Please look at a photo of Lexia and her Hydrovane. Off the self Sunbird 32 with standard Hydrovane on the stern. I have never had the main sheet interfere with the vane in some thousands of miles. jds
Postscript.
Whilst walking the dogs round the hill through the snow in the gloaming, always good for gathering one's thoughts, I remembered the following. When I bought the boat it had a quite tall structure across the stern which had been used for aerials etc. When I added the Hydrovane I found that the vane hit the top aft rail of that structure. I therefore cut down the structure to somewhat above the height of the swivel of the vane and cut out the resultant top aft rail. I then had two holes in the tops of the remaining forward uprights. I had a length of cheap flexible plastic water pipe which I cut to length and rammed into the holes to make an arch which leans backwards. It looks like the sort of arch that people have across the cockpit to keep the sweeping main sheet above the heads of the crew, however this is aft on the stern. The tip of the moving vane just clears t his arch. The arch is aft of the main sheet. It may be that somewhat by accident I have arrived at an arrangement which minimises the interference. (Sorry, but the photo on the left was taken before this arch was added! But I hope that you can get the general idea. And I think that you may be able to see it in photos from the last Plymouth rally.)