When you take a mooring, you have to think about evasive action. We also talk about turning radius. Indeed, when you drop anchor, you place your boat facing the wind and let the chain settle to the bottom, the boat then reverses into the wind bed.
Steady wind, nothing moving
It's all right as long as the wind stays steady. There are just a few small ripples which are slowly moving the boat to the right or left, but not enough to disrupt the aperitif. And when the sun goes down and you set off with your mind at rest on your bunk, the wind is always stable in the same direction, letting your boat pull gently on its anchor.
Beware of night breezes or current
But a good sailor has foresight. He will have taken into account, for example, that the daytime breeze reverses in the middle of the night. And that the wind coming from the open sea at sunset will come from the land when the land has cooled down faster than the sea. The wind will therefore turn 180°... Another case in point is the reversal of the current, which every 6 hours totally changes the anchorage configuration.
A circle with the anchor in the center
In each of these cases (and in many others), avoidance must be planned for. This involves drawing a circle with the anchor as the centre and the length of the chain plus the length of the boat as the radius. This circle represents the area that the boat can potentially cover by turning around its anchor.
Beware, in tidal waters, the tidal range decreases (high tide) or increases (low tide) the turning radius.
Same mooring length for all!
The ideal is to anchor alone in the middle of your circle. But it is often difficult because of the crowded anchorages, especially in summer. This is why it is particularly important that boats in the same area anchor about the same length of chain. This way they can all have the same turning radius and always stay at the same distance.
Theory sometimes far from practice
But this is rather theoretical, as not all boats avoid in the same way. A catamaran, for example, tends to "tack on its chain" exposing one hull after the other. A launch with little foot in the water and a big dunnage will move faster with a change in wind, whereas a boat with a keel in the water will be the first to be hit by the current. All of this can create a nice jumble of scenes that can sometimes be comical, but which have the advantage of creating encounters..
