The Offshore Voyaging Reference Site

It’s Often Better to Anchor Than Pick Up a Mooring

People are often surprised and even a little hurt when we turn down their kind offers to use their moorings in harbours we visit. They are even more perplexed as they watch us go to the trouble of anchoring outside the mooring field, often in a more exposed location.

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The holding power of a concrete block might actually be a bit worse than you have calculated. I believe the formula is F=uN, where F is the holding force, u is the coefficient of friction, and N is the normal force (1000 lbs in your example). Various internet sites put the sliding coefficient of friction for concrete on rock/sand/mud at something between .5 and .75, so your holding power is really only 500 to 750 lbs. The good news is that the initial force to break the block out will be somewhat higher, as you get to use the static coefficient of friction rather than the sliding coefficient, and as you point out on most bottoms the block will have dug itself in.


As an ex Norwegian Navy diver, I’ve inspected quite a few moorings.

Without a doubt, the heaviest corrosion occurs on the part of the chain that is lifted off the bottom with the tide. The chain may look fine at the surface and all through the water column, until you get to to bottom where the chain links rapidly taper down to barely a hair’s breadth. Under the right – or wrong conditions as it may be – a new mooring chain may be rendered useless in as little as two years or less. Needless to say, such a chain will part in the next strong breeze.

I’ve since read some research papers that seem to suggest that the greatest corrosion takes place where steel alternates between an aerobic and an anaerobic environment, i.e. between an oxygen-rich and an oxygen depleted environment. Free seawater is aerobic and mud is anaerobic. Although the mechanisms don’t seem to be quite understood, sulfate-reducing bacteria in the mud are thought to be among the factors that accelerate the corrosion process.

Anyways, what’s important to understand is that steel isn’t immune to corrosion in an oxygen-depleted environment, it just corrodes in a different manner. Also, stainless steel can be expected to fare even worse in oxygen-depleted mud than plain carbon steel.

OTOH, steel in free seawater often corrodes very slowly, and may last for a surprising number of years. A plain steel eye bolt cast into the top of a concrete mooring may look almost like new, even after twenty years or more on the seafloor.

Arne Mogstad

So, this may be a stupid question, but why could one not leave a yacht on anchor on a semi-permanent basis, and skip moorings altogether? People I speak to say they are worried to even go scuba diving while the boat is on anchor, much less to leave the boat unattended for a day or two. Having now ridden out a few storms (60+ knots sustained) and well in excess of 120 anchor-days since I bought my boat this summer, I feel pretty confident to leave the boat on anchor provided the forecast is not too bad. Is this a false confidence?