How do anchors work

An intense squall blew up last night. The wind was around 25 knots, gusting to 30 (a little over 55 kph). Not a biggie as far as these things go but was one of the strongest we've experienced. At this velocity the boat is very actively swinging back and forth on its anchor chain, it's hard to hear someone speak 20 feet away and is strong enough to knock you off your feet if you are unsteady (e.g. due to waves). After our initial surprise at the sudden increase in wind, we quickly took down anything that could blow away then let out more anchor chain and waited for the squall to blow itself out, watching other boats take similar cautionary actions. An hour later and everything was back to normal.

However, one question remained. Why did we let out more anchor chain? The boys were curious, so the next day we had a little discussion of the physics of anchors and a little demonstration.

old style

Anchors use two forces to hold boats in place. Friction and gravity. When most people think of anchors they picture a big metal fish-hook-like object attached to the end of a chain or rope. While those types of anchors are still in use, most these days look like some kind of plough or claw. However, your anchor is actually the secondary thing holding the boat in place. Most of the time (in settled conditions) you arrange things so that there is no or very little stress on the anchor itself. The primary thing holding your boat in place is the chain.  This is achieved through a bit of friction, gravity and angles.

modern plough

A boat at anchor creates a right angle triangle between the anchor on the bottom, the sea floor right under the boat and the boat's bow. Anchors hold best when the chain pulls along the sea floor in a horizontal direction, dragging the anchor and with the best designs, digging them in deeper and deeper. Anchors usually break free with little effort if the pulling force is approaching vertical. This means a smaller angle at the anchor is better. This is the friction part of the equation (anchor is using friction to stick to the bottom and dig in). In addition, the anchor chain is heavy and wants to lie on the bottom. This creates even more friction as the chain drags on the sea floor. Increasing the amount of chain that we had out last night made a smaller (better) angle where the anchor meets the sea floor which increased the amount of force (wind) required to move the anchor or break us free.

However, letting out more chain has another positive effect that isn't always obvious and this is where the demonstration came in. What I wanted to show is that the weight of the anchor chain alone significantly improves the holding and in calmer conditions is almost the only force that is needed to hold our boat in place. The anchor chain makes the top side of the triangle and is quite heavy. As the angle decreases, the force (wind) required to keep the chain (top of the triangle) straight increases (it's a non-linear increase in effort but I don't know what the factor is).

To demonstrate this we took one of our long, heavy dock lines, attached it to a cleat at the bow then ran it back along the boat deck to the cockpit. First I had the boys try to lift the line and hold it straight from 10 feet from the bow. They did this with ease. Then they stepped back 5 feet and tried again. This time it was more difficult. Then they stepped back another 5 feet. At this point Aidan couldn't get the line to stay straight except when he yanked on it (this simulates a boat surging on its anchor in the wind). They stepped back another 5 feet and now Aidan couldn't straighten the line at all and Austin could only do it by yanking. Back another 5 feet and neither of them could straighten it.

We discussed that each time they stepped back they needed to lift and keep raised more line, which increased the total weight they needed to lift. Each time they stepped back they were also making the line more horizontal, which has a similar effect to moving farther from the fulcrum of a lever (only it has the inverse effect of increasing the amount of force required rather than reducing it).