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Old 10-21-2018, 06:16 PM   #41
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TT you make a point on wet exhaust and necessary hull cleaning. On my wet exhaust systems, both genset and twin mains, I have zero soot buildup on the boat hull. I am not alone. All sorts of reasons for this I would guess. One reason proposed by many is lots of boat wax. Others could be water lift mufflers, not over propped and keeping RPM near max torque range.
I have a gas/air separater on the genset that probably contributes to the soot buildup I see.
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Old 10-21-2018, 06:28 PM   #42
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I have a gas/air separater on the genset that probably contributes to the soot buildup I see.

Interesting. Why do you think it impacts the soot buildup?
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Old 10-21-2018, 07:49 PM   #43
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Interesting. Why do you think it impacts the soot buildup?
The water goes out below the waterline, and just the gas above. My assumption is that if water was mixed with the gas above the waterline there might be less in the way of solids deposited on the hull.
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Old 10-21-2018, 10:32 PM   #44
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Keel coolers on a friend's 80' ex fishboat:
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Old 10-22-2018, 01:43 PM   #45
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Hand in hand with switching to sea water cooling, we are switching back to wet exhaust. Like you, we never had soot flakes, but we did get a coating of soot buildup around the exhaust exit. It covered all the instruments, and looked like hell. And cleaning was a major pain in the butt./ I had to climb up on the hard top, then climb the stack, and hand scrub every instrument and every branch of the tree. And all the scrum and rinse water showered down on the rest of the boat, covering it in black sooty water, necessitating a complete wash of the boat. Wet exhaust soots too, but it's MUCH easier to clean a transom and sides of the boat than to climb the stack, hand scrub everything, then the whole rest of the boat.... Never again. To me, dry stack and keel cooling was a big mistake that I won't make again.
Are other Nordhavn new build owners also choosing a wet exhaust?
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Old 10-22-2018, 01:54 PM   #46
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I have seen a couple of Nordhavn boats with wet exhaust, and one with twin engines. They are rare birds though.
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Old 10-22-2018, 02:42 PM   #47
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Much prefer half pipe or angle welded to the hull. The differences in my opinion are:

No hangers to fail
No need to zinc as it's one with the hull
Lower profile and nothing gets caught between it and the hull
Easier to clean on land or by a diver
IMO, less likely to be broken if you accidentally hit something
...
What OC said.

One could also use U or square tube instead of round pipe to get more surface area.

Professional Boat Building magazine has articles on keel cooling and dry stacks that might be of interest. The index to their back issues, and a link to the back issues, are on this page, Subject Index - Professional BoatBuilder Magazine https://www.proboat.com/subject-index/

Issue No 111, Feb/march 2008 has part 1 of a two part series on Dry Exhausts and Keel Cooling. It is by Gerr.

There is quite a bit of information in the article including the calculations needed to design a keel cooler.

Per Gerr,
Quote:
Keel coolers for STATIONARY operation should be mounted on the side of the hull or the keel, not horizontally under the bottom of the boat. That's because warm water will not flow away by convection but will be trapped under the cooler, which will result in overheating.
Gerr mentions commercial coolers as well as creating recesses in the hull for their placement or keel boxes, I wonder how the heck those areas get cleaned...

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Old 10-22-2018, 04:24 PM   #48
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I have seen a couple of Nordhavn boats with wet exhaust, and one with twin engines. They are rare birds though.

Not really. I don't know the statistics, but there are quite a few wet exhaust Nordhavns. All the twin engine boats except one that I'm aware of. And quite a few singles as well. A wild guess would be 20% of the fleet, maybe as much as 30%. I know of three single engine, wet exhaust boats in build right now.
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Old 10-22-2018, 05:10 PM   #49
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Keel coolers on a friend's 80' ex fishboat:


I am so glad I don’t have to scrape and repaint that system.
Just my eelgrass strainers took a lot of work.
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Old 10-26-2018, 02:22 PM   #50
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MOJO has a half-pipe keel cooler and is a dry stack exhaust. The original engine was a Cat D-13000, but was replaced in 1989 by a Cummins 5.9 6BTA-M1. The water pump in the Cummins is too small to move the volume of water in the keel cooler. The solution is to either run an off-engine pump for keel cooling or put a heat exchanger on the engine. The heat exchanger is the route the previous owner took and it works fine. The exhaust is still dry stack so there's no exhaust mixing elbow and no wet muffler. Here's a picture of the half pipe keel cooler. By my calculations, the 4 half pipes have a capacity of about 150 gallons!

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Old 10-26-2018, 03:10 PM   #51
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That is a HUGE 1/2 pipe cooler. 6"?



But your comment about the size of the pump mystifies me. Since it is a closed system, the size of the pump is not of any concern. There is no "head" in a closed system. No lift. As long as the pump can handle the volume over time that the engine needs, and it can because it was designed for that engine, it is big enough.
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Old 10-26-2018, 03:19 PM   #52
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That was my initial thought as well, but it turns out that a small pump pushing against a large volume of water apparently ends up creating a laminar flow that isn't efficient in dumping the heat. (I never took fluid dynamics or thermodynamics so I don't know for sure!) The previous owner (who pulled out the Cat & put in the Cummins) said they tried it and found the engine would overheat. Cummins told him that he would need to install a higher volume off-engine pump if he wanted to use the half-pipe keel cooler. The easy solution was to use the heat exchanger that came with the engine.

I don't remember for sure, but I think they're 8" dia pipes about 16' long.
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Old 10-26-2018, 03:39 PM   #53
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Using an onboard heat exchanger seems superfluous to me. Since the keel cooler IS a heat exchanger. So you have two pumps, one to circulate the coolant in the keel cooler and another to circulate the coolant in the engine and the boats movement thru the water circulates the real coolant over the 1/2 pipe heat exchanger?
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Old 10-26-2018, 03:44 PM   #54
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It all does seem odd. And wow, that must be quite a bill to replace the coolant.
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Old 10-26-2018, 04:34 PM   #55
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The keel cooler isn't being used, it isn't even plumbed anymore into the engine cooling circuit. The engine is cooled only by it's own raw water pump and it's heat exchanger. Easier and less trouble prone than worrying about a large off-engine pump. I have two spare engine raw water engine cooling pumps on-board and it's two bolts & two hose clamps to swap them out. If I were really bored and looking for a project (I'm not!), I'd plumb my 3 air conditioners into the keel cooler and be able to use fresh water for the a/c circuits. I have a 30 gpm pump for the air conditioners so I'm sure it would work fine. However, there's always ramifications of changing things! In addition to the air conditioner cooling, I use the a/c pump as a boost pump for my 33 gph watermaker. If I dedicate that pump to running the a/c's through the keel cooler, I'd have to get a new boost pump for the watermaker. Much simpler to leave things the way they've worked flawlessly for the past 18 years! It's a nice simple solution, no need for further complication.
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Old 10-26-2018, 08:00 PM   #56
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Yes, if it aint broke, dont fix it.
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Old 10-26-2018, 08:48 PM   #57
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That is a HUGE 1/2 pipe cooler. 6"?



But your comment about the size of the pump mystifies me. Since it is a closed system, the size of the pump is not of any concern. There is no "head" in a closed system. No lift. As long as the pump can handle the volume over time that the engine needs, and it can because it was designed for that engine, it is big enough.
JB
You likely know this. A keel cooler does indeed increase the total head the pump sees. There are two components. Static Head and Friction Head. By increasing the pipe length dramatically the FH loss increases likewise.
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