The Scoop On Scope
It's possible to tell if the anchor is correctly set by resting your bare foot on the anchor chain. If you feel a constant vibration it could be the anchor dragging.
Aside from the anchor itself, the angle of pull on the rode has the most impact on whether a boat is likely to stay put. This critical angle is largely dependent on "scope," which is the ratio of anchor-rode length to water depth. Scope is measured from the bow of the boat to the sea bottom; if the water is 10 feet deep and your bow is three feet off the water, the depth is 13 feet. A good starting ratio for scope is 5:1. In the case of 13 feet of depth, you'd need 65 feet of anchor rode to achieve a 5:1 scope. Scope, however, has some variables. For example, the higher the expected winds, the more scope will be needed — 7:1 or 8:1 scope, or even more, is required for heavy winds. Scope is also dependent on the type of rode used.
In calm conditions, anchoring fore and aft can prevent your boat from swinging into an
area that might not be suitable, like the shallows in front of this beach.
This is especially critical in shallow harbors, where storm surge or tidal range can decrease scope and increase the angle of pull dramatically. In his book Permanent Yacht Moorings, Ridsdale Ellis presents a graph illustrating how the holding power of an anchor drops steadily as the angle of pull increases to 25 degrees, at which point holding power suddenly takes a precipitous drop. Translated to scope, the "uh-oh" angle of degrees is reached somewhere between scopes of 3:1 and 2:1. As Ellis points out, anchors bury themselves and develop holding power when the pull on them is roughly parallel to the bottom. The more scope, the smaller the angle. But there are other ways to make the rode more horizontal in a crowded anchorage: using a heavy rode and/or a kellet.
Pros And Cons Of Chain Rode
Seizing your anchor shackles with cable ties will prevent them from working open, which could lead to the loss of your anchor.
Chain forms a catenary, or curve, in the rode that helps keep the pull on the anchor horizontal so that it digs in when tension comes on it, even with less scope. An all-chain rode has some catenary in all but the strongest (say, hurricane) winds. The catenary effect can also be achieved by attaching a heavy weight — called a kellet — to the rode.
Chain has the added advantage of also being resistant to chafe when it's dragged over rocks and debris on the bottom. The best all-around chain for anchoring use is hot-dip galvanized proof coil. High-test chain is stronger for its size, and its superior strength allows for a smaller, lighter chain — but it costs significantly more. Stainless-steel chain has the advantage that it never leaves rust marks on your deck, but it costs several times as much as proof coil and isn't quite as strong. Plastic-coated chain is easier on the deck, but it's subject to corrosion under the coating. BBB chain is similar to proof coil, but it has links sized to fit certain windlasses. Whatever you do, avoid cheap hardware store chain; it's neither tested nor galvanized to marine standards.
Depending on the wind strength, and length of time at anchor, here are appropriate
scope recommendations for a mixed rope/chain rode. For larger boats or for
anchoring overnight, two to four boat lengths of chain is ideal.
Catenary comes from weight, but unfortunately, weight is often the last thing you want aboard, and too much of it in the bow can adversely affect a boat's handling. And while chain's catenary absorbs shock in normal conditions, in storm conditions a tight chain is hard on the boat's hardware and also makes it more likely that the anchor will get yanked off the bottom. Nylon line, however, doesn't have this problem.
Pros And Cons Of More Rode
Nylon line gets its shock-absorbing properties from stretch rather than through catenary action, and it's this property, along with its lighter weight and strength, that makes it a good rode. Three-strand nylon line has the most stretch, while polyester line is about 15 percent stronger and more resistant to chafe but doesn't absorb shock as well. Polypropylene line should be avoided on board anything larger than a dinghy — it lacks strength and stretch, and it floats. Chafe is the enemy of any line and can appear at places you might not expect; one boat I know of was driven ashore after a loose thimble on a nylon rode chafed as the line worked during a blow. Polypropylene line's biggest drawback is that it has almost no weight in water and therefore no catenary, so the anchor tends to be pulled up even in light winds. To keep the angle of pull low with polypropylene line, you need to use a longer rode.
