A daft trailer question - but how have you set up your rollers? Are they quite stiff, or are they loose and 'flop' around?
I've tried both variations, and they have both caught the hull and scraped the gel-coat. If tight, the rollers don't move with the hull profile, and the ends of the front set of rollers hit the hull and scrape the gel-coat. If loose, the rear rollers can easily 'tip' as they are in the water so the hull settles on the steel end of the roller as you pull the trailer out of the water - damaging the gel-coat again.
Is it a black art, or am I just a muppet?