Polyester vs. Nylon is not one of those things that has a simple answer. I've used polyester for kedge and shore lines. I've even used it for a
bridle (in combination with a Nylon rode). What I would not do is use it as the major part of a shallow
water rode. You won't like it and neither will your anchor.
Chafe. Polyester has about 4-5 times less stretch than nylon in a given weave, but an elastic (loose) polyester weave will stretch several times more than, say, Stayset. BUT when you make a looser weave you reduce snag/chafe resistance on rocks. For example, Amsteel is less chafe resistant than Stayset if moved side-to-side on a rock. I've tested this side-by-side on a pendelum rig a number of times, with different brands. It's strange to see. We also see this when
lifelines cut. The stretchiest nylon braits are TERRIBLE on rock (I've done the testing); yeah, they stow well, but they cut twice as fast as a tight 3-strand. So a loose polyester weave rode probably is less chafe resistant on rock than you think, probably not much different from Nylon. One of the most chafe resistant materials we use is Nylon tubular webbing (think Chafepro). So the ONLY way to know if a rope is more chafe resistant than another is to test them in the mode you have in mind. A tight Nylon 3-strand will outwear a loose polyester brait on rocks. Strange but true. They won't show you that test.
Stretch. Polyester stretches less than Nylon. Tight weaves, like Stayset, are a jackhammer with only 50 feet out. I was testing in shallow
water, which was clearly stated. Now, if you are in deep water and have 200 feet of chain out (this is the example Dashew makes), then the math is different, since you've got catenary, a bigger boat (less affected by waves) and a lot more line. In fact, if you have more than 100 feet of Nylon out, the stretch can be excessive and cause more surging and yawing.
Also remember that rope manufactures understate the performance of competing rope types. They look for the worst one they can find.
So it's not simple. You can use a looser polyester weave and gain stretch, but you lose side-to-side toughness on rock. This is big deal. You can deploy a lot and get more stretch. You can go down in size to get more stretch (some advocate this) to take advantage of the greater fatigue life of polyester. But I promise if you do all of that, the line is not as chafe resistant as nylon (smaller diameter = greater pressure per area against rock).
Someone suggested using a nylon snubber with polyester rode. OK. I use a
Dyneema bridle with Nylon.
The other thing you don't see is rode tension comparisons, side-by-side. For example, the OP stated that a certain polyester line stretched 15% at working load vs. 20% for nylon. But neither stated what the working load is, so there can be no comparison. In fact, the ratio of stretch, comparing at equal load, is more like 2-3 to 1. That means the energy transferred to the anchor will be at least double in shallow water. In deep water there will be more damping; there always is.
So not simple.
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And the "poly" talk is always
funny. Obviously, they are all "poly."
-Polyester
-Polyamid (Nylon)
-Polyaramid (Kevlar)
-Polypropylene ("poly")
-Polyethylene (Dyneema)
-Polysacride (cotton)
-Polyamine (wool)
As for the rope vs. line discussion... that's just droll.