As I understand it, the smoother stainless steel is, the less chance there is of crevice corrosion starting at the molecular level and moving it...
I have not read the entire thread, so maybe this has been brought up and discussed many times, but I recently came across an interesting boat that...
Have you looked at Arch Davis's 26' "Jack Tar"? http://www.archdavisdesigns.com/davis_jacktar.html [IMG] There's a great build thread...
That's where they grow the hydroponic "skunk." And trust me, it takes "a lot" of sticky skunk weed to design something like that.
Why not just replace the existing powerplant rather than reinventing the wheel? Inboards and I/Os have many advantages over outboards (lower price...