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  #1   IP: 142.68.105.136
Old 10-04-2008, 08:10 PM
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C&C 27 Exhaust design drafts

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.
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Old 10-05-2008, 07:05 AM
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Here is another option I like: this potential design uses an inline water lift muffler that I could place aft of the little bulkhead behind the prop shaft. This keeps everything midships and fairly simple. I could maybe even smooth out the riser 90 degree corners, because I have lots of distance directly aft. Trouble is, I can't find an inline waterlift like the one Taylor Marine makes in Australia. It is made of GRP, 6" diameter and 10" long. The inlet and outlet ports are only 1.5", but unless there is reason to believe that diameter would cause a backpressure problem, then it should be OK, I guess.
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Old 10-05-2008, 09:21 AM
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rigspelt... your diagram and photograph illustrate what looks like what I have in my C&C 27 MKlll...the only slight difference is the dry stack before the waterlift muffler..mine is more direct.. the rest is the same...

I have a fresh water cooling system rather then the raw water but that should make little difference in the exhaust system.

I would just replace the bad parts with a similar design, perhaps in stainless if you can swing it. The fiberglass waterlift muffler is a bit smaller and will give you more room in the locker... but if that metal one is still good..go with it.

Don sells a stainless waterlift muffler which I think is a good choice...I do not know how big it is compared to what you already have.

David
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Old 10-24-2008, 05:53 AM
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Quote:
Originally Posted by David Masury View Post
rigspelt... your diagram and photograph illustrate what looks like what I have in my C&C 27 MKlll...the only slight difference is the dry stack before the waterlift muffler..mine is more direct.. the rest is the same...
David, which design are you referring to? The one with the muffler sitting off to the side on a little platform just aft and port of the engine (which is what I have), or the one directly aft of the engine?
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Old 10-24-2008, 05:59 AM
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I am starting now to lean toward using the existing muffler platform off to the side and getting an MMI SS muffler with top ports. If I do that, I could build a dry/wet stack like this, where the black elbows are galvanized pipe connected with galvanized nipples, ending with a short hose to the muffler intake port. Note I might be able to use a pair of 45 elbows to reduce the bend in one of the corners, and this design elminates one 90 degree bend entirely. I may not need an antisiphon valve in the coolant hose, if the loop is high enough above the waterline at heels(?). Still mulling this over.

Glad I kept the old copper fuel line from the boat -- I'm using it to help me get the shapes right. It bends and unbends very nicely.
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Old 10-24-2008, 10:07 AM
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Question Are you doing a wet exhaust loop?

Thanks for your great drawings and description of the issues. Very helpful.

I have a '75 Tartan 27 yawl with similar setup and similar issues. Except my manifold is a few inches lower than static waterline, and the top of the similar (but fiberglass) muffler is a few inches below that, and I must send my dry exhuast pipes about 6 inches forward to get the space for a vertical rise pre-water injection. Have a Yanmar made water injector there. Good solid piece.

Wondering if you decided to do a wet exhuast loop or rise after the muffler? Wondering how long is the direct run from the muffler to the exit. Do you worry about too much back pressure if you have a wet exhaust loop?

Mary
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Old 10-26-2008, 09:55 AM
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Originally Posted by HOTFLASH View Post
Wondering if you decided to do a wet exhuast loop or rise after the muffler? Wondering how long is the direct run from the muffler to the exit. Do you worry about too much back pressure if you have a wet exhaust loop?
Mary, I have not decided finally yet, but I am leaning toward a 2" ID hose from the MMI muffler to the transom, a run of 8 feet of hose. I am thinking of bringing it up vertically about 24" from the muffler outlet port, and then running it down to the transom through hull. This reduces the risk of backfilling the muffler from seawater entering at the transom and flowing back into the muffler. The two feet of vertical hose can only hold 0.16 US gallons liquid, and the mixed exhaust gas/coolant would have less liquid in it than that.

That actually shortens the distance by 4 feet over the system that was in the boat when I bought it, and opens the diameter of that hose from 1.5" to 2". The only way I'll know if there is too much back pressure is to measure it, but my new design is beginning to look like it should have less pressure than before (one less 90 degree elbow in the dry/wet muffler inlet stack, and larger and shorter muffler outlet hose), and by all accounts the engine was running OK before I started the refit.

Of course being a rank amateur I might have this all wrong, so buyer beware and all that.
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Old 05-18-2009, 04:22 PM
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Got the stack built

Finally got the stack assembled galvanized pipe) and painted with 1500 degree paint. Next step: wrap and install.
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