Overheating in Docklines and Rodes

Posted by Drew Frye at 09:20AM - Comments: (6)

What is the rate of cycle loading on a docked boat? What is the ratio of those loads to breaking strength? PS looked at load cycling rates for a report on spring lines in the July 2016 issue.

In the upcoming February 2019 issue of Practical Sailor, our testers evaluate the frequently repeated warning that under extreme loads, dock lines, drogue tow ropes, and anchor rodes can part due to overheating caused by internal friction between fibers as the line stretches and relaxes. But what percentage of a rope’s breaking strength is considered an “extreme” load? And how does the frequency of the load cycling affect heat build-up? And most importantly, how can we prevent this intra-fiber heat buildup from occurring?

Ultimately, our testers found that although intra-fiber friction is a real threat, the more troublesome threat is our obvious enemy—external friction caused by rubbing on chocks, anchor rollers, docks, fairleads etc. And the risk of internal friction on smaller diameter ropes is far less likely than it is on larger diameter ropes.

Ropes greater than one-inch in diameter are subject to internal heating if loaded past their safe working limit or loaded at high frequency. Ground anchor and sea anchor rodes are cycled at low frequency; although the core will see some increase in temperature, it will not progress to the point of weakening the rope unless the rope is loaded beyond the 10:1 safe working limit. If the anchor rode is sized right, it will never be over-strained or overheated.

Sea anchor “rodes” as well as tow lines for drogues, on the other hand, are often specified at about the same size as anchor rode, even though they are subject to much greater forces than a boat anchored in a harbor. Instead of 10:1 safety factor, a line for a drogue or sea anchor could easily be operating at safety factor of 5:1, or even less. Thus, a larger sea anchor rode can indeed be weakened by internal heating.

Could internal friction have caused Golden Globe racer Susan Goodall’s drogue to part, a failure that contributed to her boat’s dismasting? Although external friction is the more likely cause of failure in drogue or sea anchor lines, the loads imposed on a sea anchor rode can create enough internal heat to cause failure. And as we saw in our drogue evaluation, the extreme stopping power of the Jordan drogue that Goodall was using can impart significant loads on ropes, splices, and deck hardware. These loads were among many factors that prompted us to issue a warning about using metal thimbles in lines and rodes that are subjected to extreme loads.

Dock lines are more serious problem. If the boat is exposed to short-period chop from the side, the frequency can be high and the force can exceed the 10:1 safe working limit. Core temperatures above the boiling point are possible in dry conditions, and even with spray to cool the rope there may be significant weakening. Add to this considerable frictional heating at contact points and special precautions are required.

So what can a sailor do? Our testers came up with five simple steps to take to prevent heat build up in your dock lines and rode.

Anti-chafe gear type. Avoid waterproof materials. Select materials that are inherently slippery, such as nylon webbing (see “Chafe Gear for Mooring and Dock Lines,” PS October 2012).

Anti-chafe gear positioning. Avoid sharp turns because these increase friction. Often poor cleat positioning can lead to extreme bends at a hawse hole or chock. Our review of deck cleats on common production boats revealed a variety of serious problems.

Minimize the run. Reducing the distance between the anchor point and the friction point will reduce the back-and-forth motion that will increase friction. Polyester or Dyneema pendants through the chock will combat friction here. Short non-stretch pendants attached to nylon snubbers and mooring pendants reduce rode motion over the bow rollers.

Avoid slack lines and bouncing. In a recent article on storm preparation we discussed motion control (see PS July 2016 “Spring Lines for Storm Preparedness”). A boat that is bouncing on its lines increases chafe, frictional heating, and forces. While it is necessary to maintain enough slack to accommodate changes in tide, excess motion should be controlled through the use of elastic devices or slender nylon tensioning lines. Stop the bouncing.

• Choose the correct size. When truly severe weather is expected, sailors like to double up lines and install oversize lines, but the average boat does not have cleats large enough to accommodate the required dock line size. We have taken to installing strong Dyneema loops over the cleats to allow attaching multiple large lines to a standard cleat. This should not overload the cleat if it is properly installed. This setup allows the use of multiple lines in multiple directions to control motion and to keep forces within the appropriate working load limit for nylon.

Comments (6)

Could you explain, might be with a drawing and/or a photo, the pendants use and their attachments to the snubbers?

Posted by: olddog57 | January 10, 2019 11:20 PM    Report this comment

When the chafe gear melted to the roller this highlighted that the problem is friction more than line stretch. If it were line stretch, the rope would have melted internally and failed, and the rope would have melted to the chafe gear. We have seen this (rope melted to chafe gear) in testing.

For this reason, we suggest a non-stretch pendant from the cleats to just beyond the roller, so that there is less movement across the roller. Polyester and Dyneema are both good.

Posted by: Drew Frye | January 10, 2019 9:27 PM    Report this comment

I had an experience with my Catalina 36 moored in the Housatonic River (CT) several years back The boat has a double pennant setup (2 - 3/4" lines) through a double bow roller set up - to 5/8" chain to a Helix anchor. When Hurricane Irene was approaching we all (the club fleet) did our usual prep taking down sails/dodgers/biminis - etc and adding chafe guards to the pennant lines. All was well after the hurricane but I was astonished to find that the nylon chafe guards had actually melted where tthey came in contact with the bow rollers. So if anyone doubts that heat build up can be a problem let me assure them it definitely is.

Posted by: gerryfs | January 10, 2019 12:00 PM    Report this comment

Thanks for an interesting article.

Another thing that you should look at is the effect of galvanized, cast iron or cast steel cleats on mooring lines, particularly permanent lines used at a home dock.

Posted by: Norton Rider | January 10, 2019 9:35 AM    Report this comment

There is one in the works. The best use of Shockles is not to absorb surges and impacts, but to reduce motion by controlling the loose slack in the lines on the lee side. By pulling in the loose slack, rebound is reduce, while still having enough length to compensate for tide and larger waves.

Posted by: Drew Frye | January 9, 2019 11:18 PM    Report this comment

Thank you for a useful article. Where I tie-up my boat in Cleveland and other harbors on Lake Erie we can get a heavy surge during storm conditions. I have seen dock lines fail, cleats shear off, and 4"x8" dock posts snap off.

A potential follow-up article would be on an evaluation of currently available snubbing devices. I have been using Shockles on the two bow lines for four seasons on my Beneteau 321. I have been very pleased with the damping effects and the reduction in the surge of the boat in its slip.

Posted by: mark2 | January 9, 2019 8:21 AM    Report this comment

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