Exhaust Discharge

[JonRiley56]

Hi,

I currenlty have my exhaust exiting through the hull above the water line, and while it is majestic to watch it is a little noisy. Can I exhaust below the water line ? If I do so, do I need to go through a riser or a check valve ?

jon

[DetroiTug]

Exhausting below the water line is noisy too if simply exiting in a thru-hull fitting. The cold water shocks the steam and it makes a banging and popping sound. A fix for that is a piece of copper tube about two feet long along the hull that will act as a condenser to lessen this effect.

-Ron

[fredrosse]

When the engine stops, sea water will be sucked back into the engine, this is not good. Putting a check valve in the exhaust line alone is not recommended, as when the engine stops, cold sea water will rush up the line, and slam against the inline check valve. A vacuum may continue, and if the main line check valve leaks, then sea water gets into the engine again.

You should have a "vacuum breaker" in the exhaust line. This is simply a 1 inch check valve, mounted horizontally so that the flapper is normally closed by gravity, connected to the branch of a tee in the exhaust line. This valve should be mounted above the waterline. The normal flow direction thru this check valve is toward the tee. When the engine stops, sea water will be sucked back into the exhauxt pipe, creating a slight vacuum. The vacuum breaker opens and lets air in, so the vacuum is destroyed. When running under steam, the vacuum breaker check valve is generally shut, not doing anything. If you want, you could put a main exhaust check valve just upstream of the vacuum breaker, but it is really not necessary.

You should however have a hull valve for any overboard connection that is below the waterline. You have to remember to open the hull valve before running exhaust steam to the line. If you forget this, then there is a possibility of getting significant steam pressure in the exhaust line, which further complicates matters. The exhaust line then needs to be designed for full steam pressure (probably not practical), or a low pressure relief valve needs to be fitted to assure that the exhaust piping will not be overpressurized when you forget to open the hull valve. If you use automotive radiator hose and hose clamps, the relief valve must be set at something below 10 PSI, as the radiator hose is only good for about that pressure.

Another option is to have the thru-hull exhaust connection above the waterline, so no hull valve (and no relief protection) is needed. Just outside the hull a 90 degree elbow and a short piece of pipe/tube can then carry the exhaust steam underwater. A bit unsightly to some eyes, but workable. Of course, this hull penetration is not cooled by sea water, and the exhaust temperature might hurt the hull material for a plastic (or fiberglass resin) boat hull.

[Lopez Mike]

Sure a lot of trouble just to avoid a condenser and a hot well. Cough, cough.

[JonRiley56]

Hey Mike,

Can you tell me about your condenser ? I had built an onboard one but it crapped out on me. The stainless coil I was using must have split at a weld, I was getting saltwater to the hotwell.

I assume you are running under the hull ? How much length do you have outside the boat and what material did you use ? You run in salt as well, so if it works for you it should work for me.

jon

[Lopez Mike]

Mine is crude, and a consequence works well. In fact, most of the time, too well.

It's about a six foot length of 5/8" copper curving along side of the keel from perhaps three feet aft of the engine at the entry point, to just aft of the boiler where the condensate emerges. At the forward end (the lower end), there is a length of 3/8" copper that dips down to the lowest point of the condenser thus picking up most of the water.

From there it goes straight to the hot well. I have no condensate pump. The engine exhaust pressure pushes it the foot or so uphill to the well. With a simple engine the added pressure differential from a pump isn't worth it. With a low pressure cylinder on a compound it is often worth it. Not as much as some would have you believe though. It is cool to have another gauge to look at but I suspect that many condensate pumps are a source of much strong language.

Mike

[JonRiley56]

Hi Mike,

I am not sure I follow this statement:

" At the forward end (the lower end), there is a length of 3/8" copper that dips down to the lowest point of the condenser thus picking up most of the water"

Can you elaborate ?

jon

[Lopez Mike]

I'll explain the concept and the rest will make more sense.

What goes in to the condenser is steam and perhaps some air. What comes out is mostly water. That's why a condensate pump is a little different animal than regular pumps though you would be surprised at what will work. I have been.

If you exhaust steam into a cold pipe , the water (condensate) will collect at the lowest point. You want as much of the steam to hit the cold pipe as possible. If very much water accumulates in the condenser, it won't work as well if at all. Reduced working area and all that.

So we make sure that what we call the water pickup is at the lowest point of the condenser. My condenser has an elbow at each end to make the turn into the hull. At the entrance (steam) end, the engine exhaust is fed right in. Nothing fancy. At the lower (condensate) end, there is a pipe cap just inside the hull that is bored for a tight fit on a 3/8" copper tube. The tube sticks down through the cap to as close to the lowest point of the condenser as I can arrange it. Like right down in the elbow.

Just for clarification (you old hands can go back to watching YouTube), the steam going in has a large volume until it condenses. At first, this will produce a vacuum. But as the condenser fills up with water, the vacuum goes rapidly away. So we pump out the water with a relatively small pump. It seems counter intuitive at first that this great big engine is exhausting a bunch of steam into the condenser and this little bitty pump is on the output and there is a gauge showing a vacuum in the condenser. If the system is free of leaks that might let in air and is cool enough, the vacuum can be as much as 20" of mercury (about 30" is a perfect vacuum). You can't get much better than 20" because the water starts boiling on its own at such a low pressure. Your blood, at 98 degrees will boil at around 63,000 feet altitude (about 28" of vacuum). That's why they have space suits. 20" of vacuum gives a boiling point of maybe 160 degrees F. Your condensate will flash to low pressure steam as you try to pump it out. There is no free lunch, of course.

Now that you hard cores have been bored to death, I'll go watch YouTube.

Mike

[barts]

I use this method (exhaust pressure scavenging of condenser). It works well... but don't make your condenser too small. Mine is ample when I'm boating in the Pacific Northwest - 55 F or so water. When we're in 80 F water in a lake somewhere, it's still ok, but definitely warm. If we're at the dock - no way. There isn't enough water flowing past the condenser, and the hotwell starts getting steam.

My engine has about 6' of 3/4" nominal Cu pipe. or roughly 1.7 sq ft of cooling surface. This is 1/10th my boiler surface area. I would not go much smaller.

- Bart

[Lopez Mike]

I wondered why I got steam in the hot well when running very hard at the dock. Duh! I'll go now.

Mike