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How Do Anchors Work?

Ian Fortey by Ian Fortey Updated on November 9, 2022. In Boats

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The science of how modern anchors hold a boat in place relies on several factors including things like suction, pressure, weight and potentially more depending on the style of anchor and how it settles to the bottom. Most anchors are just meant to be temporary anchors. Let’s take a look at what’s going on when you drop your boat anchor and how such a relatively small tool can hold a much larger boat in place.

What Happens When an Anchor is Dropped?

When you release the anchor and the anchor chains let out, regardless of the anchor style, it will sink to the bottom and settle on the bottom below your vessel. You need to have an idea of the depth of the water below your boat for this to work to ensure you have enough anchor chain to let out, of course.

Most anchors are designed to dig into the seabed when they hit the bottom. The traditional anchor shape with hooks on either side is designed in this way so that, once it hits, the resistance from your boat or anchor winch pulling the chain rode will drag the anchor and force it deeper into the seabed. The holding power of an anchor depends on a few things.

Anchors Need to be Set With Tension on the Anchor Line

In the process of setting an anchor, some tension needs to be applied to the anchor line. In much the same way you need to set a hook after a fish bites, the anchor needs to be set by forcing it to really dig in and take hold. Otherwise the anchor may not be taking hold and your boat will drift with the anchor sitting uselessly on the bottom surface. 

Setting an anchor is a fairly easy process. You need to drop the anchor and either let the wind/current move you back the length of your anchor line, or put the boat in reverse and very slowly move back that distance. Secure the line to a cleat and then gently reverse again. The gentle force exerted on the line will pull the anchor and cause it to catch and dig into the bottom. The anchor should be set at that point. It won’t work properly unless this happens. 

An anchor may need to be reset if it doesn’t set well, which can happen on rocks, or if the wind changes direction and pushes your boat in the opposite direction causing it to come loose. 

How the Soft Seabed Affects an Anchor

When the anchor digs into the seabed it creates suction. You can get a feel for how this works anytime you step into deep mud. It’s a real struggle to pull your foot back out again and you can even lose your shoe in the process. 

The science of how mud suction works gets a little complicated but here’s a simple explainer. An anchor in mud has no force acting on it besides pressure until the anchor line is pulled, say by your boat drifting in current. The anchor pulls up and condenses the mud above it which pushes down on the anchor. At the same time, the anchor creates a gap below it and the creation of that void produces suction pressure. Pressure above from mud and water and a lack of pressure below caused by the hole. It wants to balance that pressure by pulling the anchor back down to fill the hole. The bigger the anchor the more pressure tends to be built and the greater weight it will be able to hold in place on the end of the line which, in this case, is your boat.

The anchor can work itself deeper and therefore increase the pressure as your boat tugs on the line. In some cases it can be difficult to retrieve it again when it gets a really good grip down there. The makeup of the seabed itself will greatly affect this. Thick mud can hold a lot better than loose sand or rocks.

The fluke or danforth anchor are anchors that hold well in soft bottoms. They have a sort of shovel shape that lends itself well to digging into the soft bottom of some bodies of water. You can also use a mushroom style anchor on very soft bottoms but they have less hold than a fluke and are better suited for smaller boats. Additional anchors include fortress anchors, claw anchor, plow anchor, and bruce anchor.

How a Rocky Bottom Affects an Anchor

When there is no mud on the bottom an anchor has to hold your vessel in place through other means. In the case of rocks, an anchor obviously can’t bury itself so instead the protrusions on the anchor need to snag on the rocks themselves. In this case you don’t want a fluke anchor but something like a grapnel which has three or four spikes or teeth on it, like a grappling hook and works best in anchor tests of this kind of bottom. These spiky protrusions will get wedged in the rocks at some point, under or between them, and that can hold your boat in place. 

How Does the Water Itself Affect the Anchor?

Sailboat

The weight of the water is partly responsible for keeping that anchor in place so shallow water can make it harder to get an anchor to set. Most anglers don’t like to use traditional anchors for shallow water anyway as they are noisy and can spook fish. Not to mention that they take too long to get set in the shallow water when something like a pole anchor or shallow water anchor works much better. 

Without the added weight of deeper water, it can be harder to get the anchor to set into the bottom and really get a good grip and develop the suction you need to hold a boat in place. Anchors need the weight and pressure of the water to help them work at depth. The less water there is above the anchor, the less strong its overall grip is going to be.

