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Boating Tips

ANCHORING 101

Posted 1/12/2011

Figure 9-03 A lightweight-type anchor lands with its flukes against the bottom (A). A strain on the anchor line causes the flukes to penetrate the bottom (B). Further straining on the rode causes the flukes, and sometimes the entire anchor and part of the rode, to be buried (C).

Once you’ve found your slice of heaven on the lake, how do you keep your boat in place? Easy, just lower the anchor over the side, make sure it has a good grip on the bottom of the pond, then open the cooler and break out the refreshments.

How Anchors Work
Contrary to popular belief, it’s not the weight of the anchor that holds a boat in position (with the exception of small, light boats on calm ponds), but the shape of the anchor. 

A well-designed anchor burrows into the ocean floor, lakebed, or river bottom. The more your boat pulls on the anchor, the harder the anchor digs itself into the bottom. Anchors are stubborn; you have to force them to let go. And that’s what we want them to do.

Starting at the Bottom
The anchor you choose has as much to do with what’s underneath the water as the type of boat you use. Big, heavy boats will need a different anchor than a 14’ jon boat. 

Not just any old anchor will work; you have to have the right tool for the job. For instance, some anchors hold better in sandy bottoms, while others perform well in muddy riverbeds or rocks.

Here are some popular anchors, with their common applications:

Ground Tackle
The anchor rode is the line (“line” is the nautical term for rope) that attaches the anchor to your boat. Three-strand twist or double-braided nylon line is recommended because it’s strong and it stretches under a load—such as when the boat surges from waves and wakes. Nylon is also less susceptible to degradation from rot, mold, and other organic growth, however; the sun’s ultra-violet rays can cause nylon to weaken over long periods of time, so stow nylon lines out of the sun when you’re not using them.

Braided synthetic lines are also a fine choice for anchor line. This type of line usually consists of a braided synthetic outer cover surrounding a braided synthetic inner core. Unlike twisted nylon, braided lines are easier to handle, and coil neatly without an undue amount of effort – and braided synthetic lines make good general-purpose lines (dock lines, for example).

Between the line and the anchor is a length of galvanized chain (in many applications). Two galvanized shackles (“U”-shaped fasteners) attach the line to the chain, and the chain to the anchor.


What does the chain do? When the anchor lands on the bottom, the weight of the chain causes the anchor to lie flat, allowing the anchor to dig into the bottom when tension is put on the line (by backing the boat slowly as you set the anchor, and when the boat moves in reaction to wind, waves, or wakes). The chain also can endure torturous chafing from rocks and bottom debris that would destroy the anchor line in short order.

A typical pleasure boat anchor rode consists of a long length of nylon (or similar) line, a section of galvanized chain between three to six feet long, and a couple of galvanized shackles to fasten the line, chain, and anchor together. Combined, we call anchoring components “ground tackle”, in nauti-speak.

Anchor & Rode Package

How Much Rode Is Enough?
The water depth and the weather conditions usually determine the amount of rode you have out (this is called “scope”) when you’re anchored. For instance, if the weather is bad and the water is rather deep, you’ll need to put out more rode for the anchor to have sufficient “bite” to secure your boat.

The amount of anchor rode necessary in a given situation isn’t so much a defined length of line as it is a ratio of the height of the boat’s bow above the bottom of the water.

A good rule of thumb is you ought to have a minimum amount of anchor rode to equal five to eight times the depth of the water plus the distance from the water to where the anchor attaches to the boat. 

For example, if the distance from your bow cleat/eye to the water is about three feet, and the water is twelve feet deep (fifteen feet total) and the water is fairly calm, 75 feet of anchor rode is required (5 X 15 = 5:1 scope).
On the other hand, if the water is rough, you’ll need roughly 120 feet of rode (8 X 15 = 8:1 scope) – or even 150 feet of rode (10:1 scope).

It seems like a lot of rode to keep aboard, but an anchor tends to hold better when the line pull is relatively horizontal, thus the need for long anchor lines. Shorter lines can work in shallow, protected waters, but the rode could eventually exert vertical force on the anchor, eventually causing the anchor to dislodge.

Tying Up the Loose Ends
Anchors, equipment, techniques, protocols, and the endless variety of anchoring scenarios are a deep subject (no pun intended); several excellent books are available explaining anchoring in as much detail as you can tolerate. We’ve simply provided a high-level overview; it’s up to you to delve into your particular anchoring requirements.

Your local marine dealer, well-stocked bookstores, as well as the internet can be outstanding resources – don’t be afraid to ask questions, especially of the old-timers – most of ‘em know what they’re talking about. They’ll be happy to tell you how to “get a grip on things.” Y


Resources
“How Anchors Work” and “Scope” illustrations courtesy of Chapman Piloting & Seamanship, 64th Edition, Elbert S. Maloney