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Old 07-27-2015, 07:06 AM   #1
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Hybrid dry exhaust

I met with a shipyard today about my steel trawler build. They showed me the exhaust and cooling systems which they use on workboats and tugs of about my boat's size.

The engine and hydraulic oils and also the engine coolant are cooled in a closed loop keel cooled arrangement (split pipe welded to hull). However the engine exhaust system is cooled down with salt water then the exhaust runs up through a normal dry stack.

They claim that cooling the exhaust reduces exhaust noise by half.

My configuration will be a single engine of about 180 hp, a Tier 3 compliant. I am interested in reducing the complexity of my boat and eliminating thru hulls, strainers, injection elbows, etc.

So my question is if anyone has experience with such a system and if the benefits are worth the added components?
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Old 07-27-2015, 09:28 AM   #2
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Never heard of that one. A spray ring at the hull with straight exhaust is common workboat here in US, as is dry stack. In larger yachts there is sometimes an either / or option where the exhaust can go through a wet system then out the hull or valve up the stack after the wet muffler, but there is almost always some water trap arrangement in the stack to keep moisture out of the gas as it comes out the top. This is to keep droplets of black water off the deck, nasty stuff. Also these yachts have looong vertical exhaust runs, where in contrast yours is probably much shorter.

It will be quieter than straight pipes. Is the stack fiberglass pipe? I would be concerned if it was carbon steel, won't last long. Stainless pipe is expensive and won't last forever either.

Hot exhaust gases from a diesel engine mixed with salt water is just about as corrosive a mix as you could imagine.



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Originally Posted by makobuilders View Post
However the engine exhaust system is cooled down with salt water then the exhaust runs up through a normal dry stack.
They claim that cooling the exhaust reduces exhaust noise by half.
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Old 07-27-2015, 09:59 AM   #3
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I would avoid having any sea water involved with the exhaust. A good dry muffler can be bought that is plenty quiet.

Be careful picking an engine. Most modern engines use sea water aftercoolers which are tricky to adapt to keel cooling.
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Old 07-27-2015, 10:24 AM   #4
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Originally Posted by Ski in NC View Post
Be careful picking an engine. Most modern engines use sea water aftercoolers which are tricky to adapt to keel cooling.
I assume you are referring to a turbo aftercooler. Mine will be naturally aspirated.
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Old 07-27-2015, 10:39 AM   #5
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I assume you are referring to a turbo aftercooler. Mine will be naturally aspirated.
Right. No turbo means no aftercooler. A non-issue, then.
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Old 07-27-2015, 10:46 AM   #6
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A straightforward keel cooled, dry stack engine will be the simplest. The sea water exhaust cooled system that your yard proposes adds back complexity- a thru hull, raw water pump, water spray system, water separator, raw water drain thruhull, etc. The only real advantage is that the cooled exhaust gases can be discharged upwards with much less insulation and air space around the stack than a pure dry stack system. But why not discharge them out the transom like a normal raw water cooled engine. And in that case since you have a raw water pump anyway, why not go with a normal heat exchanger/mixing elbow system.


And as Ski notes, a sea water intercooled design also adds complexity. John Deere's 4.5 liter engine will do what you want with jacket water intercooling and meets Tier 3.


How much continuous horsepower do you need? In general I would plan on an engine that is rated at about twice what is needed for normal cruising speed.


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Old 07-27-2015, 11:43 AM   #7
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What engine are you looking at? Not many NA engines out there. I prefer a turbo engine as they do tend to be more efficient, and turbo also knocks out some exhaust noise.

A mid rated turbo engine often has either no aftercooler, or a jacket water aftercooler, both have no added complexity if keel cooled. Usually only the high rated engines use sea water aftercooler, I'd avoid those for a trawler for several reasons.

Just about every problem with marine diesels comes from sea water going in the wrong places and eating expensive things up. I would avoid any sea water relating to engine, especially the exhaust. You don't need any sea water.

