The captain's confidence in the water is directly dependent on the reliability of his berthing, and many beginners make the same mistake when they buy a boat: they buy an anchor by eye, relying on the advice of sellers or visual weight assessment, an approach that can lead to the loss of a boat, especially in a night storm or on the current. Anchor calculation for the boat This is not just an arithmetic action, but a critical stage in the preparation for navigation, which requires taking into account many factors.
Unlike large boats, small fleets are much more susceptible to wind and turbulence, and a light inflatable boat or small boat can be demolished even in moderate winds if the wind is not too strong. retaining The anchor is not enough. The right gear ensures that you stay in place even if the weather changes dramatically. Ignoring the calculations often leads to situations where the anchor simply drags on the bottom without catching the ground.
In this article, we will examine the physical principles of anchors, examine the proven formulas for calculating the weight and length of anchor-rope, and analyze the impact of soil type on the reliability of parking. Understanding these processes will help you avoid costly mistakes and ensure the safety of your vessel. We will move away from myths and turn to proven data and practical experience.
Physics of the process: how the anchor holds the boat
To calculate anchor parameters correctly, you need to understand how it works. An anchor doesn't act like a hook that just hangs on the bottom. It's about burrowing into the ground. Holding power It only occurs when the legs or teeth of the anchor are immersed in the bottom to a sufficient depth, and for this, the force transmitted through the anchor-trail must be directed horizontally, parallel to the bottom.
If the angle of separation of the anchor from the ground is too large, the paws will not be able to catch and will only slide on the surface, which is why the key element of the system is not so much the anchor itself, but rather the anchor itself. anchor-chainA heavy chain at the bottom holds the rope and provides the necessary horizontal force vector. Without sufficient chain length, even the heaviest anchor can be useless in strong winds.
There's a concept of angle of attack, which depends on the design of the anchor. anchor, denforth or AdmiraltyThe slugs require different forces to initially bury the anchor, and once the anchor is buried, it is held by the mass of soil above its paws, and the destruction of this "ground wedge" causes the anchor to break down, so the weight calculation must take into account not only the weight of the boat, but also the area of sailing created by the hull and superstructures.
⚠️ Warning: Never tie the anchor end directly to the bow duck in inflatable boats without using a bursting element. In a sharp jerk (such as a wave), the boat can flip over or damage the hull. Use special anchor lears with shock absorbers.
It's also important to consider the inertia of the boat. When the wind is gusts, the boat shifts, and at this point the load on the anchor increases many times. Dynamic loads can be several times higher than static wind pressure, which is why the safety margin in the calculations is a requirement, not a recommendation.
Calculation of the weight of the anchor: formulas and standards
There are several methods for determining the required weight of an anchor. The most common and time-tested formula is that which takes into account the displacement of a vessel. For small vessels weighing up to 500 kg, the weight of an anchor is usually about 10% of the displacement, but for heavier boats this proportion varies, and a more accurate calculation is based on the sailing area and wind load.
The classical formula for calculating the mass of anchor (M) in kilograms is as follows: M = k * √W, where W is the displacement of the vessel in tons and k is the coefficient depending on the type of anchor. Bruce or Rocna) the coefficient may be lower than for older structures, but for amateur use it is easier to use tabular data adapted to the length of the case.
The following is a table showing the weight of an anchor as a function of the length and type of vessel, and these data are relevant for temperate climates and mixed soils.
| Type of vessel/Long (m) | Displacement (tonnes) | Recommended weight of anchor (kg) | Type of anchor |
|---|---|---|---|
| Inflatable boat (up to 3.5 m) | 0.2 - 0.4 | 1.5 - 2.5 | Grab/Foldable |
| Motor boat (3.5 - 5 m) | 0.5 - 0.8 | 3.0 - 4.5 | Danforth/Plow |
| Boat (5 - 7 m) | 1.0 - 2.5 | 6.0 - 10.0 | Plow/Admiralty |
| Cruiser (7 - 9 m) | 3.0 - 5.0 | 12.0 - 16.0 | Modern plow |
It is worth noting that the weight of the anchor is not an absolute guarantee: an anchor weighing 5 kg with excellent paw geometry will hold better than a 7-pound piece of irregularly shaped metal. Construction of paws And the angle of opening is crucial to penetration, and when you choose your equipment, always give preference to proven models that have been tested in real-world conditions.
Determination of the length of the anchor rope and chain
The length of the anchor cable (line) is often ignored, although it is more important than the weight of the anchor itself. There is a golden rule of navigation: the length of the etched cable should be 3-4 lengths of depth in the parking lot for daytime parking and 5-7 lengths for night or while waiting for a storm. clutter.
Why so much? If you stand at a depth of 3 meters and you erod only 10 meters of rope, the angle of separation of the anchor will be too great. As the wind increases, the boat will begin to pull the rope, and the anchor, instead of burrowing deeper, will be pulled out of the ground. The long rope allows you to create a saddle that cushions the jerks and keeps the traction force close to the horizon.
The use of an anchor chain in this equation is critical. The chain is heavier than water, it lies on the bottom of the arc. Even if the boat blows off by the wind, part of the chain must first be taken off the bottom, and only then the force will be transferred to the anchor. The recommended length of the chain is from 1 to 1.5 lengths of the boat's hull, but at least 5-6 meters. The rest can be made of nylon or polyamide cord.
