Knowing Your Anchor

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Have you ever really considered the anchor that holds you fast when moored in a bay or over your favourite fishing spot? We look at the different types of anchors and what they are designed to do.

There are certain very important attributes you should keep in mind when selecting an anchor. Will it set quickly, and hold in a variety of different bottom types? Can it withstand significant loads? Is it easy to deploy and retrieve, and is it easy to store?

A modern anchor usually consists of a central bar called the shank, and an armature with some form of flat surface (fluke or palm) to grip the bottom and a point to assist penetration of the bottom. The position at which the armature is attached to the shank is called the crown, and the shank is usually fitted with a ring or shackle to attach it to the anchor rode.

There are essentially three categories of anchor. First, there are those with deep penetrating, lightweight, pivoting flukes such as the Danforth, Kewene or the Sealock. These are great for muddy and very soft bottoms.

Second, there are the plough-style anchors such as the Bruce, Delta, Manson Plough and SARCA. These have excellent structural strength and are good general purpose anchors. On the other hand, they tend to set more easily due to increased weight. Finally, there are the ‘new generation’ anchors for the real serious anchorers. Variations on all of these traditional designs exist, and new ones seem to pop up all the time.

Sand is relatively easy for anchors to penetrate. Most anchors will hold best in hard sand. Mud, on the other hand, has low shear strength, and requires an anchor with a wider shank-fluke angle and greater fluke area so that the anchor will penetrate deep to where the mud has greater shear strength. Mud is frequently thin and layered over some other material. Thus, anchors that can penetrate through the mud to the underlying material will hold better. Fortress anchors (an aluminium danforth pattern type) work very well in mud, because they can be converted to a broad fluke angle.

Rock and coral bottoms present different challenges. Here, holding power is more dependent on where you happen to drop the anchor, than on the type of anchor you have. As a general rule most anchors will stick (get stuck in) to rocks well but this is the best ground for a grapnel. Grassy or weedy bottoms present tough challenges for all anchor designs. Here, it is the tip weight and cutting edge of the anchor, which is the most important factor.

If you try and set an anchor too fast, you’ll find that it will skip along the bottom, or suddenly set and could cause damage to your winch or capstan. This is quite likely with the new generation anchors, as their design will cause them to set and stop you pretty quickly, so passengers had better be holding on!

Many people regard the anchor in its own right as just an anchor. However, they should regard it as part of an anchoring system. An anchor will work better with chain behind it, and an anchor straight on the end of a piece of rope won’t work as well. You have to have a matched set.

It’s important to realise that the main purpose of the chain is to provide sufficient weight to keep the angle of attack of the anchor within its design limits, and that the harder it’s blowing, or the faster the current flow, the more chain is required. The weight of the chain also provides a dampening effect so you don’t shock load your anchor out of the ground or winch/cleat out of the deck.

It also pays to buy quality rather than buying to a price. For instance, there are a lot of knock-off ploughs, Bruces, danforth anchors and chains coming out of China, which the experts say, generally aren’t up to the standard of most Australian and Kiwi made anchors. When you consider the value of the boat an anchor is protecting and the fact it stops you from waking up on the rocks, an anchor is no place to economise. It pays to get the best performing anchor system you can.

How To Anchor Securely
By Tom Burden –West Marine Advisor.

Anchors dig into the seabed to hold a boat in position. They serve a safety role by keeping boats out of the surf or off the rocks. They also allow boaters to secure the boat temporarily while fishing, having lunch or spending the night.

When an anchor penetrates the surface of the seabed, suction generates resistance, created by the bottom material plus the weight of the material above the anchor. As the boat pulls on the anchor rode, the anchor digs in deeper, creating additional resistance. In rocky or coral bottoms, anchors can’t dig in, but rather snag on protrusions and hold precariously.

To ensure that an anchor “sets” well, apply tension to the rode so the anchor penetrates the bottom. Do this by making fast the line and applying power in reverse. If your boat moves, reset the anchor and try again.

Many boaters make only a half-hearted attempt to set the anchor by putting the boat in reverse for just a few seconds. To be sure the anchor is set you must put a reasonable strain on the rode for a reasonable length of time. Your boat should surge forward when you back off the power, indicating that you have put some strain on the rode to test the anchor set. We know of no way to ensure that your anchor will hold other than by pulling on it hard.

Scope is defined as the ratio of water depth (plus freeboard) to anchor line paid out. Most anchoring texts and anchor manufacturers agree that a scope of 7:1 achieves the anchor’s designed holding power, and more scope is better than less. In theory, 7:1 scope is great, but at a crowded anchorage most cruisers scoff at the idea of paying out more than 3:1 or 4:1-there just isn’t that much space for boats to swing. When an anchor is securely set you can consider shortening scope in a crowded anchorage. 

Once an anchor has been set, it will almost always hold the same amount of tension that was used to set it, even if the scope is reduced. This means that you can pay out long scope, pull hard on the anchor rode using the engine, and then shorten scope to reduce swinging room. However, if your boat swings and the anchor has to reset itself, it will have to do so at a reduced scope. This is known as Anchoring Russian Roulette.

It’s fairly easy to set an anchor when wind and current come consistently from one direction, but if they veer, some perform better than others under varying angles of pull. Any anchor can become dislodged from the seabed if the boat swings far enough. It pays to have an anchor alarm on your chartplotter so it can alert you if the boat swings too far from position where it was when you set the anchor.

Despite claims to the contrary, no single anchor design is best in all conditions. Boaters voyaging to areas where there is a specific type of bottom must carry an anchor(s) suitable for that bottom. For all but very small boats, we recommend that all boats carry at least two anchors.

This means that you’ll have another if one anchor is lost and different anchor types work best for different conditions.

Two anchors allow you to anchor bow and stern in tight anchorages. Inspect your entire anchor system frequently for chafe, loose shackles, and bent flukes. The system is only as reliable as its weakest component. Store at least one anchor so that it can always be used immediately. Even the strongest anchor won’t do you any good if you can’t deploy it. Quickly deploying even a small anchor can keep you from going further aground.

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