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Go Back   SailNet Community > On Board > Gear & Maintenance > Hot/dry exhaust installation
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Topic Review (Newest First)
02-01-2005 02:21 PM
owenmccall
Hot/dry exhaust installation

DelmarRey wrote:
"You always want to eleminate as mush back pressure as possible. It can burn and/or deposit up the valves."

This is what I have always heard too, but a sailor friend of mine who designs gasoline engines for a living laughed when I told him of my worry that a waterlift system could potentially generate damaging levels of back pressure. I will quote him: "Exhaust backpressure is absolutely not an issue. Period." He says that typical back pressure for an automobile exhaust system is around 3psi. (This corresponds to lifting a solid column of water 83 inches!) He says one year the OEM Chevy Corvette exhaust system, as designed and marketed, gave 7psi back pressure (that''s sixteen FEET of water column). His recommendation was to make the exhaust loop as high as needed to eliminate any chance of back siphoning and to not think about it again. He did say that there would be a slight drop in engine performance, but not anything significant. (We are talking about gasoline, not diesel, engines here, and naturally aspirated, not turbocharged, though even with those, while more of an issue, he thinks backpressure is not a major concern.)

So I now reverse my plans and have begun installing a waterlift system after all!

Owen McCall
S&S30
01-03-2005 03:06 PM
owenmccall
Hot/dry exhaust installation

Thanks, that sounds like great advice. I will proceed just as you say.

But how do you support the system? Using pipe it will weigh quite a bit.

Owen
12-25-2004 03:38 PM
FalconEddie
Hot/dry exhaust installation

As far as the hot-dry exhaust, it''s dangerous, but you can do it if you do it right. First, it REALLY needs to be well wrapped with insulation, and I mean completely. I have done this with thick fiberglass tape for many customers, and have a sort of system that I like. I carefully wrap the entire system with one layer, then wrap over it with stainless steel safety wire. Then I completely wrap it again in the opposite direction and do the wire thing again. If I have gotten thick enough fiberglass and feel safe (I like at least 3/8" of thickness, and extra pads anywhere the system gets support. If the fiberglass isn''t thick enough, I keep wrapping until it is. It''s better to take the time and be safe than to have a fire break out just when you''re trying to make it through a difficult inlet in rough weather, and bad things always happen under the worst conditions. Last, I use that red High-temp gasket sealer and put dabs of it all over the glass on the seams, mashing it into the surface to help prevent the glass from migrating around due to expasion/contraction and vibration, but don''t try to seal it or it will hold in moisture and rot the pipe, and don''t try to make a hard skin by putting anything like resin on, because it''ll catch fire.

You can make a good solid exhaust out of plumbing iron pipe and secure it well, so long as you don''t transmit sound to the hull, but you have to use a section of about a foot to eighteen inches of flex pipe between the engine and the solidly mounted pipe to allow for engine mount flex. Wrap that the same as the rest of the pipe, but check it more because it flexes, and keep anything like fuel lines or electrics well away from it.

I know it sounds like I''m a know-it-all saying all this cautionary stuff to someone who probably knows it anyway, but nothing scares me more than propane except a dry exhaust. Last thought - fitting at the transom or hull needs to be insulated.
12-21-2004 10:20 PM
DelmarRey
Hot/dry exhaust installation

Owen,
I really see your problem here with the motor being right in the middle of the solon.
You always want to eleminate as mush back pressure as possible. It can burn and /or deposit up the valves.
As for the lift, the exhaust thruhull is usually a few inches above the waterline. The riser loop from the lift only needs to be above the engine exhaust manifold but at least 18" above the waterline. It must run down hill from the top of the loop to the thruhull. And the lift itself should be at least 12" below the manifold outlet, or the loop in the exhaust outlet.

There are several ways to install a wetlift system. The main thing is you need to know is where your exhaust mainfold is in relation to your waterline.

If it''s above, that makes it easier. You can put the wetlift closer to the thruhull and less loop.
12-21-2004 06:12 AM
owenmccall
Hot/dry exhaust installation

I would much prefer to do a waterlift, and the bilge is deep enough to position such a muffler below engine level. However, the side discharge means that the top loop of such a system would have to go up to almost saloon overhead level (to prevent siphoning when heeled). This would result in excessive backpressue in the exhaust, I fear. Although the authorities seem to be divided on this point; some feel back pressure is a critical issue, while others advise that it is essentially a NON-issue!
This boat, an S&S 30 was built by Aquafibre, Ltd., of Rackheath, England in 1971. The S&S 34s (e.g. Fastnet racer Morning Cloud of Prime Minister Edward Heath fame) also have this side exhaust (See Good Old Boat cover, issue before last. That boat, Redline, is an S&S 34, and you can see smoke exiting just above the bootstripe on the port side. (I wonder if they flipped the negative over - my boat has the exit on the starboard side.))
The other problem with a waterlift setup is the potential to siphon through the water pump. The engine is located dead center in the middle of the saloon (under the table). There is no room above the engine for an anti-siphon loop unless it penetrates the dinner table!
Therefore I have reluctantly decided on a long hot/dry run to the head/saloon wall where the standpipe is located.
Owen
12-20-2004 06:42 PM
DelmarRey
Hot/dry exhaust installation

Especially with a gasoline engine you would want a wet system if you can manage it. Unless the engine is well below the waterline a wetlift is the way to go. Dry exhausts are fire hazards.

A dry exhaust are common on trawlers with the stacks comming straight up thru the deck and is insulated with another pipe surrounding it, which is air cooled.

I''d recommend properly redoing the wet system.

And what kind of vessel is this that came with a dry exhaust?
12-20-2004 01:54 PM
Sasha_V
Hot/dry exhaust installation

Your exhaust run should not touch the hull at ANY point, but it has nothing to do with the risk of fire.

NOISE.

Noise and lots of it. Think of your hull as a large kettle drum that you happen to live ont he inside of. The thrud-thrud-thrud sound of the exhaust will reverberate beautifully throughout the vessel''s interior.
Even through foam lagging.

I recommend using the rubber exhaust hangers that are used on cars and trucs and building some support frames for them into several parts of the hull. This will suspend the exhaust an inch (or even 1/2 an inch will do) away form the hull at all points between the engine and the exhaust through-hull.

OUr boat had someone''s idea of a "clever" exhaust system when bought it too (not nearly as clever as the fuel system though, but that is another horror story). The sound of our 20hp Yanmar has dropped by about 60% since elevating the exhaust system away from the hull surfaces. It will get even quieter when I build the soundproofed companionway engine cover.... (I''ve been busy, okay?)




Sasha
12-20-2004 06:52 AM
owenmccall
Hot/dry exhaust installation

I need to undo the previous owner''s "improvement" to my engine exhaust system. He had replaced the original design (which included a very long hot/dry run) with a waterlift setup where the water is injected nearer to the exhaust riser. However, the engine location does not really permit this so I want to revert to the original design with the long hot/dry run. My question is this: Can the hot/dry portion be permitted to touch the hull at some points without risk of fire? It will of course be covered in lagging, but should additional protection be provided at those points where space constraints force close proximity of the hot/dry portion with the hull, or am I being a worry-wart? (The engine is an 11hp gas 2-cylinder, and the hot/dry exhaust has an outside diameter of 1 5/8".) Thanks for any advice.

Owen McCall

 
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