I’ve recently been to a marina, and it got me thinking about anchors. I don’t really understand how they work.
You drop a curved T shaped hook, and it is supposed to lodge in whatever is at the bottom - which could be a variety of surfaces, including sand, silt or rocks. I can’t see how it would get a firm enough grip - or any grip at all in some cases. But let’s assume it does; if it’s stuck firmly enough to secure what could be a very large ship against possibly very strong tides and winds, how is the anchor released when needed - and only when needed?
Can a boat drop an anchor anywhere? I’m assuming the Mariana Trench is out of the question, but presumably you’d need sonar or charts to tell you if it isn’t too deep for your anchor and what is on the bottom?
It all seems a bit hit and miss. How do they work?
The main thing about an anchor’s function is that it works against a force exerted against it by the ship/boat, via the anchor line/chain, in a horizontal direction. For an anchor to hold, you need to veer out enough chain/line so that the last part of it lies on the ground (rule of thumb we learned for small boats: at least depth times five for anchor line/depth times three for anchor chain - other sources say ten times/five times).
So a ship/boat does not lie above its anchor but downstream/downwind of it (assuming only one anchor). To pluck the anchor out of the ground, you take the boat above the anchor, ideally (if it is not snagged) it cannot hold against a vertical force.
I have found a drawing of anchoring a sailboat here - note the last part of the anchor chain tugging at the anchor horizontally (the German text says: at 10 m depth at least 30 m of anchor chain) - only if the drawing were to scale that sailor would have committed a grave error; if ever the wind changed direction he’d find himself stranded. That’s why in practice boats at single anchor are anchored much father off shore.
I have read somewhere, sometime, that it’s not the anchor as such that holds the boat in place, it’s the weight of the chain lying on the bottom that does the trick, but this doesn’t explain how it works if you have something else than a heavy chain connecting the boat with the anchor.
Simple…silt (which most bottoms consist of) gives great grip. You shove a bloody great hook into it, pull on it to dig it in properly and make sure you’re always down wind of the anchor. Larger ships will in fact actively back away from their anchors to make sure that they dig in properly. To release, you simply run the ship above the anchor, digging it out and releasing the grip of the silt.
Non-silt bottoms, such as sand or gravel, may require different anchors. Many ships carry different anchors for different bottoms, or may have a jack-of-all-trades anchor (as all larger ships do).
Charts tell you both depth and bottom type. If you’re in a badly charted area, depth sounders can tell you whats in front/below you.
Naturally, anchoring isn’t done at random. You will find anchorage where there is a suitable bottom and where you have enough cable to reach it. You will also be aware of tides in the area (anchoring on a taught cable in low tide can have rather sucky consequences) and you will anchor in such a way that you can swing with the wind if needed.
Anchoring 15-30’ boats (and some larger) goes like this in my neck of the woods (Atlantic Coast and Intercoastal waterways from Maine to FLA:
Danforth anchor (fluke type) with chain connected to rope. Drop over bow. Let line out until it hits bottoms. Let more out. Let more out as you drift (rare that you don’t drift, so you can bank on it). Drift so much and let out so much line that the flukes are dragging/digging/burrying themselves. Suddenly, the anchor is so well dug in that you don’t move.
To raise anchor, pull the line up until you are over the anchor and the anchor will give to the upward pull. It does not have up/down holding power, because its holding power comes from the seabed.
You can lose an anchor; it can become snagged. It happens, but most of the time you can free the anchor by going past the anchoring spot and pulling from the opposite direction.
As mentioned above, it’s a 1:3 ratio. For every 10 meters of depth, you should let out 30 meters of line.
All information above is correct but I’d like to add that there are different anchors depending on the composition of the bottom, but if you generally sail in one region (or even continent) you only need to carry one with you.
On our small (20’) boat we have a Danforth anchor with three feet of chain on a 100’ line. We are on Lake Erie and will anchor in about 50’ at maximum, we don’t try an anchor at greater depths. I know we don’t use the maximum length of line for those depths but it keeps us in one place very well. We also have a much smaller Danforth we use off the stern, this keeps the boat from swinging with the wind so you are pretty much staying in one place. As Philster has said when you want to bring the anchor aboard you pull the line until directly over the anchor and pull up, sometimes it will take a good deal of strength depending on the bottom conditions but we have never had a problem. Often the flukes are covered with black mud and you have to swing the anchor back and forth in the water to clean the gunk off before bringing it aboard.
I’ll just add that boats will naturally swing around to face the wind, which means they will try to pull the anchor horizontally, lodging it into the sea bed. (When sailing, you can tell pretty much which way the wind is coming from by simply looking at which way the moored boats are facing.)
Aside from mushroom-type anchors (which are essentially just heavy weights shaped like an inverted umbrella) the shape of an anchor makes it dig deeper or grab harder when a load is applied along the axis of the shank (the central part that the rope’s tied to and everything else extends from), similar in concept to a finger-trap.
It also borrows from the pattern of a properly angled tent stake in that it’s hard to pull out from one direction, but very easy from another. If you pull on an anchor at a mostly horizontal angle, it tends to dig deeper into whatever medium it’s embedded in. If you pull at a mostly vertical angle, it comes up relatively easily.
By backing off from an anchor as they pay out cable, you’re making sure that the boat (or ship) is always pulling on the anchor at mostly horizontal angle. Also, since wind and waves rarely have abrupt reversals, a change in wind or weather will move the craft around the anchor in a circle. Of course, your anchor will turn with the boat (which is why you see ones that have 4-5 teeth spitting out in different direction), but as soon as the load steadies, it just digs back in.
