A lot of harbour masters on the East Coast would disagree with you. I am in 12-15' of water with 30' of 1" bottom chain and 15' of 5/8" top chain and 18' of pendants. It takes a lot to lift the bottom chain.
Mooring rigs are designed to operate with much shorter scope than an anchor. With a rode of 150' the shock loading is distributed over much more line. If all your elasticity is in 20' of stretchy line, it may over heat and fail. We saw this during the hurricane, and thus advised against nylon pendants in favor of low stretch pendants and increased the weight of the chain, thus increasing the catenary. Eric On Sat, Mar 14, 2015 at 10:58 PM, Joe Della Barba via CnC-List < cnc-list@cnc-list.com> wrote: > I wouldn’t be so sure about the catenary providing much shock absorption. > In about 30 knots of wind my mooring chain is pretty much stretched out > taught. I have 30 feet of chain in 12 feet of water. Speaking of stretch, > in Hurricane Charlie we had 150 feet of ½” nylon in 10 feet of water. It > was like being on a giant rubber band J In the morning the anchor was dug > in so deep we needed a CG patrol boat to help get it loose L From the > looks of the line the anchor was at least **6 feet* under!* > > > > Joe Della Barba > > j...@dellabarba.com > > Coquina > > WWW.CRYC.ORG > > *From:* CnC-List [mailto:cnc-list-boun...@cnc-list.com] *On Behalf Of *Rick > Brass via CnC-List > *Sent:* Saturday, March 14, 2015 9:59 PM > *To:* 'Paul Baker'; cnc-list@cnc-list.com > *Subject:* Re: Stus-List Deck hardware for mooring > > > > 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, they’re 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 don’t > 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. > > > > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > >
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