Go dry, and do it right.
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Old 07-27-2015, 02:06 PM   #8
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"Hybrid" exhausts are very common and very safe and troublefree on a very large number of very high quality yachts of all sizes. There is no threat or problem using seawater to cool and silence engine exhaust. We use a mix of stainless steel and glass fiber tubing as appropriate to the location and achieve long life from all the components. A good system is not cheap but it works very well and no water carries over to the dry side.

The exhaust outlets shown in the first photo are generator and main engine exhausts. They discharge dry (and quiet) exhaust at the waterline.

The second photo shows a pair of water separators for 99kW generators. Note the location of the water injection that cools and silences the exhaust before it enters the separator. Dry gas exists at the top and a drain at the bottom allows water to discharge below the waterline.

The third photo is taken overlooking the top of a generator at the bulkhead mounted version of the generator water separators. The grill like device in the center is a window for viewing the water level and to add a bit of bling. The main engines in this case are MTU 4000s so the technique is applicable to a wide range of power and exhaust flow.

By the way, those photos are of an installation in the engine room of a sailboat, not a multi-story megayacht engine room.

Today we completed another installation on a yacht in Cannes that is identical to those shown in the photos. The exhaust is cool, it is quiet, and it is clean and dry.
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Old 07-27-2015, 02:10 PM   #9
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Every system has pluses and negatives. The salt water injection and separation exhaust is often referred to as a "water-drop" system. It's really popular on generators, for those who dislike the puff-pop of mixed gas and water. The result is a very quiet, cool, dry gas that can be routed anywhere. Out the stern, up a mast, wherever.

Downside is it's not simple. Reality is any marine exhaust will require attention at some point. Yes, a straight dry pipe with muffler is simpler, also lots of fires associated with same.
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Old 07-27-2015, 04:04 PM   #10
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Tad:


Considering the boats where you had a hand in the design or construction supervision, how many had wet exhaust systems, how many had the hybrid type, and how many had the dry stack type?


You spoke of fires on dry stack systems. Isn't that only possible on poorly designed or maintained systems?


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Old 07-27-2015, 04:15 PM   #11
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Welcome back RickB!!!!
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Old 07-27-2015, 04:48 PM   #12
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Easier to do dry stack on a steel boat. Just less opportunity for hot pipes to be close to wood or FG.

Dry stack can accumulate soot over years of running, then if power is increased, it can ignite the soot in the muffler. I've personally had this happen, it is exciting and could be dangerous, but can be controlled by shutting down engine.
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Old 07-27-2015, 07:28 PM   #13
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Quote:
Originally Posted by djmarchand View Post
Tad:


Considering the boats where you had a hand in the design or construction supervision, how many had wet exhaust systems, how many had the hybrid type, and how many had the dry stack type?
Honestly, I have no idea. Certainly the vast majority (just in numbers) would be production fiberglass boats with standard wet exhaust systems with water lift (sailboats mainly) or straight-through (powerboats mainly) mufflers. But I've spent hundreds of hours engineering water-drop and wet jacketed exhaust systems.

Quote:
You spoke of fires on dry stack systems. Isn't that only possible on poorly designed or maintained systems?


David
When it comes to boats....if stuff can happen, it will! I grew up with nothing but dry stacks, it's personally my favorite. Properly done it will be very reliable and safe. But it's not foolproof or without possible problems. Those problems occur over a period of time, vents get plugged, or the prop gets changed and the stack temperature goes up, injectors get dirty and soot builds up. It's like the cooling water flow, just something you are aware of.
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Old 07-28-2015, 06:50 AM   #14
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Welcome back Rick B.!!!

\
Dry Muffler science has come a long way , just listen to your car.

The best really simple silencing system deems to be for emergency gen sets.

Google "Hospital Critical" mufflers for an idea of simple and silent.

I am lazy so one more seawater system to maintain and winterize might be quieter , but ,,,no thanks.

Exhaust Silencers - Maxim Silencers

www.maximsilencers.com/exhaust_silencers.html


In these areas, installation of an M32 on an engine exhaust is intended to bring ... Compact chamber silencers are engineered to provide “critical to "hospital" ...
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