I saw this on Cruisers' Forum and thought it was worth sharing here but suspect vast majority of OCC Members know how to rig a preventer correctly, still if anyone is not sure here it is:
https://www.maritimenz.govt.nz/commercial/safety/accidents-reporting/accident-reports/documents/Platino-mnz-accident-report-2016.pdf

and the relevant diagram:

My take is that you want a little give in the preventer to avoid any impact loads, so we use some very stout 5/8" double braid penants on the boom and then the main preventer lines are attached by bowline to the penant and are made of 1/2" sta-set double braid. Ours is a 44ft boat with 650 sq ft main.

Bill Balme

+x

 

Bill Balme - 9/5/2019

My take is that you want a little give in the preventer to avoid any impact loads, so we use some very stout 5/8" double braid penants on the boom and then the main preventer lines are attached by bowline to the penant and are made of 1/2" sta-set double braid. Ours is a 44ft boat with 650 sq ft main.

Hi Neil,
I agree w/ Bill. Nylon, in reasonable sizes, might allow the boom to go full swing if the tip is dipped in a wave as it stretches a great deal. HM lines are like wire and would make transmit shock loads directly into the preventer system un-modified by any cushioning. Double braid Dacron/polyester in ½ inch is my choice (significantly smaller main), but size should be chosen commensurate with load estimates. Mine is regular untreated Dacron by a good manufacturer. Sta-Set is likely also fine: it has been worked to decrease stretch, but does not approach HM line, I believe. My experience w/ Sta-Set X is that it is stiff and hard to handle, less so w/ Sta-Set: fine for halyards, but less user friendly for lines needing handling like sheets etc.
It would be interesting to have a rigger weigh-in. If anyone is near one (I am not), one might ask and give a report. Make sure they are experienced with offshore boats and not just coastal cruisers.
My best, Dick Stevenson, s/v Alchemy