An outboard of 5 to 20 hp, on a retractable bracket, is quite a common choice for auxiliary power around here on sailboats of 18 to 26 feet or so.
It's a logical choice for a boat with few systems, low electrical
demand and limited space, that will only be under power while in the marina. Some of the common problems- hard to lift
up, etc.- can be avoided with balancing springs, clever linkages, etc. A well might be tricky to seal in such a way that the water
flow under the hull
isn't disrupted, though, and takes up space just like an inboard. I would be much more inclined to just add a bracket on the transom, unless the hull was originally designed with a motor
well. Several o/b manufacturers have "big foot" or "high thrust" lines with larger diameter props, steeper gear
reduction, and sometimes a better alternator
, that are better suited to a sailboat then the standard versions.
The downsides, of course, are that powering in a rough seaway is problematic, and the electrical
output of these small outboards can barely keep a battery
charged well enough to run nav lights and a VHF
. And converting an existing, inboard-engine boat to outboard-in-well or outboard-on-bracket is unlikely to do anything nice to its resale value.