Have I got the book for you or what!
It's about the 'Rescue Ships' and they did convert a few corvettes to this role but they found them troublesome as the tumbleholme? and flare to the bow and stern made bringing a lifeboat there a bad idea for guys who've been in an open boat. [They'd have had not only had to climb up, they had to climb 'out' as well.]
So they prferred to use the small mail boats that had more of a vertical sides.
But I have images of a modified corvette in this book and I'll sell you scans of them... we'll just have to agree on a price.
Actually, I've jst ran and got it, there's one full shot of the 'EMPIRE REST' with the nets (6) just hang over the midships section. They are, as ian says, about 12" spacing. On rescue ships, they'd also hang one off a boom, swing the boom over the side so it was 90° to the ship and they'd scoop up any survivors who fell off from up fore, or it'd catch a helpless boat that was drifting along the side.
The net is just rope that's been woven into a net. [you could 'trawl' the internet for net-making how-tos for fishermen. Sorry, couldn't resist!]
If you think they'll be of use, I'll scan the later when I've a min. There's 2 that you may find useful?