What if an anchor can't reach the bottom?

A drogue, or sea anchor can be used in conditions where an anchor can't reach the bottom of the ocean to help keep the ship stable in open water.

onesteppower.com - How do anchors work? - OneStep Power

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It’s Fun Fact Friday and this Friday it’s all about anchors. First we’ll talk about what they actually do and the difference between temporary and permanent anchors. Then we’ll take a look at anchor basics: what were the first anchors made from and what they are made of now. We’ll learn how to set an anchor and why there are different types. Then it’s on to sea anchors that help keep ships stable in inclement weather.

What does an anchor do?

At its most basic level, an anchor is a device to keep a vessel in one place. To keep a vessel in one place, anchors are used to combat wind and currents that want to move the vessel off course. Anchors can be temporary, like those most frequently used by boats and ships in shallow water, or permanent, like those used for oil rigs. A drogue, or sea anchor can be used in conditions where an anchor can’t reach the bottom of the ocean to help keep the ship stable in open water. Some advanced research, construction, and oil vessels use dynamic positioning to hold position, but that is usually reserved for larger, more expensive ships that operate in deep water and can’t afford to move off course at all. Those vessels can justify the more complex, computer controlled system of thrusters and propellers. Even single anchor vessels should always carry at least two temporary anchors of different types. Even if one is normally used, there is a backup in case of failure as well as another option for different bottom conditions. Larger vessels carry multiple anchors to keep them moored in place at docks and in ports, and can anchor in the ocean if the water is not too deep.

Anchor basics

Temporary anchors are usually made of a heavy material, with a design to dig into the bottom and hold a boat. According to Britannica, “Ancient anchors consisted of large stones, basketfuls of stones, sacks filled with sand, or logs of wood loaded with lead; these held the vessel merely by their weight and by friction along the bottom. As ships became larger, they required a more effective device to hold them, and wooden hooks that dug into the sea bottom came into use as anchors. Iron replaced wood in their construction, and teeth or flukes were added to help the hooks dig into the bottom.” Eventually curved arms and better balancing were designed to make them more effective and promote easier storage.

What type of material are anchors made of?

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According to Marine Insight, “Anchors are generally made of metals resistant to long-term corrosion that use suitable methods of protection such as electroplating and galvanization. However, they can also be made from fibre-reinforced composites or polymers such as carbon-fibre. The advantage of using such materials is that they have a high strength to weight ratio. This means that compared to generic metals, even light reinforced composites structures can handle enormous amounts of load or strain.” The difficulty with strong, lightweight materials is part of the holding power of the anchor is in the weight itself. Anchors that are too light may not dig in or hold well enough to perform their function. Cost is also a factor when less expensive, heavy anchors do a good job of holding a vessel in place.

How anchors work

Modern anchors have a chain near the anchor followed by lighter cable or rope up to the vessel. The anchor chain helps give added weight so the anchor can be set with horizontal force to dig in and stay put. The captain should “set” the anchor by applying reverse power to make sure the anchor digs in and won’t go anywhere. Proper setting ensures that the boat actually stays and doesn’t drift off course. Recommended length of rope and chain is 7x depth.

Types of anchors: time and conditions

There are many different variations of anchors. They have evolved heavily over time, but the main concern is how they perform in different bottom conditions. Assessing those conditions is the most important part of proper anchor function. According to West Marine, “An anchor's ability to develop resistance is entirely dependent on its ability to engage and penetrate the seabed. We have participated in several anchor tests, and despite varying results, there always seems to be one undeniable conclusion: the selection of a suitable bottom for anchoring is a much more critical factor than the design of the anchor.” Let’s take a look at what the anchors are digging into. Sand is the one of the best materials for anchoring a vessel. Anchors grab easily and stick the best in hard sand. Mud doesn’t work as well as sand, and requires a broader fluke angle and more area onto which it can grab. There is usually another material below the mud, so penetrating past the mud is the most effective method of anchoring - Fortress anchors are a good choice. Rocky bottoms do best with an anchor that has high structural strength and the ability to latch onto something on the bottom. There is more luck here as the anchor needs to grab something rather than dig in. Shale, clay, and grass bottoms are difficult to get a good purchase. A potential danger is an anchor may grab roots or other vegetation that holds temporarily, providing a false sense of security. Lastly, a sea anchor can be used to help a boat stay put in rough weather. According to Wikipedia, “A sea anchor[‘s] (also known as a drift anchor, drift sock, para-anchor or boat brake)...purpose is to stabilize the vessel and to limit progress through the water. Rather than tethering the boat to the seabed with a conventional anchor, a sea anchor provides drag, thereby acting as a brake.” They are basically a parachute for the water, and function so well they need a tripping line to retrieve them. Another name for this device is a drogue, basically the same concept - create drag to slow the vessel and increase stability.

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There is no one anchor that works well in all conditions. Carrying at least two anchors is the best way to ensure you are ready for any type of conditions. Inspecting equipment such as the anchor, chain, rope, and mechanisms is an additional layer of insurance. Having at least two anchors ensures options for multiple conditions, a backup in case of a broken or lost anchor, and the ability to drop an anchor bow and stern for maximum holding power. We hope you learned a little more about anchors, their shapes, and how they are used! PS - Don't have anchors? Give OneStep Power a call about safer, more reliable testing your Dynamically Positioned vessel for closed bus!