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.
Here is an example of why: Recently I have been trying to get a mooring put down in a sheltered inlet in Nova Scotia that will be heavy enough to withstand fall storms and even a hurricane. The first person we contacted, a reputable diver who does many of the moorings in that inlet, suggested a 2000lb (900kg) concrete block for our 26 ton boat. He said that was the heaviest weight he uses.
Now let’s think about that for a moment: Concrete loses half its weight in water and a block has virtually no form drag, so we are talking a total holding power of 1000lb (450kg). That is much less holding power than a good pattern 25lb (11kg) anchor provides. Would you trust your 56’ boat and home to a 25lb anchor, even in settled weather? I know we would not. Sure, when the block sinks into the mud the holding will go up, but not that much, and if the block moves at all from the snubbing of the boat, that benefit goes to zero.
I’m not trying to dump on the diver, particularly since, after a bit of discussion, he agreed that a lot more weight was required for our boat. But the point is—I know, you thought I would never get there—that many, maybe most, of the moorings in that inlet are at best 2000lb blocks.
Contrast that with our 120lb (55kg) SPADE that I would conservatively estimate can, when well set with plenty of scope, withstand a drag load of well over 8000lb (4500kg). That even makes a 1000lb (450kg) mushroom anchor (considered a heavy mooring weight on the east coast of the USA) look pretty wimpy. Even if it does move a bit, a good anchor, like a SPADE or Rocna, will reset itself, whereas a concrete block or mushroom anchor, once moved at all, will exert little more drag than its dead weight in water.
Finally, I know that our chain is in good condition and all the shackles are properly wired. Given all that, we know which ground tackle we will choose to give us a good night’s sleep.
Further to the above, when bad weather threatens we are always amazed how many cruisers pick up a mooring they know little or nothing about rather than heading for the security of a snug cove and their own anchors.
{ 3 comments… read them below or add one }
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.
Hi Jim,
Great information, thank you. It is great to get these kinds of things properly calculated by someone who, I’m guessing, has engineering training? If you see anything else on our site that could benefit from your expertise, we and our readers would be very grateful for your expertise.
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.