Cast aluminum water jacketed exhaust manifolds were very common on earlier performance boats.
Peter, did you have any experience with these? How did they do over time, any corrosion?
Chris, yes I ran a set of aluminum manifolds on my drag boat for a while. They worked fine for lower performance engines. Due to the log design, exhaust flow was very restricted — tubular headers were a much better choice for hopped up engine combinations. A very bad point is that if one has a coolant failure to the manifolds, the manifolds will instantly melt & possibly light you up !! Guys that ran in the ocean experienced some corrosion problems as most smaller boats do not have compound cooling systems. In fresh water I never experienced any noticeable corrosion.