Originally Posted by whiteminiman
This is from a rescue boat angle... so I feel that the "fail safe" of having a second engine is quite important.
I think you just answered your own question!
When I was planning for a new boat (Hurricane 590), I spent a lot of time researching this. Interestingly, both the Hurricane dealer and the folks at Mercury talked me out of it... increased cost (initial and ongoing maintenance), decreased performance (according to both of them, you get less speed from twins with same horsepower...) and increased fuel consumption.
In my case, the question was answered when a gently used boat came along already equipped with a single 150, so I bought that.
Having said this, if I was replacing the boat, I would still consider twins just for the redundancy. In 10 years, I have only had engine breakdowns twice. Both were major (blown head) but conveniently, both times I was close to home....
I think the other advantage of the twins is apparent when towing. If you are going to be using your boat as a rescue boat, presumably this is also an advantage.
I don't recall ever seeing a boat smaller than mine (5.9m) with twins, but I suppose they're out there. The width of the transom is probably the issue...
In Canada, I don't recall ever seeing a Coast Guard RIB that didn't have twin engines. Of course they don't need to worry about cost, but obviously they feel it's worth having the twins...