Rode Elongation

Chain  doesn't stretch (enough to matter in the math), but if enough is deployed and depending on the wind strength, the catenary formed by gravity absorbs energy. Nylon can stretch a great deal, according to the weave, about 7% at 12% of breaking strength, its practical fatigue limit when wet. (AKA working load limit, or WLL)

 

 

Polyester stretches less, only about 3% at 12% of breaking strength, but its fatigue limit is much higher, so it is safe to work at higher load factors. In fact, the 5% stretch of polyester at its WLL of 20% compares very favorably to to nylon's 7% at its WLL.

The chafe resistant of polyester is about 2-3 times that of nylon. Not load related, but important.

8-strand polyester is not listed in this graphic, but if you compare DB nylon and polyester, and compare 8-braid nylon with DB nylon, you get the same picture.

Next, consider that an all-nylon rode may actually stretch too much, increasing yawing and thus increasing rode tension. I studied this for Practical Sailor and came to t he conclusion that somewhere between 60 and 100 feet of nylon snubber resulted in the lowest loads, and that beyond that, the load increased with longer snubbers. The conclusion seemed to be that nylon is too stretchy beyond some length.

One solution is oversized nylon. That is what I did for my F-24; the required anchor line is only 3/8-inch, but 1/2-inch is easier to haul by hand and reduces stretch. But not a good answer for larger boats or if a windlass is used.

Another solution is a polyester rode. Popular in England, as long as there is a good length of chain, it works. What is a good length? Optimally, at least 100 feet. Steve Dashew was a proponent of polyester rode after several hundred feet of chain. 

Finally, remember that combination rodes with 15-50 feet of chain rely on the rope portion to absorb energy. The corollary is that there must be enough nylon to dissipate the energy, and rodes have failed when there was only 5-10 feet of rope out. That means about 25 feet minimum with nylon, and probably about 50 feet with polyester. 

The forces on the anchor and deck fittings will be slightly higher with polyester rode. The risk of chafe and fatigue failure will be less.

I need to look into this further. Polyester has many merits, but perhaps it is less idiot proof, particularly in shallow water.