Since no one else has said it....I will. Your boat and trailer set-up was designed for use with a sling, and not a ramp. While I applaud you and the others for trying.....the boat ramps in my part of the country are far too busy for that kinda messing round. lowering\moving a trailer out from your tow vehicle with a strap will surely bring the harbour patrol out and tickets will be written.....doesn't matter what there for but it will ruin your day.
Best of luck with that.