I am knee deep into figuring out the optimum exhaust design for this 1974 Mk II C&C 27. I don't know yet what the original exhaust design was.
When I got the boat it had a galvanized hot stack with a 7" vertical loop to the deckhead under the cockpit, a 2-3/4" drop to the raw water injection point, and another 3" drop to an elbow turning toward a 5" rubber hose attached to a side-entry Onan metal waterlift muffler of uncertain vintage (see photos below). The manifold outlet is 2" below the waterlift inlet. The muffler is 12"x12" and sits on a wood platform built into the hull a foot and a half to port of the aft end of the engine. I have no idea whether that is original purpose of that shelf. From the muffler, there is a 12.5', 1/5" ID hose looping up 26" vertically to run under the toe raill, dropping at the transom to the overboard through hull which is 8" above the water. The waterline is about the middle of the engine and waterlift, so the manifold and waterlift nipples are above the waterline at rest.
There is very little about C&C A4 exhausts on the Internet, so maybe this sailboat design is fairly tolerant of exhausts people have cobbled together over the years.
I have been doodling with the maze of exhaust paramaters: see draft drawings below. These drawings will change as I learn more about this complicated little business.
The main goals are to safely carry away the hot gasses, minimize back pressure, and prevent water from backfilling into the valves through the manifold. Apparently, the exhaust on this boat was working OK, since there were no complaints of engine problems or water flooding prior, as far as I know. Given that the waterline is below the manifold and top half of the waterlift, and that the waterlift is not far to the side, then presumably the 7" dry stack vertical lift (more than 12" above the static waterline) and 2-3'4" injection point were OK.
I have to decide whether to live with that Onan of uncertain age. The upside is that I could have a new dry/wet stack made of SS using the old one as a template. The downside is waiting for the muffler to corrode out and leak -- not a pleasant concept.
There are fibreglass mufflers, but unless I design a much bigger stack giving an injection point much further away from the muffler, then the water-exhaust mix will be too hot for the GRP muffler.
MMI's waterlift is attractive because it is half the size, but I would need to design a new dry/wet stack, and to get into the vertical inlet on top of the muffler with decent drop from the top of the loop to the injection inlet, I would have to run the stack up into the locker, not under the cockpit deck.
I could move the muffler closer to midships to keep everything under the cockpit and out of the lockers, but it won't be easy to build a new shelf for it around the stuffing box, I don't want to block access to the stuffing box (easiest by far from the port side because the engine angles that way), and the base of the muffler would not be much lower than it is now on that port-side shelf. It would be a lot harder, if not impossible, to build an adequate stack for the top-loading muffler in there, I think.
I plan to loop the coolant hose up into the side of the locker and add an antisiphon valve. At present it attaches at the same level as the manifold outlet, but 2-3/4" below the top of the stack on the down side.
I can cut the 12.5' exhaust hose run down to 8', and increase the ID from 1-1/2" to 2", which hopefully should further reduce backpressure risk, if this boat had any (that's a long run for a 1-1/4" exhaust outlet at the manifold, according to a rule of thumb I found on the net). I have all kinds of room in the locker to run that hose vertically >12" from the waterline at all heel angles, and <33" from the muffler, so that part is easy.
I'll add a flapper to the transom throughull.
When I got the boat it had a galvanized hot stack with a 7" vertical loop to the deckhead under the cockpit, a 2-3/4" drop to the raw water injection point, and another 3" drop to an elbow turning toward a 5" rubber hose attached to a side-entry Onan metal waterlift muffler of uncertain vintage (see photos below). The manifold outlet is 2" below the waterlift inlet. The muffler is 12"x12" and sits on a wood platform built into the hull a foot and a half to port of the aft end of the engine. I have no idea whether that is original purpose of that shelf. From the muffler, there is a 12.5', 1/5" ID hose looping up 26" vertically to run under the toe raill, dropping at the transom to the overboard through hull which is 8" above the water. The waterline is about the middle of the engine and waterlift, so the manifold and waterlift nipples are above the waterline at rest.
There is very little about C&C A4 exhausts on the Internet, so maybe this sailboat design is fairly tolerant of exhausts people have cobbled together over the years.
I have been doodling with the maze of exhaust paramaters: see draft drawings below. These drawings will change as I learn more about this complicated little business.
The main goals are to safely carry away the hot gasses, minimize back pressure, and prevent water from backfilling into the valves through the manifold. Apparently, the exhaust on this boat was working OK, since there were no complaints of engine problems or water flooding prior, as far as I know. Given that the waterline is below the manifold and top half of the waterlift, and that the waterlift is not far to the side, then presumably the 7" dry stack vertical lift (more than 12" above the static waterline) and 2-3'4" injection point were OK.
I have to decide whether to live with that Onan of uncertain age. The upside is that I could have a new dry/wet stack made of SS using the old one as a template. The downside is waiting for the muffler to corrode out and leak -- not a pleasant concept.
There are fibreglass mufflers, but unless I design a much bigger stack giving an injection point much further away from the muffler, then the water-exhaust mix will be too hot for the GRP muffler.
MMI's waterlift is attractive because it is half the size, but I would need to design a new dry/wet stack, and to get into the vertical inlet on top of the muffler with decent drop from the top of the loop to the injection inlet, I would have to run the stack up into the locker, not under the cockpit deck.
I could move the muffler closer to midships to keep everything under the cockpit and out of the lockers, but it won't be easy to build a new shelf for it around the stuffing box, I don't want to block access to the stuffing box (easiest by far from the port side because the engine angles that way), and the base of the muffler would not be much lower than it is now on that port-side shelf. It would be a lot harder, if not impossible, to build an adequate stack for the top-loading muffler in there, I think.
I plan to loop the coolant hose up into the side of the locker and add an antisiphon valve. At present it attaches at the same level as the manifold outlet, but 2-3/4" below the top of the stack on the down side.
I can cut the 12.5' exhaust hose run down to 8', and increase the ID from 1-1/2" to 2", which hopefully should further reduce backpressure risk, if this boat had any (that's a long run for a 1-1/4" exhaust outlet at the manifold, according to a rule of thumb I found on the net). I have all kinds of room in the locker to run that hose vertically >12" from the waterline at all heel angles, and <33" from the muffler, so that part is easy.
I'll add a flapper to the transom throughull.
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