It may be better to use multiple hydraulic rams/jacks to support the weight along the roof length. They can be positioned inside the roof too.
Mind the wind doesn`t catch the roof while its open or it could be gone ;)

Sucker bait,
You have not given us any dimensions so it is difficult to give you a valid reply. With that in mind; I see no reason you could not use your boat wench. A lot depends on the length of the roof you want to life. Where do you purpose to anchor the wench? What will be your point of attachment for your pully(s) and I would suggest more than one point of attachment (to the roof section).
As Hex mentioned, the wing coulld be an enemy so I would also suggest that you have a LIMITING CHAIN or cable attached to keep the roof from opening ANY MORE that the 18" you are previding for.

Give us some more detals and we can give you a better answer

1eyedJack and the Dawg

thanks for the replies. a limiting chain is a definate. the greenhouse is 10x20 and the pitch is 5 3/8 / 12. i have the roofed framed as one hinging unit with polycarbonate roofing. it is fairly light i would like a central crank position with one pulley, but i am imaging a split somewhere to outside posts would stop swinging. i have a lot of images in my head, however they all don't seem perfect yet. i can figure out all the details. i'm just looking for a simply layout regarding my pulleys and cables. thanks again.

I would do two positions with the lifting points about 5'-7' in from each end of the 20' roof edge. This will help prevent sagging. If you can mount external posts to the frame at these points, offset a little so the roof edge clears, you could put a pulley atop each one; then run cable from the two lifting points, up through the pulleys, and converging in the center -- probably to a central pulley -- and then to an externally mounted crank.

thanks, that's kind of what i was thinking, with the central pulley as well. i hadn't decided where to put my external posts yet so i am going to go with your 5-7' suggestion. thanks again. anybody seen any pictures of this sort of roof on the net anywhere?

Hi suckerbait
Sounds like it should be do-able.
Chains and cables would work but i would worry an area that large snatched by the wind could easily tear our the anchorage points.. especially with 18" of distance to build up speed and force. Unless its fully open it may be possible for the wind to lift and drop the hinged section repeatedly :)
To maintain a positive grip on the roof during opening and closing, standard type scissor jacks connected in series along the roof would be very sturdy and easier to operate :)
Fix a sprocket to the end of the series jacks and use chain drive down to a crank at a comfortable height.

A winch handle that gets away from you can`t be stopped and could well break your hand. A worm drive won`t move unless you turn the crank and self holding at any position ;)

Just a thought.