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Old 07-22-2017, 10:31 PM   #21
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I've seen metal spring devices used on dock lines. Expect them to reduce shock on the lines.

Hmmm. One-inch lines for the typical TFer boat seems like overkill. I'd lean toward multiple lines of smaller diameter. Thinner lines stretch more (helping to reduce shock) and fit on cleats better.
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Old 07-22-2017, 11:24 PM   #22
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Originally Posted by tadhana View Post
Well there have been some very interesting comments here about lines braking in the middle while under load. I have never see that and am interested to know if anyone can provide any more input as to how and why. 50+ years in boatyards and marinas i have seen a lot of dock lines fail in storms. But it was always at a chafe point. What am I missing? also amsteel has no stretch, zero, nada, and in rough conditions the entire shock load is directly applied to the points of attachment. A very strong rope, but might very well break the cleat at one end or the other.
Chafe, in my mind, implies line being abraded over a rough surface...which isn't the case at all. What's weakening the line is heat generated by the nylon fibres abrading against each other as they stretch over a hard edge or sharp radius.

Following link & quote explain a bit:

Nylon, also known by its chemical name polyamide, has been widely used in marine mooring and towing lines since the 1950's. It has the lowest stiffness modulus, and thus it is favoured where high extension is very important. It is the strongest of the common fibres when dry. However, wet nylon fibre loses about 10% of its strength, and wet nylon ropes can lose up to 20% of their strength. Wet nylon ropes also suffer strength loss due to creep and internal abrasion during tension cyclic loading.(Flory, 1982) The resulting short service life of large nylon ropes generally makes them unsuitable for permanent deep water moorings.

Polyester ropes are very durable in cyclic tensile fatigue loading. (Parsey, 1982, 1985) Very strong polyester rope with relatively high modulus can be made with the newer high-quality polyester fibres now available. They can be as strong as nylon when dry. Polyester ropes do not lose strength when wet and are generally stronger than nylon in wet condition. Thus polyester ropes are now supplanting nylon in many critical conventional marine applications and are good candidates for deep water mooring systems.
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Old 07-23-2017, 06:28 AM   #23
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I too cant recall docklines ever failing except at knots or chafe points, unless the dockline already had a bad spot in it.

But a few pictures have surfaced on this forum through the years including achor bridles that failed in randon spots IIRC.

I have seen tow lines fail in random spots, but they were on very old and abused towline znd probably fom weak spots generated from various issues.

Once saw a 3 or 4 inch poly line melt straingt through near the middle. Strangest thing.... was in a tug a war between 2 cub scout troops and funny as all get out when they all went flying. A couple of pretty experienced leaders all looked at the melt point and at best figured it was a defect that allowed the melt point a head stsrt as internal friction and heat built up....wierd but gut wrenching funny to watch....
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Old 07-23-2017, 07:17 AM   #24
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Double or triple your line keeping the same tension.
When I worked on the Columbia River hauling 240' wheat barges we had two loaded barges side by side. I took our barge line and wrapped back and forth between the barges using the length of the line. About eight loops. Each barge had 3100 tons of wheat. The tug Captain cut a corner and grounded the barge he was pushing. The side barge wanted to keep going. We broke one 2" steel haul back winch line, three 2" cross over steel winch lines. But the poly line I tied held the load. It was amazing.
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Old 07-23-2017, 07:31 AM   #25
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The kids broke polypropylene line (not known for strength), not polyester (used in towing hawsers).....
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Old 07-23-2017, 08:14 AM   #26
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You dont know the history or quality of those lines so it is hard to say what happened. IMO stretch is as important as strength.

i prefer three strand with a tight lay because it will stretch a bit and always use doubled lines in storms or when I'll be away for a while. better grade three strand lines have a colored thread that breaks when the line is over stretched for an early warning that the line has been overloaded.

you may have had excessive shock loads so some sort of shock absorber is worth considering. As is keeping lines short enough to prevent momentum build up. Always a balance as longer lines add stretch because the amount of stretch is proportional to the length of the line.
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Old 07-23-2017, 12:33 PM   #27
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What is the strongest 1" line for dock tie up ?

Definition of a tight fit.
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Old 07-24-2017, 09:22 AM   #28
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Originally Posted by O C Diver View Post
First, double up for storm lines. The first line should slow the boat as the line stretches, the second should stop the boat with only minor stretching. Repeated shock loading destroys a rope internally. In rock climbing, they use to (may still) rate ropes for number of falls. After so many falls (shock loads), the rope is retired.

Second, avoid short rope runs between dock and boat. The velocity at which the boat stops (a contradiction of terms), hugely impacts rope degradation. You want to slow the boat to a gradual stop, not jerk to an abrupt hault. Shock loading destroys the cords and can break loose boat hardware.

This is how we rig for storms as well.

The fact that the lines were of unknown origin, age and condition and the fact that they were not doubled-up is probably the root cause of this issue IMHO.

Nylon lines have a fixed amount of stretch. Eventually they don't stretch and rebound any longer. They effectively become 'static' lines. When this happens the 'hit' becomes big.

We spent years in a rough marina subject to swell and storms were rough. At the beginning of the season, you could feel the stretch in teh lines. By the end of the season, you could feel the 'hit' because the stretch was no longer present.

I never snapped a line, but spent countless hours over the years running around the marina shoring up boats with parted lines and pulled cleats. The lines that snapped (and cleats that pulled) were all very short runs and/or very old line with no stretch left in it. then there were old chaffed lines as well.
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Old 07-24-2017, 10:27 AM   #29
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I would expect undamaged 1" docklines to pull the cleats out of the boat or dock before they broke. I don't know what's appropriate for your boat but the good rope manufacturers have tables to determine the appropriate sixe for your situation.

Stretch is important for dock lines. The ability of a line to stretch takes the strain off the cleats. Think of what would happen if you used chain for docklines and a surge occurred.
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Old 06-09-2018, 04:01 PM   #30
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I seriously doubt if 1" nylon which is used for dock lines actually broke without first chafing. 1" has a minimum breaking strength of about 20,000 lbs. That will lift a lot of boats out of the water vertically. With four 1" lines on the corners of the boat, the piles or the cleats would give way first.

Chafe was probably the biggest factor.
This spring I saw FOUR 1-1/4" 3-strand nylon lines part in the middle during a freak storm with 50-60 mph sustained winds and 70-80 mph gusts for 3 days. The 35 ton boat was side tied to a T-Head with two bow lines, fore and aft midships springs, and two stern lines. Only the two stern lines held! All four lines that parted were parted in the middle, and NOT due to chafe (unless you consider internal friction as chafe). My assumption is that the repeated jerking on the lines over 3 days due to wind/wave action caused the 3-strand lines to repeatedly twist/untwist causing internal friction, heat and finally failure as the nylon fibers melted. The boat was well fender'd and amazingly, there was no damage to the dock or pilings.

I've read about this type of failure on moorings and anchor lines during hurricanes in the Caribbean, but had never actually seen it until now.
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