I purchased an 18' (from tip of trailer tongue to tail lights) on craigslist for a very nice price.
I wanted a 14' x 7' utility bed, but the size is difficult to find used....new trailers are too expensive. I decided I would convert the cheap boat trailer into a bigger utility trailer. No cutting of the boat trailer was needed. I wouldn't want to damage the integrity of the frame by doing so. I simply purchased some angle iron and 2x2 11 gauge steel tubing. I framed in the trailer and welded it down. I followed by adding the steel tubing as support for the boards. After welding on the support, I taped off sections of the trailer, scraped it down and applied a base coat primer of rust-o-leum to help prevent rust. I followed up with another coat of black rust-o-leum. I also added some 45 degree 2x2 11 gauge steel tubing from the front outer corners of the angle iron frame back to the curved frame of the boat to give the front outer corners of the trailer more support. The boat trailer curved in too soon so the corners wouldn't have been properly supported. They are barely visible in the pic below.
I then cut the 2x10s to size, used a ratchet strap to move the boards as close together as possible and then started bolting them down with carriage bolts.
I wanted a 14' x 7' utility bed, but the size is difficult to find used....new trailers are too expensive. I decided I would convert the cheap boat trailer into a bigger utility trailer. No cutting of the boat trailer was needed. I wouldn't want to damage the integrity of the frame by doing so. I simply purchased some angle iron and 2x2 11 gauge steel tubing. I framed in the trailer and welded it down. I followed by adding the steel tubing as support for the boards. After welding on the support, I taped off sections of the trailer, scraped it down and applied a base coat primer of rust-o-leum to help prevent rust. I followed up with another coat of black rust-o-leum. I also added some 45 degree 2x2 11 gauge steel tubing from the front outer corners of the angle iron frame back to the curved frame of the boat to give the front outer corners of the trailer more support. The boat trailer curved in too soon so the corners wouldn't have been properly supported. They are barely visible in the pic below.
I then cut the 2x10s to size, used a ratchet strap to move the boards as close together as possible and then started bolting them down with carriage bolts.