Dry vs. Wet Exhaust ???

[Cichlidgeek]

Hey all,
Due to the board reconfiguring my previous thread asking about the pros and cons of a dry vs. a wet exhaust did not make the cut to the new board format.

So......I did see that I had a few replies, but did not get a chance to read them.

I am thinking that a dry stack is a bit simpler, but perhaps more cumbersome than a wet exhaust. I am a big fan of "the fewer through-hulls the better" mentality.

May I have your thoughts again, please??

Thanks!!

 

[Downeaster]

Based on necessary holes through the hull, the two systems don't differ by as much as you might think. The engine has to be cooled from some external source and - wet or dry - that source is the ocean.

With a wet exhaust, ocean water is circulated through a circuit that includes a heat exchanger - a water cooled radiator so to speak - which cools the engine(s) cooling system coolant and then exits the spent cooling water out of the exhaust where it provides a second service (preventing the exhaust system's hoses and fibreglass tubing from going up in flames).

A dry exhaust setup doesn't pull ocean water inside the hull but cools it by circulating ocean water through an external cooler which is typically mounted very close to the keel. Actually, with this system, the engine's coolant is sent outside the hull to circulate through that externally-mounted cooler.

The problem is that the dry system requires two substantial holes through the bottom of your hull - one more than is needed for a wet exhaust. Another problem is the drag that the keel cooler itself introduces. Finally, there is the matter of insulating the exhaust and then plumbing the finished exhaust pipe through the work and living area of your boat.

The are techniques that will mitigate some of the issues mentioned (the keel cooler could be recessed into a pocket moulded into the hull or an internal tank could be created in an aluminum hull that could cool an engine without cutting any holes into the hull at all) but in the end - at least with small boats - there are substantial reasons for the popularity of the (engine-mounted or internal) heat exchanger system.

Let me jump up on my soapbox here and preach the virtues of an accurate exhaust temperature gauge and alarm system. They are almost unheard of but they do so much. When the company commissions your new diesel, what are they measuring? This system could head off an embarrassing exhaust system meltdown and it is, IMO, the easiest and most accurate way of determining how efficient your throttle settings are and also allows you to accurately tune your trim tabs. Soapbox off.

 

[steveinak]

Let me jump up on my soapbox here and preach the virtues of an accurate exhaust temperature gauge and alarm system. They are almost unheard of but they do so much. When the company commissions your new diesel, what are they measuring? This system could head off an embarrassing exhaust system meltdown and it is, IMO, the easiest and most accurate way of determining how efficient your throttle settings are and also allows you to accurately tune your trim tabs. Soapbox off.[/QUOTE]

DE are you talking about a temp gauge for the wet exhaust system or a pyro for the engine exhaust?? Actually i think both would be a great addition to any rig and in fact i will be installing a pyro on my rig as soon as i figure out where the probe for the pyro should go ? I'm thinking somewhere on the exhaust elbow just before where the water is injected into it. Any thoughts? let me have them

 

[CEShawn]

I am amazed at how little data we have on our small engines. I work on commercial engines up to 33,000HP and it amazes me. Yes these small high speed engines that we run, one hiccup and boom they are done. It is hard to tell before something happens that it is happening. Talking diesels here...

You really need to have an exhaust pyrometer, easy install on almost all engines. A boost pressure gauge to tell how the turbo is working. Those temp switches going to a single alarm are great on the exhaust elbow, first sign you'll probably see if you have a salt water cooler issue. Of course the list goes on from here too. I really like a remote gauge on my fuel filter too, simply because so many times in the middle of the night coming into a harbor after a 3 day trip, knowing I burnt through one tank and at the end of another, having to open up the engine hatch and take a look.

I am not dead familiar with dry exhaust on small vessels but we deal with it all the time on ships. I think one issue that wasn't addressed is the cooling issue. Yes you can put a keel cooler on, do fancy things to dress it into the hull etc. I do think that you do not get the same cooling capacity though for our higher horsepower engine's. I know several commercial boats that have them but they are doing 8-12knots. I think there is a thermal issue with getting enough surface area for the heat generated at those higher speeds. That combined with drag I think you have that lower horsepower where you see this application. In the back of my mind too, I also know from working with steam plants, that if your rate of flow is to fast, you get less thermal transfer and wonder, but not proven, if you are doing 20 knots, if that just isnt there. The other thing when you start going faster less boat is in the water even in some displacment. Our H&H 40, had a thruhull just by the helm area and would get air in it in rough weather because the wave would fall out from the boat at speed. Generally thinking that could be another issue.

Now the dry exhaust part has always intriqued me as I am used it and has great reports if done correctly of extending turbo life. I think its neat too because you can rig it to be a little bit of a simple heater for cold operations.

The other thing, haven't touched on this lets talk tuna trolling. I've heard boats with dry exhaust have fish coming right to the back of the boat. Mind you, in our old H&H 40 we had that too. Matter of fact one day we had a 400lb blue marlin eat a 18" Moldcraft less than 3 feet off our transom right behind our exhaust...

Just my two cents, I do not always have it right, but I do play a Chief Engineer in the Merchant Marine...

 

[BillD]

EGT and raw water alarms

Steve and DE.
On both my 370 Cummins, I have ISSPRO Turbocatur EGT/Boost gauges.

EGT is THE BEST engine "parameter" you can measure. As DE said, increasing EGTs (from a baseline EGT when ALL engine parameters are correct) will indicate increasing loads on the engine. For instance I have the 370s on my Blackfin "under propped".

OK the single most impostant "alarm" for a marine diesel is a "loss of raw water alarm". Again I have this alarm on both my engines.

Very simple and inexpensive to install. Borel Manufacturing makes a good one along with AquaAlarm.

I have 180F temp switches on both the engines' exhaust mixing elbows.

Should an engine lose an impeller or the engine sucks up a bag on the raw water intake or for ANY reason raw water coolant flow is lost, the exhaust mixer heats up real quick and the alarm goes off.

The alarm prevents two things.

1. an overheated engine and
2. an exhaust hose fire.

 

[Downeaster]

Good question and I must admit to having not made the distinction - brain cramp here. While I was thinking about the differences, I came across an article that is far more thorough than anything that I could come up with (not real enthused about the gauge pictured with it though). Sorry but I can't get the damned link thingie to do anything but tell me that I'm trying to upload an invalid file!

Banks Power | Why EGT is Important

 

[Crabman]

Not all dry exhausts require keel coolers and many have the same arrangement as wet exhaust whereby the exact same raw seawater pump supplies the cooling seawater to a the multiple heat exchangers. The only difference is at the exhaust: the wet system goes through a cooling exhaust injector where the hot exhaust mixes with the seawater circulated through the heat exchangers and in the dry system the raw water is just pump overboard while the hot exhaust is directed through the exhaust pipes to a muffler or silencer.

I have this system in my 36' BHM. It's the KISS principle for exhaust. No hot exhaust and saltwater mix so corrosion is minimal. Some other advantages are: exhaust fumes are exhausted above the boat, no fumes over the transom and on my boat and a lot of working lobster boats, you can have your warm water engine discharge water near the hauler and helm to rinse your hands off and also see that you are pumping cooling water through the engine.