I have a 1974 Ericson 27, Silver Maiden. Her Palmer p-60 finally gave up the ghost 2 years ago. I converted her to a yanmar 1gm10 which pushes her along just fine and sips fuel compared to the old Gasoline P-60. I gave a thought to both Electric and Outboard when I was sorting out what to do.
Thunderstruck was not helpful. I contacted them to find out about a very slight modification to one of their brackets to better work in Silver Maiden. They never responded in spite of repeated attempts. I did do some calculations on what I would need for batteries and the battery weight was going to be more than the weight of the old P-60, which was 425 lbs.
There is a fellow on the
EricsonYachts.org: The Starting Point on Ericson Yachts! site that did convert his 27 to Electric. He has batteries in all the lockers under the port and starboard setees leaving very little storage. My impression from his writings is that he does not require any long motoring.
Had I gone electric on Silver Maiden I would have installed a generator to charge the batteries. Gasoline or Diesel still has a higher energy density than batteries.
With the switch from the Gasoline engine to the Diesel my under power range went from something like 120 miles to over 600 nm with the same tank.
I can only think if you are doing anything more than in and out of the marina you will not be as happy with the outboard as with the diesel inboard.
As has been stated above with a boat your size handling it in close quarters will be an issue with an outboard. Trying to lash the tiller amidships and steering with the motor will leave you in an uncomfortable position, and steering with the prop behind the rudder by using the boats rudder means always being way ahead of the boat and sometimes sculling with the rudder to get a turn started. I know because while I was trying to get my old palmer working, through the completion of the engine swap I hung a small outboard off a custom mount off the transom. I was very pleased to be back to inboard power when I got the job finished.
Of course if you have any non-trivial wave action you can expect the prop to spend plenty of time in the air and cavatating with maybe 1/3 of it's time taking a useful bit out of the water.
I am just starting the restoration of a Mcgregor Venture 22 with an outboard on it. I have to move it about 70 miles and 1/5 of that is shallow twisting IC waterway where sailing is painful. I am not looking forward to taking her at least 35 miles under power with an outboard, but since the boat is small, light, and designed for an outboard maybe it will not be that bad.
I do know on my 5800lb 27 foot Ericson while I might mount the dinks outboard if I lost the diesel and could not sail for whatever reason it would not be a pleasant or economical way to traverse a long distance. Since most of my sailing is solo I tend to use the motor when there is no wind and I have to be someplace, or when it is blowing like merry hell and I can not handle the sails and helm by myself due to conditions. In those conditions I would prefer my 1GM10 to the 7.5 horse dingy outboard I can sling on the back if need be.
Additionally if I were buying a boat such as yours and it had an outboard mounted and the inboard either did not operate or was removed I would knock a fair amount off the price I would pay figuring I had a repower in my future. I am handy and could do it myself, but I would knock off what a shop would charge because my time is worth something too.
I hope this helps you sort things out.