I saw this thread was active so I just thought I would share a schematic drawing of the exhaust on my 68 Columbia 36.
Not all the features on this boat were first class, but the exhaust seems to be well thought out and nicely made. It is dry all the way to the transom with the water only injected just before it leaves the boat. The hot gases are cooled very well with a water jacket heat exchanger and then the water exits the heat exchanger by a hose and is injected into the exhaust after a high loop at the end of the pipe. This stuff is all bronze or brass and looks pretty healthy after all these years.
The high loop at the end is just single wall exhaust pipe but it always runs cool. The previous owner had fenders and ropes and stuff crammed in the lazzarette (sp?) against the heat exchanger with no problems of burning or melting. And, the single wall pipe at the end does have some fiberglass insulation around it just for extra safety.
We realized the beauty of this system when we were cranking the engine a lot to chase down an ignition problem. We thought we might fill up the engine with seawater through the exhaust so we stopped
. After analyzing the system I was really happy to find out that would be very unlikely. A following sea in rough weather might be the only problem here, but a simple flapper might fix that.
This all responds well to the first rule of boating; "Keep the water out of the boat"
Russ