line between the anchor and Pesto’s bow. With that, I was able to tie the fenders to the chain as I deployed it from deck, and the end result was satisfactory. It does require a bit of deskwork beforehand (the geometry calculations), then there is the preparation on deck (setting each floater with the corresponding line length), and the deployment of the chain gets a bit more complex (each floater needs to be tied to the chain at specific lengths). But I still saved 1:30hrs of work on the water, and several dives to the bottom to tie up and set the fenders. It was well worth it.