It depends, of course, on where you spend your time sailing and anchoring. In the Mediterranean and the Caribbean, 60-80m is usually more than adequate. However once you transit the Panama Canal on your way across the Pacific, you will definitely want to consider putting on more.
In anticipation of doing our circumnavigation, I specified 120m of chain for our yacht when she was built. The shipyard suggested (graciously!) that was too much and all the other yachts they’ve built have put no more than 100m on. Their premise was the chain weighed a lot and carrying more chain than you would ever need would hurt performance in the long run. Sound thinking for a yacht destined to rotate between regattas in the Mediterranean and the Caribbean but, in my opinion, not for blue-water yachts planning to circumnavigate the world.
As it turns out I was mostly right. Since we arrived at the Galapagos Islands last April and, so far, all the way across the Pacific to the Kingdom of Tonga. We have had mostly very deep anchorages. Usually, we are happy to see 30m of depth to anchor in while still keeping us far enough away from hitting a reef or the shore. Sounds crazy perhaps but if you look at the charts you’ll see most of these Pacific Islands are the peaks of ancient volcanos and come off the seabed from thousands of meters to a sharp point (i.e. the island above the water level). If you are looking for a 15m depth to anchor in you need to have a draft of 2.5m to be able to have the ‘swing-room’ you need in case the wind shifts. Even then, you will be hard pressed at times to find shallow anchorages like that. For a yacht like ours with 4.0m draft, we need to be much further out.
So, getting back to the length of anchor chain you need. If you have a draft of 2.5-3.0m I would put no less than 100m of chain on. For a draft of 4.0m (like us) actually 120m has been short sometimes. There have been a number of situations where we would have liked to have another 20m of chain. So, when we get to New Zealand, we’ll put that 20m on to have a total of 140m available for our next season in the western Pacific. The comfort of knowing you have a safe anchorage with good swing room and an solid connection with the bottom trumps the extra weight for 20m of chain! Tonga has been the most challenging for anchoring as all the islands have very steep drop-offs and we consider ourselves lucky to find a small number of places where we can anchor comfortably.
Perhaps a number of readers will say “Well that’s what you get for having such a deep draft” and the deeper draft does affect our choices for anchoring locations however its big benefit is the performance to be able to make 240nm days on passage and average 10kts so, it’s a trade-off we are pleased to be able to make.
To be honest, I thought the deeper draft would prove to be more troublesome than it has turned out to be. Now, just get me another 20 meters of chain and I'll be really happy!