You do need to understand two basic principles about anchoring. The first is that the task of the anchor is to keep the end of the anchor chain, not the yacht, anchored to the same spot. To do this, it has to be properly bedded in.
The second is that there needs to be enough weight of anchor chain on the sea bed so that, no matter how strong the pull from the yacht, the last few metres of chain always remain on the sea bed, so that the pull on the anchor is always horizontal. No matter what depth you're in, the pull transferred from the anchor chain to the anchor should always be parallel with the sea bed.
In absolute flat calm, with no tide, the anchor chain will leave the sea bed at almost 90 degrees, but there should always be a length of chain lying along the sea bed. As the wind gets up, or the tide starts to show its strength, the yacht will pull against the weight of the anchor chain, causing a curving effect, known as a catenery.
This curve is essential. For the anchor to hold properly, the horizontal pull on the yacht at the surface needs to be transferred through the catenery of the chain to a horizontal pull on the anchor at the sea bed. In other words, you need enough weight of chain lying along the sea bed so that the catenery does not end at the anchor, but a few meters back from it.
No matter how strong, the pull from the yacht should not overcome the weight of those last few metres of anchor chain, so that the pull on the anchor always remains horizontal, parallel with the sea bed. If the pull increases to the point where the whole chain starts to leave the sea bed, then the chances are that the anchor will break out and drag.
So the stronger the wind or tide, or the deeper your anchorage, the more chain you need.
Anchoring depths can be made to seem more complicated than they need to be. Really, all you need to know is how much water you want under the keel at low water, and how much the tide will drop by then. Here's how I approach the business:
- I find a promising looking sheltered area on the chart, making sure that I'm not on a lee shore, and note the composition of the bottom to check the holding
- Next, I decide what depth I need under the keel to stay safely and confidently afloat at low water, given that the yacht will swing around the anchor. I check the chart to get an idea of the slope of the sea bed towards the shore. Then I decide on a safe depth. Let's say I choose four metres at low water
- Next, I look at the tidal curve for the day, and check to see how much the tide will fall between now and the next low water. It doesn't matter if the tide is rising - it is the drop between now and the next low water that matters. Let's say the tide will drop two metres between now and low water
- I add the two metre drop to the four metre safety margin, which gives me six metres
- I simply sail into six metres of depth, and anchor. If I can see the bottom I can pick my spot - if not, I take pot luck. I can then be confident that at the next low water I'll have four metres under the keel. Simple
Once you know how much depth you want to anchor in, you can decide on how much scope to lay out. It's a hotly debated topic, but I usually use five times the depth of water for chain. Some say that's a bit of over-kill, and leaves me vulnerable to a wide swing. But I've found it works for me.
For all the reasons stated above, I prefer to have a bit more chain on the seabed - and most yachts will swing together (unless you have the misfortune to be anchored close to a catamaran, which can lie differently). Here's how I approach the scope:
- Decide on the scope. I use 5:1 with chain
Next, check the tidal curve to see how much the tide will rise between now and the next high tide, even if the tide is falling. Let's say the tide will rise one metre between now and the next high tide
- Add this figure to your anchoring depth - the depth on the depth gauge at the point you anchor. I am anchoring when the depth gauge reads six metres, so I add one metre for the one metre rise, and that makes seven metres. Five times seven is 35, so I will flake out 35 metres of chain - or read off 35 metres as I anchor
That's all there is to it. Except for one thing. Once the anchor is down and the required amount of chain has run out over the bow roller, I secure the chain and motor very gently astern. You don't need anywhere near full power - just enough to pull a good catenery into the chain and make sure that the anchor is well bedded in.
Too hard astern with today's powerful yacht engines, and the chain will go bar-taut, and you will pull a perfectly well bedded anchor out again. A good way to check that the anchor is bedded and you're not dragging is to put just a few revs on astern, wait until the yacht holds against the catenery, then watch the catenery carefully.
If it holds its shape, you can be fairly sure the anchor is well dug in. If, however, the catenery changes shape, dipping and forming, then the anchor is likely to be dragging.
If you think it's dragging, there's little option but to pick a different spot and try again. But if you've got a good anchor, and have chosen your spot carefully, nine times out of ten it will be fine.
Richard Thomas holds a Commercially Endorsed RYA Yachtmaster and Cruising Instructor (Sail) certificate. He runs his own delivery business, www.yachtmovers.co.uk
He is available for deliveries, assisted passages, own-yacht tuition, and yacht management.