The Optimal Rode
Strength, catenary, reasonable weight, a measure of stretch, and chafe resistance on the bottom are a tall order for an anchor rode. The solution is to combine a length of relatively heavy chafe-resistant chain attached to the anchor and then a lightweight, strong, stretchy line attached to the chain so that you get the best of both worlds. One to two boat lengths of chain is sufficient for most purposes, although more is always better. Be aware that with a rope/chain rode, only certain windlasses can bring in rope and chain on the same gypsy. Those that can will require that you use a rope-to-chain splice, which is more vulnerable to chafe. To create an effective rode, use goodquality shackles to tie the system together, and seize the pin to prevent it from unscrewing at a bad time. But just dropping the anchor and rode anywhere is an invitation to failure.
Location, Location, Location
As wind speed doubles, its force quadruples, so protection from the wind is obviously important. But protection from wave action is just as important as being shielded from the wind. Finding a good spot to anchor takes some planning. Anchor your boat like you'd circle a parking lot in your car, looking for just the right parking space. This approach might get you some funny looks on the water, but that's exactly the frame of mind you should be in when anchoring. Not only do you need an anchorage with good protection; you also need a bottom into which the anchor can dig and an area providing you with sufficient scope so you won't swing into other anchored boats.
Anchoring inside another boat's swing radius will create a conflict when the wind shifts.
1. The different lengths of the all-chain rode on the white sailboat on the left and the longer nylon rode on the nearby tan powerboat will result in the boats overlapping, and bumping, when the wind shifts.
2. The boat approaching the anchorage should have anchored near a boat with a rode of a similar length.
3. Or it should have anchored at least a boat length behind and off the other boat's quarter.
Before dropping anchor in a new area, check your chart to make sure that the bottom offers something your anchor can grab. Most charts have abbreviations that identify the type of bottom; if the chart says the bottom is "hard," your mud anchor may not set. The meanings of the abbreviations are usually shown in a legend box on the chart.
Once you've located a suitable spot, determine where the anchor should be dropped in order for the boat to end up where you want it. One way to estimate swing room is to think of your rode in easy-to-picture sizes,like boat lengths. Subtract the depth of the bottom from your rode length, then divide that by the size of your boat. For example: 110 feet of rode minus 20 feet of depth gives you 90 feet, or a swinging diameter of about three boat lengths around the anchor for a 30-foot boat. Boats typically oscillate to the wind every 1 to 5 minutes and, alas, not at the same time, which means that the entire anchorage might look different every few minutes. Take your time to select.
Note that the boat should be anchored from the bow. There have been a few sinkings of small boats that were anchored from the stern, something usually done for convenience while fishing. In most cases, the boats filled with water from waves or a wake coming over the already low transom of an outboard-powered boat, and the boat capsized. The bow is the strongest part of a boat and the most able to withstand and ride up and over waves; it should be the only place an anchor is tied on a small boat.
Setting The Anchor
There are two important rules for dropping the anchor: 1. Don't get your feet tangled in the line when the anchor is over the side, and 2. If your boat has a windlass, keep your fingers well away from the gypsy. Once the anchor is safely overboard, the only way to know for sure that it's set into the bottom is to dig it in by motoring gently in reverse; this lets the anchor plow itself into the bottom. When it feels like the anchor has dug in, give it a stronger and steady pull in reverse to tighten up the line. If your boat springs back toward the anchor, you're probably set. If you simply end up backing through the anchorage, you'll need to haul up the anchor and try again, maybe in a different spot or with a different anchor.
When the wind rises, how do you know if you're dragging? Many GPS models have an anchor-watch alarm that sounds if your boat moves out of a specific range. Some depth sounders will alert you if your depth changes substantially. You can also take bearings to shore with a hand-bearing compass. And if you're getting closer to all the other boats downwind, it's a safe bet that you're the one who's dragging.
There's an unwritten rule that requires each arriving boat at an anchorage to take into consideration the boats already anchored. Sleepless nights and a lot of frustration could be reduced if everyone were to follow this rule. Even when anchored at a comfortable distance, you must be aware of how much scope you and everyone else have deployed. Too little, and someone is liable to swing into you if there's a wind shift. Too much, and you could swing into someone else. Don't forget your anchor light, which isn't just so others can see your boat; it's required by law.
Finally, when it's time to leave the following morning, the anchor may or may not come up with a few hefty pulls. Remember to make sure you're directly over the anchor before you pull. If that doesn't work, try snubbing the rode and bumping the engine forward from various directions. Make sure the rode is taut so it doesn't wind up in the prop. If the anchor still won't budge, bring the entire crew up to the bow and snub the rode. The crew then parades aft and — bingo — the anchor pops out. When it works, it's quite the party trick. When it doesn't work, at least you won't be the only one on the boat who's irritated.