Certainly there is a size limit where outboards are not practical. But they have powered boats crossing major oceans (26 foot boat accross the Atlantic). I know of 35 to 40 foot sailboats where the inboard was removed and a transom mounted outboard was used (I mistake in my opinion--but done). I have seen large houseboats both on the rivers and Inland Passage powered by high thrust outboards. The documentation of 4000 hours in four stroke outboards has been on "The Hull Truth"--from several charter boat operators--and claimed also in a Coast Guard vessel. 600 hours is extremely low for the longivity of an outboard--1000 to 2000 hours, if well cared for-- is more likely. Even though the diesel is more expensive--consider the over all cost of operation; The over twice as much cost of the diesel over the outboard will take many thousands of gallons difference of fuel consumption to retrieve. In a small boat I suspect it would be difficult to justify on economics alone. I lived with an outboard in a well--that is far from ideal. There are fume problems--and a flush plate must be set in the well to seal off the lower leg. Bob, I currently own 4 outboards from 115 to 150 hp and have yet to drop a wrench into the water while working on these. The only service problem I can see with the outboard is lower unit oil change and impeller change with a boat in the water all of the time. Since these boats are light weight--hauling on a trailer for an hour or so for this maintance--or on the grid in AK--is simple and inexpensive. These outboard powered boats are coastal travelers--not ocean crossers. After living with the four strokes for a few years--I am not going back to inboards for boats less than the mid 30 foot size. Bob Austin