Like some of the others, I suspected your plan to use at least ¾ pendants
to be overkill for a 27 foot boat.
Someone else pointed out that you want some stretch to cushion the shock and
¾ line will have no stretch to speak of at the loads your boat will
generate. But the desirability of stretch is more appropriate for an anchor
rode and not a mooring pendant. At a mooring, the cushioning is a result of
the extra chain and catenary between the mooring ball and anchor; your
pendant would probably only have a few inches of stretch over its relatively
short length.
To see if ¾ line is really overkill, I spent a few minutes doing research
on the Boat US site, Fortress and Mantus anchors sites, and a site having
engineering standards for different types of rope.
There is an ABYC table on the Boat/US site showing the following for the
estimated load on an anchor and rode based on boat size and wind speed. The
data is:
Wind Speed 30kt 42kt 60kt
Working storm
violent storm
Boat size
25 490lb 980lb
1440lb
30 700lb 1400lb
2800lb
35 900lb 1800lb
3600lb
These numbers are pretty consistent with what I found on the sites for the
anchor companies for anchor load numbers.
The rope specs show the following for minimum break strength for medium lay
3-strand nylon:
7/16 ½ 9/16 5/8 ¾
4320lb 5670lb 7200lb 8910lb 12780lb
Recommendations vary on the working load to max break strength ratio. As low
as 3:1, 4:1 seems common, but Mantus says 8:1.
At 4:1 the working loads are: 7/16= 1080 ½=1420
9/16=1800 5/8=2230 ¾=3195
At 8:1 540
710 900 1115
1600
For boats anchoring in 30 knots of wind, Mantus and others recommend:
20-30 & 11000pounds 9/16
30-35 & 15000 ½
35-40 & 20000 5/8
West marine suggests 1/8 of diameter for each 8 of length for heavy boats
with high windage. So for a 32 boat that would be that would be ½
diameter. Add another 1/8 for storm conditions.
Boat/US recommends the following in 30kts.: 25 boat=3/8, 30=7/16,
35=1/2. But, hey, theyre mostly talking to power boaters.
On one of the sites there was a note that deck cleats are generally
installed in such a way as to resist a minimum 3400 pound load. I dont know
if that is some sort of standard or just an observation.
You plan to run a single pendant, with a somewhat longer backup in case the
first breaks. For a 30 boat, at 4:1 safety factor, at 42kt winds, it looks
like you need ½ line. In a hurricane ¾.
Or you could use the system I did when I was on a mooring and use two ½
diameter pendants of equal length. Smaller diameter would let you install a
pair of chocks instead of heavy cleats with a high load. And you can use
your existing deck cleat. The pendants would be less expensive and easier to
handle when picking up the mooring. Two pendants would easily handle the
loads from a severe storm. And the equal length pendants on either side of
the bow would act, to a certain extent, like a bridle to reduce the extent
the boat sails from side to side on the mooring.
Good luck
Rick Brass
Imzadi C&C 38 mk 2
la Belle Aurore C&C 25 mk1
Washington, NC
On 2015-03-13 9:28 PM, Paul Baker via CnC-List wrote:
So, I am going to be moving my boat from a nice, safe and secluded slip, to
a mooring buoy out in the harbour. Yes, the mooring is big enough, yes I
will be using at least 2 unequal pendants of good quality (probably at least
3/4"), and yes I will be using chafe guard where appropriate. Assume also
that any hardware would have backing plates as large as practically
possible.
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