Overheating in Docklines and Rodes
Published on February 5th, 2019
by Drew Frye, Practical Sailor
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. – Full report.