Anchor Shape Affects How Anchors Work

We’ve touched briefly on the shape of anchors here and how they can affect how the anchor works in general. There are literally dozens of anchor shapes that a boater can choose from, some are very similar to one another and some are extremely different. Hat stereotypical old timey anchor shape is barely even used by anyone any longer.

Different shapes, sizes and designs of anchors have different purposes in terms of making the anchor work. Some, like the fluke, are designed for soft bottoms. The grapnel style is good for rocks. A plow style anchor may work best on a bottom that’s covered in weeds.

A mushroom anchor will fill with substrate to hold in place but this is better suited for very small boats or for use as permanent anchors, while flukes dig in and grapplers get wedged. Companies are constantly designing new anchors to bring to market that have features which can be more useful or appealing to certain boaters depending on where they are trying to anchor, for how long, and so on. 

How The Boat Itself Affects the Anchor

Though the anchors on large ships can weigh as much as 20 tons, the cruise ship it’s attached to might weigh 200,000 tons so there’s a huge disparity there. The anchor for your fishing boat might only be 10 lbs. So contrary to what some people might think, it’s not always the weight of the anchor that allows it to hold a boat in place and, in fact, it’s also the weight of the boat itself that gets the work done.

In windier conditions, a heavier anchor can be ideal because the strain put on a lighter anchor by the wind pushing the attached boat can be enough to dislodge it. 

The anchor needs to be pulled on to both set and hold its place in the seabed. The weight of the boat adds more stress to the line and puts more pressure on the anchor itself as wind and current try to push it along. So the weight of the boat is transferred to the anchor which is what pulls it to create that suction pressure we talked about. 

There’s some room to play with anchor weight and size here to achieve the best results but a heavy anchor is clearly not always the best choice. If your anchor is too heavy for your boat, the boat will have a harder time pulling and keeping pressure on the anchor so it won’t have as much force holding it in place as a result. So it’s possible a heavier anchor can do a worse job holding a boat than a lighter one. 

By the same token, an anchor that is unnecessarily heavy takes up too much space on your boat.

How Does a Shallow Water Anchor Work?

Shallow water anchors are meant to work fast and prevent a boat from drifting at all. Many of these are actually poles like the Power Pole that extend and stick into the bottom, holding the vessel in place with no rope needed. Because the water is shallow enough, it’s just like driving a stake into the ground that’s attached to your boat. Typically these are very quick and easy to both deploy and release but they cannot work in deep water.

You Can Anchor Bow and Stern for Tight Anchorages

When you have limited space you may not be able to let out a long length of rode. If space is limited due to other boats or you’re in a lagoon, close to shore or in a narrow space for whatever reason, you may need to use two anchors to anchor your boat instead of one. This stops your boat from swinging around the anchor. You can deploy an anchor at the bow and stern or two off the bow with one 180 degrees from the other. The tension of one anchor rode can help set the second one.  Under normal circumstances you don’t want to anchor at the stern.

Anchor Rode/Anchor Chain Length

The length of your anchor rope, or scope, should be at least 4 times the depth but 7 times is a much more standard length to go by. Some boaters will go as much as 10 times the depth. When you don’t let out enough line for your anchor you increase the tension on the line. If it’s extremely short it obviously won’t set in the bottom at all, or it won’t set enough, but if it does set on a short length, the motion of your boat can put too much strain on the anchor and it will be pulled free too easily. In other words, you won’t be able to anchor your boat at all. So the length of the anchor line can be as important as the type of anchor itself when it comes to how the anchor works.

A long line allows the strain to be dispersed more evenly between the boat and the anchor. So when wind and current push the boat, the weight is distributed more evenly and it’s not all jerking on the anchor at once at the end of that short line, forcing it to pull free. 

The Bottom Line

Anchors work to hold boats chiefly by the pressure of the material on top of them and the suction caused below as force exerted by the boat pulls on them. A small anchor is therefore able to hold a much larger and heavier boat in place thanks to these forces.

The size, shape and style of an anchor changes how it holds depending on the bottom material that it digs into, as some anchors cannot hold a boat in some situations. This is why having at least two anchors onboard is a good idea, so you can adapt more easily.

The proper length of anchor rope and ensuring the anchor is properly set will help ensure the anchor stays in place and does its job.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

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