Similarities between the O/B R-23 & the I/B R-21EC?
Outboard gas engines are comparably inefficient using twice as much fuel than larger, stronger Diesel inboard engines that carry a 1/2 size fuel tank full of Diesel. +Outboard engines don't last half as long as Diesel engines.
Doing the maintenance on a small, wide open, 3 cyl Diesel engine is a breeze compared to bigger Diesel engines or any 'ol/new gasoline engine.
There are alot of good things previously mentioned in this thread about gasoline engines that hang off the stern. But Ya just can't beat a modern, smooth clean running, little Volvo Penta Diesel Engine inboard a R21-"EC", with an updated flat-planing, wave-slicing hull. Eight (8) knots are plenty for a scaled down RangerTug. With 30 HP, the I/B Diesel eng. matches the hull speed of O/B Cabin-Cruisers, clear to Catalina Island.
According to the Official Volvo Penta Diesel Service & Routine Maintenance Manual... these new little I/B Diesel engines like to run hard, forever.
*It ALWAYS costs a lot more time, money, & effort to re-engineer a boat for any major changes, and completely finishing them up to your own specs & satisfaction, than it is to find one already done like that.
"You can't always get what you want" - The Rolling Stones.
:x Bill 🙂 (R-21EC w/sm.mods.)