Exhaust design

[Peter]

Hi all,

Patiently (NOT!) waiting for the MANUAL to arrive.

Am performing an engine change and will redo the exhaust while I am at it. The picture below is a schematic of my current system.

I will add an anti-syphon device to my cooling water injection loop.

Question #1 - does the injection point need to be as high as possible? On the existing exhaust this was the case and it resulted in a nipple that was completely threaded being used and of course it rusted out.

Question #2 - the "loop" on the transom exit is not very high on my boat - are there any guidelines for how high it should be?

Thank you,

Peter

[Al Schober]

Yes, keep the injection point high - avoids a LOT of problems.
Second, I don't think you need any loop on the exit. Also, I don't think you need that water lift - just go from the injection point and out the back. If noisy, you may want a muffler.

[ATTACH]11135[/ATTACH] ABYC Article P-1 Exhaust Systems

[ndutton]

Given the drawing you provided I agree with Al that the waterlift is unnecessary but please consider a standpipe system. The advantages compared to a waterlift are:

• Little to no exhaust backpressure
• Minimum length of hot pipe
• Superior backflooding prevention

The geometry involved is important so research is in order. Probably one of the best resources for a standpipe system on the internet is this forum not to mention the principal component is available from Moyer Marine.

http://www.moyermarine.com/cgi-bin/shopper.cgi?preadd=action&key=EXHT_01.0_527

[joe_db]

If you really have that much room and height, do yourself a huge favor and follow the suggestion in the last post and get a standpipe. Failing that, you can make it a powerboat style exhaust with no waterlift.

[JOHN COOKSON]

A plug for my Catalina 27 exhaust system. (I'm RWC)
Drop dead simple . Works well. Easy to replace when its life is over. No back flooding of the engine if you crank no start with the raw water valve open. No engine noise if done correctly - no muffler needed. (details available on request)
On my 27 there is no loop in the hot section -> stern of boat. There is a slight dip just before the stern exit where water accumulates which acts as a muffler.

TRUE GRIT

[tac]

Waterlift guidelines

If you decide on a waterlift exhaust, here is a good summary (Dave Gerr is a noted Naval Architect and former head of Westlawn Institute).

http://www.goodoldboat.com/reader_services/articles/Marine_exhaust.php

[sastanley]

A plug for my Catalina 27 exhaust system. (I'm RWC)
Drop dead simple . Works well. Easy to replace when its life is over. No back flooding of the engine if you crank no start with the raw water valve open. No engine noise if done correctly - no muffler needed. (details available on request)
On my 27 there is no loop in the hot section -> stern of boat. There is a slight dip just before the stern exit where water accumulates which acts as a muffler.

TRUE GRIT

John,
What is the purpose of #5 & #6 in your diagram..some kind of baffle??

[JOHN COOKSON]

John,
What is the purpose of #5 & #6 in your diagram..some kind of baffle??

I don't know why the tail piece is there. The boat came from the factory that way. I've always included the feature when I've rebuilt the hot section. I assumed it was some sort of resonating chamber. Standing waves and all that.
The PDF file came from MMI. It's buried in the forum somewhere. I saved it on my computer so I would have it if and when I need it. Scroll down on the lower right.
An interesting observation is MMI did not include the tail piece in their Catalina 27 like hot section. P\N EXHT_01.0_562 in the online catalog.

TRUE GRIT

[tac]

It could be a cleanout port, since crud tends to collect around the injection point.

[JOHN COOKSON]

It could be a cleanout port, since crud tends to collect around the injection point.

The hot section is made up of a bunch of plumbing nipples. After a short period of time the salt water and exhaust corrode the threads so badly one would never be able to remove the end cap to do the clean out.
That said maybe it was designed to be a clean out as you mentioned.
A bigger problem is the injection port is brass and the pipe is iron. The same injection port has been in three different hot sections rebuilds on my boat. The two dissimilar metals make a nice battery (electrolysis). The iron is the sacrificial metal. The answer of course is to make the whole thing of iron. Next rebuild.

TRUE GRIT

[toddster]

Say... that appears to be the drawing of my boat!

Which was designed for a standpipe exhaust. The only difference is the exit port from the original standpipe was near the top, and the exhaust hose could slope downward to the through-hull. The Moyer unit has the exit near the bottom and the hose ends up running slightly up-hill. Seems to work OK though. Maybe the water "batches" a bit more than before.

I've considered putting a full-port valve on that through-hull for putative off-shore sailing, but it would take a "pooping" wave that pretty much fills the cockpit to force water backward through the standpipe. And in that case, what would happen to the gas tank vent?

[sastanley]

John, I've noticed the same problem with my hot section..the iron is the weak metal and falls apart, except where it is connected to the more noble parts like the injection assembly and the flange..

I thought maybe a resonating chamber too, given the lack of a "muffler", which is why I asked, but did not want to assume.

Carry on...hopefully our OP Peter gets some ideas based on our discussion.

[Peter]

Thanks to all for advice.

This is season one with the boat and I have had exactly one day of sailing so far and am WAY over budget already. So, a totally new exhaust system is really not in the cards or my mental capacity at this time... I think I can cope with a new hot section. This winter will see more engine work and perhaps then.

What is the big drawback to exhaust back pressure? My exhasut flange does have the removeable plug that would allow for a pressure measurement.

If it is a case of power, the few times i have actually had the boat out under power it has rocketed along - 6 knots with the throttle at most half open. Boat is an Ericson 29. 2 blade folding prop of unknown make or size (I will check tomorrow)

Peter

[JOHN COOKSON]

Thanks to all for advice.
so far and am WAY over budget already. Peter

Didn't they tell you a boat is a hole in the water into which one throws money?

Since you have a new boat that is an unknown, unwrap the hot section and have a look. You are looking for holes and weak spots. If you have any questions put up a post.

Try twisting and moving the various parts of the hot section. If any of the plumbing breaks with this treatment it means it was thin and near the end of it's service life.

It's good that you are on this. Carbon monoxide is sneaky stuff.

TRUE GRIT

[toddster]

Yeah, my previous standpipe seemed OK and functioned right up until I put the boat back in the water, after a six-month refit, during which I spent every dime I had on it, and then some. Then the exhaust promptly disintegrated. Apparently the rust was all that was holding it together.

Around post 87 here...