long term mooring design - an engineering question
The solution is to get the hell outta Amer Samoa and go cruising. You
been there too long already.
"mitch" wrote in message
. com...
I have a mooring problem maybe someone has solved befo its a bit
long trying to explain the situation...
Background:
Weather conditions in winter are re-enforced tradewinds, 20 to 35,
blowing down a bay 2 miles long. Wind generated waves reach 4 to 5
feet, max. Many days of calms in between stretches of wind. Summer
conditions are mostly calm, with often occuring squalls from any
direction, and an occasional hurricane. Early January saw cyclone
Heta graze by, with winds of 90 to 100 for about 12 hours.
Our boat is on a mooring, in 40 feet of water, at the head of a bay
fed by a small stream. The mooring is a 1200lb ships anchor, with 150
feet of 5/8" chain. The chain is attached to an 8 inch diameter
galvanized steel ring at appx 10 feet below the surface, to which is
attached a 3/8" chain with a mooring ball which holds the chain "up".
Attached to this same galvanized steel ring are my one-and-a-half inch
diameteer, led thru the bulwark hawsepipe and secured to the sampson
posts.
The chain/float/mooring lines connection is at 10 feet below the
surface due to the high corrosivity in the water here in the top 6 to
8 feet; the junction below that level greatly extends chain/shackle
life.
There is no swivel, as the weight of the chain (about 6 lbs per foot)
binds the swivel upon deployment. I tried this for two years, and can
confirm that the swivel is useless. Additionally, of the ten boats
here on moorings, two in the last two years have gone aground due to a
mooring failure, and each time it has been swivel failure.
OK, now for the problem:
The wind here is highly variable, with some periods of calms
in-between. Boats are oriented by tides, multi-directional winds, and
(in calms), by the stream. This results in the boats being rotated
around their moorings continuously, with the result of mooring lines
twisted around the mooring ball chain. And THIS requires an almost
weekly "untwisting" of lines and mooring ball chain. Either in the
water to do the untwisting with mask and snorkel, or spinning the boat
around the mooring with the dinghy. Its really pretty humourous
watching three dinks spinning three boats either clockwise or
counter-clockwise at the same time on calm days. I sometimes wonder
what people on shore must think...
Unfortunately, when its blowing 25 to 30 for three weeks, this is
impossible. And if the line is severly twisted when it starts
blowing, the coral/barnacle growth on the line combined with the
tension caused by windage on the boat literally saws the mooring lines
in half.
The only solution I can see is to eliminate the mooring ball, and
shackle the mooring lines directly onto the 8 inch galvanized steel
ring ten feet below the boat. This eliminates the tangling/knots that
result, but then puts the mooring chain load directly onto the boat
via the hawsepipes. Two boats here have now been doing this for a
year, without any apparant problems. They niow wave merrily from deck
when the rest of us are "rotating our boats". All boats survived
Hetas 90 to 100 knots, but there was adequate time to prepare.
Every reference source I can find indicates that using a mooring float
is the "correct" procedure. But I cannot get around this snarling of
mooring lines and pennant! Other than the danger to the boat, it is
getting expensive replacing mooring lines...Anchoring bow and stern is
not possible, as we can see winds from the entire quadrant in excess
of 40 knots on any particular day.
Any ideas welcome.
Mitch
sv KOMFY
American Samoa
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