There seems to be poly confusion about the various kinds of synthetic fiber
ropes with "poly" in their name. I must admit that I was confused myself. I
spent an evening reaching back into my memory as a rag boater and researching
manufacturer's web sites and now all is clear. Since I started boating back in
the days of manila rope and cotton sails, any line that can be put away wet
seems a miracle to me.

Let's start with nylon. OK, nylon isn't a "poly" but it is a good baseline.
Nylon line is strong, stretchy, and has good sunlight and UV resistance. It is
modestly abrasion resistant. A 1/2" twisted nylon line has a dry break
strength of 5,750 LB but loses up to 10% of its strength when water saturated. Under
load, nylon will stretch up to 25% before breaking. Nylon is almost perfect
for anchor and dock lines where the stretch reduces transient loads on cleats
and anchors. On the other hand, the stretch makes precise positioning of loads
difficult. Nylon does not float.

Polyester, sometimes known by the trade name Dacron, is almost as strong as
nylon and does not lose strength when wet. It is very abrasion resistant and
has limited stretch, about 3%. Breaking strength of 1/2 twisted polyester is
5.720 lb. Dacron fabric is almost universally used for recreational sails because
it holds its shape very well and is resistant to sunlight. Polyester is used
for running rigging on sailboats (halyards, sheets) and in situations where
abrasion resistance is required. Braided line can be supplied with a nylon core
and a polyester sheath making a composite, but expensive, anchor line.
Polyester does not float.

Polypropylene line is relatively low priced and is used in large quantities
in the construction and towing industries. It has fairly high strength and low
stretch. A 1/2" twisted line has a breaking strength of 4,200 lb. It has poor
resistance to sunlight and tends to become brittle, the outer fibers of a rope
forming nasty splinters after a few years exposure to sunlight. Polypropylene
line has many uses aboard a boat as long as you don't expect it to last
forever. I use it to tow my dinghy, but i make sure to replace it every couple of
years. It's cheap. Polypropylene line floats.

Polyethylene line is the favorite for water skiing. It has a soft hand, is
moderately strong, and does not splinter after sunlight exposure. It is also
quite cheap. A 1/2" twisted polyethylene line has a breaking strength of 3,500
lb. It is usually sold as a braid rather than in twisted form. Polyethylene is
very slippery and does not hold splices or knots well unless special knots are
used or extra tucks are taken. Some formulations of UUHD polyethylene
(Spectra) are extremely strong offering breaking strengths of 16,000 lb for a 1/2"
twisted rope. The stretch is very low and it has been used for standing rigging
in racing sailboats. The downside is that it is quite expensive and short lived
in sunlight. Polyethylene line floats.

There are about a half dozen other synthetic fibers used in rope, some
offering extreme strength characteristics, but there is little use for them aboard a
trawler. They are not "polys" either.

Larry Z

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To add on to Larry's explanation, I would like to point out that the three
major manufacturers (New England, Yale, and Samson) all make a variety of
Polyester sheathed braided ropes with a variety of cores. These cores are
mostly intended to reduce stretch and/or add strength. Although I use New
England Nylon, braided dock lines, I like Yale's website best as it clearly
shows each type of  line, its characteristics and intended applications. The
industrial portion of the website shows even more types of cored line.

Ron Rogers