Re: PSC 37 running backstays...
Are there winches available for tensioning? If so you won't need much of a tackle.. if not I'd think 6:1 should be adequate and have seen plenty of 4:1s, but perhaps on smaller boats.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)