Mike-
We have two moorings- one on the Merrimack River and one in Rye Harbor NH. Booth moorings utilize ~ 8 foot pendants which are of adequate length for pick up with a boat hook. Both mooring areas are crowded, and we are restricted on the overall mooring tackle scope that the harbormasters allow. The Merrimack for instance runs ~ 4 knots where we are moored and has 28 feet of depth at high tide. There's quite a swing at the mooring when wind and tide oppose each other. I run 44 feet of 1/2 chain from the mooring block to the buoy. I'd like to run a longer length to ease strain on the bow cleats in bad weather, but can't because we'd swing into other boats. The pendants are 3/4" line (I use two), I forget the brand, but it's very high-end (Spectra??) and they show no degradtion after 8 years of use. They are also somewhat protected by UV becuase we use pipe insallation to float the lines and prevent them from wrapping on mooring chain. In both locations, we get about 3 years of life from the chain. We drop the chain to the harbor bottom in Rye (slows the rusting process by being buried in the muddy bottom) and haul out the mooring in Newburyport (to prevent ice from cutting the tackle).

This is all probably more than you'd like to know.

Plymouth seems like it would be a calmer harbor than ours.
  

we have on board 2 long pennants with a length about 20-mtr and 2 short ones with a length about 10-mtr.
material is polypropylene 16mm.

sufficient for our Siler for safe mooring everywhere in our sailing area.
 

With all due respect to Durk, polypropylene line is not recommended for mooring lines of any length. They do not stretch and give  as do nylon, providing shock absorption.

As far as length, our harbor (which owns the spaces in the water we "rent"... we pay for all of the tackle) dictates the length.

Well Chuck, agree, there is polyprop and polyprop. When you have the cheap ones it it shit.
But as long as polyprop is used a lot on commercial ocean going vessels I don't see any reason to have doubts that this quality wouldn't suit to keep our Catalina alongside. The good thing is also that it is floating...
 

Durk.... your right about it floating and I know they use large sizes on tugs where they're working with it daily and monitoring its condition but to use it as a mooring pendent where the boat is left maybe for a week or 2 , depending on the weather,.... I still don't recommend it.

How do others feel?