I carry a 15 HP Mercury (which I believe weighs the same as the 9.9 HP--about 80 lbs). I use a crane which is depicted in one of the photos. I use a 4-purchase block on the crane to hoist and lower the outboard. As you'll see in one of the photos, the wooden mount is made up of two thick pieces of teak that I bolt together from both sides of the rail on the starboard quarter. I used a router to cut out the portion that sits directly over the rails.
My railing is a bit beefier than OEM since I have buttressed it with a mount for a wind generator, so I really don't experience any issues with sturdiness. For a while I carried a 20 HP 4-stroke, which was far too heavy for the set-up, so I went back to the 15 HP 2-stroke. It has done well there for all conditions, and it's enough for us to get up and plane using our Avon as a "sport boat."
As an aside, I often use the same 4-purchase block to lift our Avon (3.4 meter model with wooden floorboards) onto the sundeck using the end of the boom. I swing it over the port side, attach the shackle on the end of the block to the lift straps, and then swing it over the sundeck using the boom once I get it raised. It's a bit of work, but easier than muscling it up and over the rails on the port quarter.
Cheers,
Paul
