When it comes to dropping the anchor, most of us follow the usual rules. But besides the general provisions, wind and tidal currents must also be taken into account. Anchorage is a key part of a cruise sailor's arsenal - at least for those who are not going to take shelter in the marina every time they make a stop. Therefore, it is so important to rely on reliable [...]
When it comes to dropping the anchor, most of us follow the usual rules. But besides the general provisions, wind and tidal currents must also be taken into account.
Anchorage is a key part of a cruise sailor's arsenal - at least for those who are not going to take refuge in the marina every time they stop. Therefore, it is so important to rely on reliable and adequate knowledge about how and how much of the anchor chain must be given in a given situation. It is often difficult to find clear and comprehensive information - on this issue, different experts argue with their vision. I will not pretend to any special "expertise", but I will share my personal experience of sailing.
We usually need a convenient rule of thumbwhich can be easily used in most cases for safe anchoring... Of course, by its very nature, such a rule cannot cover all situations. But it is surprising that many boaters overlook some very important considerations simply because they find it difficult to find a simple working formula.
Everyone has their own idea of how much anchor chain to use. The simplest - and perhaps the most frequently used method - why leave the chain in the box at all, are we wasting it ?!
In fact, this usually means using the maximum safe length - at any anchorage there are rocks, shallows and other boats either already at anchor or about to do so. Give little - can pull, give a lot - the risk of an overturned chain increases. Therefore, the length should be sufficient. The question, as usual, is how much is "enough"?
So, it is necessary to determine what is safebefore you anchor. Traditionally boaters use a multiple of water depth to determine the length of the anchor chain to use. RYA offers at least 4: 1, others say that 7: 1 is needed, but in crowded parking lots and 3: 1 is quite common.
Practice, however, says that having a changing environment, one cannot fully rely on the rule if it does not sufficiently take into account the main forces acting on the boat. Namely wind and tidal currents.
Often it is the wind will be the biggest problem, and this must be taken into account. And also be prepared for the maximum expected wind strength. And there is one problem: there are very few materials that would tell how to take into account the strength of the wind when setting up the anchor.
So I ended up with a very simple guide, an empirical rule that takes wind and currents into account.
If you see nothing more than the upper limit of four Beaufort points (16 knots) and anchor a 10m yacht in fairly shallow water (less than 8m deep) you should end up with 16m + 10m = 26m chain. That is, the calculation is as follows: by the meter of chain for each wind knot plus the length of the boat... But if you expect the wind to increase up to 7 points (33 knots), try setting the chain 33m + 10m = 43m. This rule of thumb works in most anchors relatively close to shore where the water is quite shallow, but for deeper depths (around 10-15m) you will obviously need more chain.
How much more? The answer to this question is simple: you need to use a factor of 1.5 to the wind speed to ensure the best result.
So here the first formula (which only works for shallow depths, 4-8 meters!):
Chain length required = wind speed in knots + boat length in meters.
The second formula for depths (10-15 meters):
Required chain length = 1.5x wind speed in knots + boat length in meters.
Above 15 meters, use a factor of 2.
Using these formulas, it is easy to see that the uncritical principle of 3: 1, 5: 1, or 7: 1 does not always work. So, for a depth of only 4 meters, but with 5 Beaufort points (21 knots), for a 10-meter yacht we need 32 meters of chain. That is, the ratio will be 8: 1!
Length calculation chains: a rule of thumb
- Simple equation. The most basic equation is wind speed in knots + boat length in meters = chain length in meters. This works for depths up to 10 meters. Beyond this depth, use a factor of 1.5 to the wind speed.
- Calculating the impact of tidal currents... The tide must also be taken into account by converting the tidal force applied to your boat into a relative wind speed. Using a motor with different headwinds and then correlating the speed of the boat at the same speed in calm conditions can provide a surprisingly accurate understanding of how the speed of the current affects the boat (more on that below).
- Let's add something else. It is advisable to add approximately one additional boat length to our calculations to provide sufficient chain weakness to resist the yaw that results from pulling the anchor up.
- Plan ahead. Make sure you consider not only the conditions at the time of arrival, but what they might become. Will the wind forecast increase significantly? Is it likely that the anchorage will get significantly more busy while you are there? How many tidal currents can pass when the tide changes?
The actual holding force of the anchor is not taken into account here. This is important and discussed in many other articles.
How to choose an anchor for your boat
Different types of anchors: pros and cons
The traditional "fishing" anchor is flat, takes up little space and is easy to store. It grips well on rock and grass, but weak feet will slide on any other bottom, making it too risky for a main anchor.
CQR, Delta and Kobra II anchors can pull under extreme heavy loads, plowing the seabed in soft sand or silt. But in general, these are anchors with a high holding capacity.
The original Bruce has been out of production for many years and many copies have been produced, often from poor quality, fragile and weak materials. The original anchor is said to hold well on soft to medium soils and cling to rock, but not grass.
Danforth, Britany, FOB, Fortress and Guardian have a large surface area for their weight and hold well on soft to medium ground. On hard bottoms, such as compacted sand and pebbles, they can slip and generally do not “switch” when tide or wind changes direction of thrust.
This category includes Bügel, Manson Supreme, Rocna, Sarca and Spade. Their designs have increased holding power and are designed to continue to hold the boat even when the wind or tidal current changes direction.
The second force acting on the boat is tidal currents... And you can easily measure it yourself.
On a windy day, move slowly against the wind, screw on the engine and find the rpm that just balances the wind force. Then, on a calm day, notice the speed of the boat at the same revolutions.
For example, my boat in the wind at a full 4 Beaufort points (16 knots) required 1200 rpm to balance its power, and in calm conditions at 1200 rpm the speed reached 4.2 knots relative to the ground. Thus, a tidal current of 4.2 knots would correspond to a 16 knot wind, which requires a 16-meter chain to balance it, that is, about 4 m of chain per current knot.
So now we have a more complete formula.
Anchor chains are usually marked at 10 metersso a practical approach is to round the calculation to the nearest 10 m of the circuit.
With an added boat length of 10m, this gives an easy-to-use table (below).
It seems odd that most articles on anchoring pay so little attention to how to take into account the strength of the wind and current.
Yes, there are great articles on anchoring in general, but few attempts have been made to apply this to sailing practice. Hopefully this material will provide more clarity on how to find out the appropriate length of the anchor chain under various conditions.
In fact, it is not that difficult.
Wind and tide formula for shallow waters (4 to 8 m):
Chain required (m) = wind speed (knots) + 4 x current speed + boat length (m)
Calculations based on a 10.4m boat Jeanneau espace, chain 10 mm thick. The values will be reasonably similar for most production yachts, provided the chain size increases with the size of the boat. Don't forget to use "Depth factor" and may the power be with you!
Author: Christopher Smith
What are the types of anchors? How much does a good anchor cost? Which type of anchor is suitable for which soil? Do I need a spare anchor?
Watch our detailed video review of all types of anchors available on the market. In this video, Sasha Goron answers all possible questions about anchors for your boat:
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