There are a couple reasons I am wanting a second engine. The first one is for redundancy on a boat I'm wanting to take trips of a month or two where an engine issue isn't a trip killer, I'd like to be able to continue on with whatever it is I'm doing. That also includes going upstream on whatever river system I'm exploring if need be. Oh, I should also mention that I'm a certified master mechanic, formerly certified master engine machinist, and state certified welding instructor. I have 6 seasons experience working as a marine mechanic/electronics tech on Lake Michigan and the Mississippi & Illinois rivers and a couple decades now in the transportation industry.
Anyway, I want 2 engines and since my mercruiser Bravo requires 100 hour changes for both engine oil and gear oil if I'm on a long trip I'm going to have to do them mid trip somewhere. PITA and another reason for having a different motor do most of the boat pushing. So if I have another motor to do most of the cruising work, how big should I be looking at? As mentioned I want it to be a 4 stroke for a few reasons but being plumbed into the main tank is a big one.
For the rebuild here, everything including the existing deck is going to be removed. I am going to be moving the helm from midships to all the way to the front, so I am going to have to lengthen the control cables from 20 ft to 35 ft for starters. Not really planning on saving any of the existing engine instrumentation.
At any rate, it's a couple years from being in the water and I can toss a 2 stroke 20 hp on for a start to see how that works and go from there either up or down. The main thing I mentioned is it was one of my kids ideas, not mine, and he's going to be building just as much of it as I am. I think that with how today's generation seems to be wary of anything involving hard physical work he's going to be developing a skill set hard to match in his age group, not to mention that we're going to be doing some cool trips around the US with this.