☑️ Checking the anchor system
⚠️ Warning: Never use an anchor line without a chain insert if you plan to anchor on a rock or shell bottom. The rope will quickly RUB against sharp edges and you will lose anchor. The minimum chain insert is 5 meters.
To calculate the total length of the rope, use the formula: L = H * K, where L is the length of the rope, H is the depth in the parking lot (plus the height of the board above the water), K is the coefficient (from 3 to 7). Always have a margin of length. A rope break or rubbing is a common cause of emergency situations.
Effect of soil type on the holding force
Soil at the bottom of a body of water is a variable that can't be changed, but needs to be considered when calculating, and different types of anchors show different performance depending on the bottom relief. Slurry soil It is considered ideal for most modern anchors, as it allows the paws to dive deeply and create a powerful holding effect.
The sandy bottom is also good, but here the anchor can slip if the angle of attack is wrong. On a solid clay bottom, the anchor is difficult to hook initially, requires a strong initial load to plow the ground. The worst is on a rocky bottom or a seaweed bottom. Algae can block the anchor's paws from reaching the ground, and it will lie on top without carrying any load.
- 🌿 Sludge and soft clay: Perfect for anchors like Plow and Danfort, maximum holding power.
- 🏖️ Sand: They hold anchors with a wide paw area well. They need to be properly installed.
- 🪨 Stone and pebbles: Require anchors such as "Grab" (cat) or admiralty with a bar, which can get stuck between the stones.
- 🌾 Algae: Problem for all types: It is recommended to look for clean areas or use anchors with high mass.
When planning a parking lot, always look at navigation maps or local destinations, which often indicate the nature of the soil, if you are in an unfamiliar place, try to probe the bottom with a bugger or sonar before anchoring, which will save you nerves and time.
Algorithm of correct anchoring
Even a perfectly designed anchor won't work if it's not properly installed. The anchoring process is a well-established procedure. First, the vessel is nose-driven against the wind or current (whichever is stronger), then the anchor sinks to the bottom, but doesn't throw itself to avoid entanglement of the rope.
Once you touch the bottom, the boat is slowly drifted back, etched the rope, and when the required length is etched, the rope is fixed and reversed at low engine speeds, an action called anchoring causes the paws to burrow into the ground, and if the boat is standing still when the engine is running, the anchor is laid down correctly.
It is important to monitor the position of the vessel relative to other objects. When the wind changes, the boat will describe the circulation around the anchor. Make sure that the radius of this circulation (equal to the length of the etched rope) does not overlap the parking lots of neighboring boats and does not threaten navigational signs.
Anchor equipment care and safety
The anchor is subjected to tremendous stress and aggressive environments: salt water, sand, silt, all destroy metal and synthetics. Regular inspection is a guarantee of safety. Pay special attention to the connection of the chain and the vertebra, and the attachment of the fry bolt to the hull of the boat. Weakening bolts is a common cause of loss of anchor.
The synthetic rope also requires maintenance, and after each trip it must be dried, and the wet rope, which is put in the knob, starts to squirt, mold and lose strength. Nylon cord It is afraid of ultraviolet light, so it is better to store the anchor in a closed box or case, rather than under the open sun.
It is also worth checking the strength of the knot or kows that attach the rope to the anchor. If you use knots, make sure that they do not crawl under load. For polyamide cords, it is better to use special sea knots or pressed couches.
Common mistakes in selection and operation
One of the most common mistakes is to buy a back anchor by weight, and many people think that if you have 3 kg in the table, you have 3 kg, you take it, but the conditions can change: wind will increase, rain will change, the current will change. Always take an anchor with a margin of 20-30%. The extra kilogram in the runduk is not noticeable, and on the water can save the situation.
The second mistake is ignoring the vertebrae, and without the vertebrae, the anchor rope will inevitably swirl around the chain, especially if the boat changes position relative to the wind, and the twisted rope loses strength and can burst at a critical moment. vertebrate It is an essential element of the modern anchor system.
The third mistake is the use of inappropriate materials. Some people try to use construction ropes or rusty chains found in the garage. This is unacceptable. Sea gear must have adequate safety margin and corrosion resistance. Save at anchor can cost you the whole boat.
How do I calculate the length of the rope if I don’t know the exact depth?
If the exact depth is unknown, use a sonar or a simple lot (lining with a load with tags). In extreme cases, for small boats in calm water, you can focus on the average depths in the area of the berthing, but always leave a reserve. It is better to etch more rope than less. The minimum safe reserve for unknown depths is 15-20 meters of rope for small craft.
Can two anchors be used?
Yes, double anchoring (e.g., tank or runaway) greatly improves parking reliability, especially in strong side winds or on the current. However, for small boats, this is often redundant and takes up a lot of space. Double anchoring is justified when sleeping in the open or waiting for a storm.
Which is better: a chain or a rope with a piece of chain?
For small boats (up to 5 meters), the full chain will be too heavy to lift by hand and will create an excessive load on the nose. The best option is a combination of 5-7 meters of chain at the anchor to press the rope to the bottom and then a high-quality synthetic rope.