Your method is quite crafty and certainly will be strong. An aversion to drilling holes in the hull of a Boston Whaler boat is a good credo to follow in most cases.
On the other hand, when I had a 1976 SPORT 15, I also added two cleats near the transom. I installed them using conventional screw fasteners on the inboard face of the hull, just forward of where the liner curves to meet the transom .
- Fig. 2. A cleat added to the inwale of a SPORT 15.
- AftCleatSport15.jpg (50.34 KiB) Viewed 1813 times
The cleats were not attached to the hull in an extraordinarily firm manner, that is, unlike factory cleats on a Boston Whaler boat, they were not through-bolted. The
wood locating diagram for that hull shows there are embedded plywood boards about 10-inches long in that area. See CALLOUT J on the drawing. As I recall, I could visually see some evidence of the wood backing, particularly if I placed a very strong light source to illuminate that area and carefully observed the gel coat surface for any evidence of a slight anomaly. The screw fasteners seem to have good purchase. Also, I liked the position below the gunwale, as it kept that surface free from a cleat that could snag a line and the stern quarter was kept clear so you could step aboard there from a dock if necessary.
I only used the cleats for very temporary mooring lines, or to hold a fender in position. For really strong mooring lines, I had dock lines with an eye splice on one end. I just passed the bitter end of the dock line through the stern towing in, then through the eye spice. This attached the dock line to the towing eye in an extremely strong manner.
I sold that boat in 2002 to a fellow. Oddly, about 10-years later we ended up working at the same company. He still had the boat. He had a summer home on a small lake and said he had thought of selling it to get a pontoon boat, but his wife insisted they keep it, as she just loved the SPORT 15. Those cleats are still in use on the boat and have never failed.