I'm in the middle of just that conversion - IO to OB - two Merc 165 hp 4 cyl, to two 175 hp Suzuki's. I ran the I/O's for 10 years with no problems, but age caught up with them, along with noise
& vibrations. I have consulted with a number of others who have made the switch - most are very happy with the results. My OB's are mounted on a custom designed aluminum
bracket bolted thru the transom with high strength bolts. We put a lot of engineering into the set up taking into account changes to weight distribution as well as buoyancy. Total empty weight is down by nearly 300 lbs - but the CG has moved to the stern by 20" +/-. My hull is not cored, but solid fiberglass
. A cored hull could represent a real weak spot. I would research
that very carefully. If your target is a balsa core
design - check it for moisture very carefully. Any sign of moisture would be a show stopper for me. I've had a few issues of wet cores in decking. That can be fixed, but a wet cored hull (converted or not) is just not acceptable to me.
It's true the transom was not likely designed to handle the engine (thrust & torques) and should be reinforced with structure to transfer those forces to the original engine mounts and stringers. Should be pretty obvious looking in the old engine room. Mine consist of 4 vertical - 2' wide epoxy/coosa board cored gussets between the transom and hull and horizontal "U channel", gunnel to gunnel, from the same material - which also serves as a battery
shelf. Because of CG issues, I had to move starting batteries
to the transom. Also added two 3/8 Aluminum
gussets that are bolted to the bracket (thru the transom) and secured to old engine mounts. I've added a couple of strain gages on these gussets to monitor
for any movement.
I'm not back in the water
yet, but two others that I know of are and both report good handling results, no unexpected problems. One had to change props for proper engine loading, but otherwise it worked pretty well out of the box.
Soooo, it's not unheard of, done more often than generally noted. It can be done with good results, but does require some thought and engineering. There are some potential serious problems. I would have a recognized surveyor look it over carefully.
Old sage, 'Never fall in love with something that can't hug you back - or you wouldn't drill a hole in - especially a boat!!"