Winter Mooring Log

In the U.S. North East, it is standard practice to remove the mooring float and attach a log for the winter. This anticipates the sea freezing : a large spherical buoy is much more likely to be caught up in the ice, which can then drag the mooring, or even break the buoy free. Traditionally, a length of tree trunk or old telephone pole is used; the straight parallel sides give little purchase for the ice.

Others use plastic pipes for the same purpose; I decided to break with tradition also; the main justification was that I wanted the "log" to be under water at anything above low tide, to further reduce the chance of ice capturing the marker. I wanted to do this by judging the bouyancy required to float the depth of chain for the mooring, and I thought I could calculate the buoyancy of plastic pipe more easily than wood.

Calculations are fairly basic - you need to know the internal diameter of the pipe, the density of sea water, and the weight of the hardware attached to the log.

Schedule 40 DWV (Drain, Waste, Vent) 4" d. pipe is the obvious choice. I checked the buoyancy of the pipe material with a small piece : in fresh water it was almost neutrally buoyant, so I decided to discount it for the purposes of buoyancy calculations; in sea water it would be a little buoyant, but this would provide a safety factor - better to be over-buoyant than under.

I used standard bathroom scales to weigh the hardware :

• 2 end-caps
• 3 shackles
• stainless steel eye-bolt, nuts and washers
• 26 ft. ¼" chain

Sea water has a density of approximately 1026 kg/m³; over here in the Colonies, we'll say that's 0.037 lb/in³; the approximation is because of variations in salinity and temperature, but as you'll see the final calculation is rather inexact anyway, so this is an acceptable approximation.

Following Archimede's "Eureka!" principle, we need to find the volume of sea water that is equivalent to the weight of the log hardware : weight divided by density equals volume.

The pipe is 4" internal diameter, so the area of the pipe's cross-section is :

    pi * (4/2)² square inches = 12.57 in²
    

    length = volume / area

    or :

    length = (weight / density) / area

    so :

    length = (20 lb / 0.037lb/in³) / 12.57 in²

           = 43 in
    

I drilled a hole through one end cap and fitted the eye bolt, complete with sealant; I used some gunk called "E6000" because it was on the discount trolley when I was buying the hardware, and the blurb on the tube was convincing, but this seems an ideal opportunity for that nasty 3M 5200 stuff if you have a tube to hand.

Note that the shackle on the end of the log chain should be large enough to accommodate the (probably larger) mooring chain.

The end caps were glued to the pipe with standard PVC cement; I reasoned that this effectively produced a mechanical bond which would be strong enough to withstand the buoyancy and ice forces - seems I was right (so far)... I considered putting a threaded rod through both end caps, but that seemed excessive, and complicates buoyancy calculations. I also could have filled the pipe with foam - but I didn't.

There's a wrinkle, of course; the sharp-eyed reader will have spotted the third shackle (one on each end of the chain, plus one). Having calculated flotation for 26 ft. of chain, I knew that my estimate of the mooring depth wasn't too accurate, and that I needed to take the area's 10-ft. tides into account, so I used the third shackle to make a loop to ensure the "log" was weighted with 26 lbs. of chain even if the actual depth was shallower (again, a rough measure but still good enough). I adjusted the length of chain in the loop when I put the log in place, going by feel to assess when the log chain (now attached to the main, and much heavier, mooring chain) was on the bottom - or nestled in the mud, at least.

Of course, if you are confident of the depth of your mooring, this adjustment isn't necessary, but it does mean you don't have to faff around trying to get the chain length exactly right for the flotation you are aiming for.

In summary, I knocked this up after discussion with fellow mooring maintainers with a range of solutions, and some thought. By the end of the winter, I was pessimistic about my chances of finding the log still attached to the mooring, but it turns out it was only my pessimism that was misplaced - sometimes it's good to be wrong...

Then again, there's the chap next to me who left his buoy out all winter, complete with whip, and just picked it up this spring - all intact. Bastard.