Anchoring Part 03: How Anchors, Chain & Line Work
How to set an anchor, what to feel when setting and why anchors, chain and line work the way they do; why anchor rode has to be so long and how to calculate it.
Tags: Anchoring, How Anchors Work, How Much Chain, How Much Line, How Much Rode, How Much Scope, How To Set Anchor
September 24th, 2009 at 5:30 pm
Anchors dig into or hang onto a rock on the sea floor. The boat pulls the anchor and both the speed and the angle the boat is pulling are most important. Anchors are designed to set only when the shank (arm) is precisely horizontal. When the chain pulls the anchor along the bottom, the anchor catches and digs in.
At first, just the tip(s) dig in and it is easy to pull the anchor too fast, skidding the anchor across the bottom rather than slowly pulling the anchor and allowing it to dig in at the angle it was designed to. You will feel this as the anchor initially pulls steadily and as load is increased or as the same load is maintained, the anchor pops out.
The more the anchor digs in, the deeper it buries itself. The deeper it digs itself, the harder it is for the anchor to pull out because the soil above the flukes (wings) of the anchor is more substantial and compact. Because of this, it’s important to gingerly start backing down, only increasing power once you’re sure the anchor has dug in fully at that amount of force. Slowly adding just a bit more power, you will dig it in a bit deeper and will feel the line smoothly tighten. As you pull more, if the anchor is digging in well, the line will take the strain smoothly, tightening. When you have half power for thirty seconds and the line is steady, you’ve set your hook!
If you feel the line tighten and pop out, often several times, a few seconds to maybe ten seconds apart, the anchor is trying to catch but is skipping across the bottom. Most likely, you’re initially starting too fast but you could have your anchor fouled with weed or even fouled with its own chain. it’s also possible that you’re using an ineffective anchor type or that your anchor is simply too small.
You have chain for three reasons: Durability, weight and dampening. Rope cuts easily on coral, rock and wrecks, so it’s necessary to have chain to prevent this – It’s a scary thing to suddenly be adrift on a lee shore with a cut anchor rode!
Chain is used is to allow the boat to pull the anchor precisely along the bottom. If the anchor is pulled up, it disengages, digging itself out of the soil and that is how we retrieve them: Drive up to the anchor, pulling the rode in and when you’re near it, you’ll feel the anchor give, lying on but not attached to the bottom. When we anchor, we use a length of chain that is too long and too heavy for the boat, bobbing on the swell, to be able to pull the anchor up, unseating it.
Think of placing the anchor on the end of your dock and laying the chain and line out along the dock. Now imagine being able to walk up a stairway to the depth of the water you’re anchoring in: ten feet is a building’s story, so if we’re anchoring in 30 feet of water, we’ll be on top of a 3 story building. Now imagine pulling rhythmically on the line, like you’re playing jump rope but straight up and down. It’s easy to pull the line off the dock and the first part of the chain but if the chain is long enough and heavy enough, no matter how hard you pull, you only drag the anchor along the dock because the end of the chain by the anchor lies quiet on the dock. Now imagine you have all the power of a swell, quickly lifting your boat – You need a lot of length and weight in that chain to keep from lifting it all!
Additionally, you can see how the amount of line you have laid out creates the angle you’re pulling at. Because we anchor at different depths every time, this length is measured in relation to the depth of the water we’re anchoring in. Because we’re pulling from the bow and that increases the pulling angle, the height of the bow is added to the water depth and it’s also nice to know if the depth sounder is calibrated for the depth it is underwater and that added if needed, too. Let’s say we’re anchoring in 15 feet (where we drop the anchor, not where we end up) and add another five for the height of the bow, for a total of 20 and want to be reasonably secure. We need to pay out a total of seven times that figure (20) of rode, so that’s 140 feet. If we’re on my boat (A Catalina 30), we’ll pay out my chain (two boat lengths or 60 feet because my boat is 30 feet long) and a further 80 feet of line. Remember we’re only in 15 feet of water! It’s common to have deep anchorages in California and 50 feet is really quite normal – So to anchor reasonably safely, I’ll need 7 x 55 = 385 feet of rode (Remember to add that height of the bow before multiplying!). If the weather’s rough, I need to increase it to ten times the height of the bow above the bottom or 550 feet of rode. Now you know why anchor line comes in 600 foot spools of line – You need it!
For a all chain rode, the minimum ratio of depth (from the anchor roller!) to the ground is 4:1 and in rough weather, 7:1.
The minimum length of chain is a boat length and two boat lengths is recommended for any swells or breeze more than twenty knots. Any less than a boat length is generally ineffective in keeping the anchor pulling into the ground.
As the force on the anchor increases, all the parts need to be stronger and heavier and I’ll describe that in the next part of this series.