I went through this process a few years ago with my 32 foot Chris-Craft. When I ran the numbers, I found I needed to use the boat about 1000 hours per year for 5 years for the cost of fuel to offset the cost of the diesel engines. I had twin 210HP Chris-Craft inboards that were built on Chevy blocks. The problem with the cost analysis was that I only averaged using the boat 200-300 hours per year. At that rate I would probably never recover the costs of the diesels.
In addition, the diesels were considerably heavier than the gas engines for comparable power output, and Chevy-based parts are sooooo cheap! I looked at Cat, Cummins, DDA, Volvo, and Yanmar. I really liked the Cummins B series, but just couldn't make the numbers make sense... Sometimes it's not really about what makes sense though, it's what you want!
Hope this helps!
