Anchor rode scope
Have a question? E-mail it to firstname.lastname@example.org or send it to Soundings Editorial, 10 Bokum Road, Essex, CT 06426
Q: How much rode should you use when you anchor?
A: The general rule is a scope of 5-to-1, which means 5 feet of anchor rode for every 1 foot of depth. “Depth,” however, should include the height of your bow above water. Therefore, if the water is 10-feet deep and your bow is 5-feet high — and, therefore, the roller through which the rode passes is 5 feet above the water — the scope should be 75 feet, not 50 feet. As you might expect, there are significant exceptions and variations. Here are a few.
Another general rule is that the more rode you have out, the better off you are. Longer scope allows the boat to pull on the anchor more horizontally, allowing it to dig in easier and increasing the likelihood of it staying put. Longer scope also helps absorb shock from wind and seas that can jerk your anchor out. And special occasions often call for a lot of extra scope, such as a storm or high winds.
But I’ve seen some skippers come into a harbor and pay out 200 feet or more of rode in 10 feet of water, just because they feel it makes them safer. Maybe it does if there are no other boats around, but usually there are, and collisions with boats in storms can be just as dangerous as dragging anchor. Boats don’t necessarily “swing together,” and everyone anchored should anticipate this. Also, if that anchor drags anyway — and they all do sometimes — you’re going to have a heck of a time getting in that 200 feet to reset the anchor before you hit the beach.
There are several tactics that help hold the boat with less scope, but you should always use adequate scope when you can. These tactics are commonly used when there isn’t enough room for the scope wanted, or even to simply help the rig do a better job when there’s plenty of scope deployed. Some skippers will secure the rode to a through-bolted eyebolt on the prow very close to the waterline. They use a chain hook (if it’s a chain rode) or other means to attach a line from the rode to the eyebolt. Of course, the end of the rode still goes up to the roller and to the windlass, but the point of pull is lower. This would remove some of that 5 feet of extra depth I mentioned above. But I’ve seen a boat pull out a big hole in its stem in a storm. The bow section and eyebolt must be built to take it.
People frequently suspend a weight (kellet) from the rode — particularly nylon anchor rode. The weight may be specifically designed for the purpose, or jury-rigged when the need arises. It essentially slides or rolls down the rode and is held in place by a line attached to the bow. The weight holds the rode down closer to the bottom and allows more of a horizontal pull. It also provides more elasticity to the rig. As the wind or sea jerks the boat back, the weight has to be pulled up before the jerk reaches the anchor. Within reason, the heavier the weight the better. I use an all-chain rode with nylon snubber, and I create this weight by looping chain between the point where the snubber is attached to it and the point where the chain goes over the roller.
Another factor relevant to scope is bottom contour. For example, if there’s a nearby reef or shoal that will snag your keel if you swing onto it when the tide or wind changes, you should adjust your scope accordingly. This isn’t to say you should use less than 5-to-1. If that’s necessary, you should choose another spot. Successful anchoring involves many factors, and shortchanging any one can cause you to drag.