Christopher, might be easier to comment on the specific boats you are looking at with outboards. I don't know of any boat which has done a diesel
to outboard conversion, but I have owned and cruised a few thousand miles on both (diesel engine
cats and outboards driven cats). Most boats designed with outboards have been designed with narrower hulls (12/1 for PDQ
36 to 14/1 for slipstream) because they don't need the greater bouyancy to compensate for the heavier engine, which has obvious advantages in terms of speed and sailing.
The outboard "can" have several advantages, in weight, ease of finding a qualified mechanic
anywhere, lower replacement costs, better access to the complete engine by again being able to completely lift
it up, having a completely retractible prop (and no metal parts
touching when retracted so little electrolysis
risk while at anchor
and dock), isolation of the motor
and fuel system from the living space of the boat and therefore isolating the gas, smoke, from the interior
driven cats do have dedicated engine rooms way aft, but pay for that in terms of hull
shape), or simply being able to have a replacement engine mailed to you on a distant island (been there, done that) and while a diesel is theoretically simpler than a gas engine, an outboard is far less complex than a typical modern diesel and needs much less expensive components such as filters. PDQ is a great example of a nice catamaran
with retractible outboards...
diesels do provide better fuel economy for a given horsepower and SHOULD HAVE greater long term reliability
, but I know of catamaran
owners with diesels in 3 year old boats which are on their second sets of diesel engines, and other owners who have 20 year old reliable diesels, everything depends on the specifics. Was this engine cared for well? Is it a reliable make and model? The biggest lesson I've learned with outboards is to budget
in replacement costs for the outboards after around 1500 hours of use before repair costs become too high (or say replacing every 5-7 years with good use each year), but for an engine whose replacement costs are around 3k it's not that big of a deal, that's my diesel engine mechanic
bills after 3 years and you would get a brand new engine! Keep an open mind about retractible outboard engines, yamahas are very reliable and very fuel efficient, but after a certain number of hours the corrosion
in saltwater will make them need to be replaced.