I'm thinking of doubling up the purchase on my backstay tensioner. Will I achieve anything useful? Will I rip the fittings out of the hull?
It depends how hard you pull on it.
if its anything like the purchase on the 38 woudnt bother,I too thought about increasing it when first got boat , but soon found out plenty !!!, bought a nice shiney new rope instead .
only advantage makes it easy to pull on too much ;)