The stock 3:1 downhaul on the H18 isn't very effective. You'd need to upgrade to a 4:1, 5:1 or 6:1 system to get it to perform properly.
The older the sails, the more they tend to stretch and the more downhaul you need to apply to remove the wrinkles. If you're worried about tearing the sail, maybe it ought to be repaired or replaced.
Now, if you're just looking to have some fun on the water, the boat will sail without any downhaul whatsoever... your sail trim won't be right, and it won't be very efficient, but it'll sail just fine for your needs. It's up to you to decide whether you'd like to make due with what you've got, or try to upgrade. I built a 5:1 downhaul for ~$175, using all parts listed in the hobie catalog.
I borrowed centralmichigansailor's design (viewtopic.php?f=13&t=36064&start=30
). I like that it was simple to install and didn't require any drilling on the mast. I'm not racing, so I don't mind that it's not more fancy. I believe srm has a really nice system that he's outlined on the forums... 8:1 purchase, and adjustable from either side of the mast, for ~$250.