This question does not come with a simple answer. You asked about “cheapest to rebuild” and “afford to maintain”… and then, you throw in the word “easiest” which, by the way, adds another dimension to your question. I’m not exactly sure how to answer, but, I’ll start with the “easiest”.
To me, this would be an engine that has the least amount of peripheral equipment: hoses, add-on coolers, piping, guards / shields, complex wiring, fuel systems, etc. This would allow the easiest maintenance as “less things” equals “less work”. Popular engines that may meet this criteria might be unacceptably large (in relation to output HP) or old fashioned to be applicable to a small vessel. Or, the simplest in modern engines would be one that may or may not have turbo charging, would not have sea water after cooling, add on engine oil coolers, fuel coolers, air heaters, combination exhaust manifolds / expansion tanks, and would be fairly close in design to their industrial counterpart. Modern engines that you may consider (around 200+ HP) with these designs are made by John Deere and Cummins.
The 6BT 210/220 Diamond is definitely one of the simplest in design of popular engines around 200 HP.
I suspect you are assuming that “cheap to rebuild” and “afford to maintain” go hand in hand, but realistically this is not the case. Maintenance during the lifetime of a diesel is more or less associated with oil changes, adjustments, replacement of hoses, belts, pumps, starters and maybe some other peripheral equipment. You may find that the marine engines that do not have a large installed base of industrial or on-highway counterparts tend to have higher prices for maintenance related parts, parts specific to the marinization of the engine, and base engine parts. I don’t feel the cost of “maintenance parts” should be too much of an overall concern in engine choice as recreational users should base an engine choice more on local support and not the cost of an oil filter, etc. Maintenance parts DO NOT add much to the overall lifetime cost of recreational use of a marine diesel, and shouldn’t be a determining factor in engine selection choice.
As to the cost of rebuilding, most definitely, there are many popular engines that the rebuild cost can exceed the cost of a replacement engine. Regardless of the design of an engine, rebuilding is strictly more of an economic thing than whether the manufacturer advertises that this engine is rebuildable because of its design features. But, if you were to choose an engine that was truly designed to be simple and affordable to be rebuilt, I think the 2-stroke Detroits have just about everyone beat. This engine is not for everyone though, and if you are looking at the more modern engines that have substantially higher output in relation to their size and weight, you will find that generally the cost of “marinization” type parts and “base engine parts” go up with the power density of the engine.
A higher output engine is more complex in design, and parts just cost more than parts for the same basic engine w/ lower output. High output and sophistication come w/ a price. Very high output Cummins, Yanmar and Volvos have some very complex design features that make them more expensive to maintain and service than lower horsepower counterparts. I think Yanmar has the highest output per “cube” right now, and with this comes a very complex and “busy” engine, along w/ some very expensive marinization and base engine parts. But, this engine allows performance that was not attainable at any price just a few years ago. So, one could say that it’s cheaper now.. The 43/44 Series Volvo is extremely popular in marine use. This engine is also a very high HP per cube engine with a lot of very complex engineering features that may make for a more expensive engine to maintain per hour of use when compared to some other engines. But again, you cannot get the features and performance w/ these “less” type engines.
So Andrew, I don’t know if I’ve answered your question or just made you scratch your head even more. For my cup of tea, I like simple engines that are run easy. From the business standpoint, I make more $$ on fast boats/engines than with slow ones, and my customers are always demanding more performance with “less engine”, an engine that seems to be getting more complex each year. Buy an engine/boat combo that fits your performance requirements/desires, and one that has the local support you need to maintain and service it appropriate to your specific needs. Depending upon your specific requirements, you’re going to find that all of the modern engines from Cat, Cummins, Volvo and Yanmar, etc, offer features that may make them more desirable in some circumstances than others.
I’ll recommend a Yanmar or Volvo diesel package for the guy who wants a “state of the art” diesel stern drive, a Cummins 6CTA 8.3 for a 10-12 knot commercial boat that runs 2000+ hrs per year, and a 4LHA 190 Yanmar for a 28 Bertram repower. What’s the most affordable, the cheapest, or the easiest?? Spend some time searching thru the 100’s of past postings in the forums/archives of http://www.boatdiesel.com , and you will be able to answer this question for yourself better than I can.