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Old 12-27-2018, 11:05 AM   #1
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GenSep - exhaust water above WL?

I have a 5kw NL in a sound shield - it's quiet enough but the exhaust splash is bugging me. I can )barely) fit a GenSep under the gunwale where the vented loop is now, and my question is: has anyone seen this where the water exhausts above the waterline? I can install that in my slip, otherwise I need to haul out for the through-hull.

My guess is that it would be about the same as the generator water exhaust - a steady stream that is noticeable but not as annoying. I suppose I could start with that route and do a new underwater outlet later.

Any comments?
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Old 12-27-2018, 11:38 AM   #2
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I would never exhause underwater , you will still hear the exhaust , but with bubbling added.


Placing the exhaust in the water line paint stripe might stop the falling water noise as its so close to the water.
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Old 12-27-2018, 12:06 PM   #3
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I have not seen it done that way, but can't see why it would not work. I'd put the water discharge as close to the water line as you dare to minimize splash.

Like you said, worst case you could move it at your next haul.

Also nice to look over and see it is pumping.
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Old 12-27-2018, 12:27 PM   #4
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I would never exhause underwater , you will still hear the exhaust , but with bubbling added.
My genset (also a NL 5K) has underwater drain for the water separating muffler, and there is no splash. It is very quiet outside the boat. You may be able to heel your boat in the slip to get the water outlet very close to, or under the at rest waterline.
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Old 12-27-2018, 12:28 PM   #5
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I would never exhause underwater , you will still hear the exhaust , but with bubbling added.


Placing the exhaust in the water line paint stripe might stop the falling water noise as its so close to the water.
Ah, I wasn't clear - the GenSep separates the cooling water from the exhaust gas. The exhaust gas would be above the WL, the issue is where the cooling water goes. Per the Centek documentation:

"The water drain is the smallest pipe on the bottom of the Gen-Sep. It can be exhausted above or below the LWL. The water drain line should be as short as possible."
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Old 12-27-2018, 12:30 PM   #6
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My genset (also a NL 5K) has underwater drain for the water separating muffler, and there is no splash. It is very quiet outside the boat. You may be able to heel your boat in the slip to get the water outlet very close to, or under the at rest waterline.
Good point about heeling the boat. I did that to raise the stern - put a 50 gallon waste can on the bow and filled it with water - worked a peach to raise the stern at least an inch.
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Old 12-27-2018, 02:42 PM   #7
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You can put the water outlet anywhere above a line 6 below your max loaded waterline. When we are light on fuel ours actually drains above the water but its not too noisy and there is no annoying pulsing. One spec you need to pay attention to is the minimum height the gen sep/water sep is above the waterline. I think our min spec was 18 from the top of the box. Youll also want to keep the hose as short as possible and always angled down even if you are rolling.
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Old 12-27-2018, 03:52 PM   #8
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You can put the water outlet anywhere above a line 6 below your max loaded waterline.
I had to read that a sentence a few times - what's the point of not going below that line? It's not obvious to me.

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When we are light on fuel ours actually drains above the water but its not too noisy and there is no annoying pulsing.
Good to hear - I'll probably try that route. Right at to about a foot below the waterline is a problem for me since my hull has a double chine - there's a concave carve out. I could do the transom however.
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Old 12-27-2018, 03:55 PM   #9
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We have one on our boat and love it. It exhausts the cooling water beneath the boat and into the water. The exhaust air/gasses come out above the water line. If you're standing on the dock right next to where the exhaust gas comes out you can barely hear it (15KW Westerbeke diesel gen) and all you hear is a slight hum sound.
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Old 12-27-2018, 09:01 PM   #10
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We have fisher Panda 15 Kw with a gen sep that we put in(upgraded to) a few years back now. We absolutely dump our water below the water line (through a dedicated through hull). You want to put a value on it so you can partial close it to adjust the flow of the unit (and to close it for servicing as well). Love the Panda and gen sep makes it super quiet. The only suggest I have is to make your exhaust port bigger in size then the Gen Sep, the bigger the opening the less you hear the exhaust. (also avoid hard elbows (we had to due to space, as test we did a bigger hose radius turn and it much quieter, but we do not have the space for that.)
Last thought, we also did a through hole with rubber flapper, The idea was to keep water out when not being run under way, not sure we needed it since Gen Sep is so high up above water line, but better safe then flooding a generator.

Ps we mounted our exhaust through hull in the boot stripe as well and helps hide things and keep it looking cleaner.
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Old 12-27-2018, 10:13 PM   #11
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My boat originally came with a large engine with an 8" exhaust. The generator has a water separating muffler with the gas tied to the engine exhaust as it exited the side, and the water exit below the hull. During the refit and repower, the engine exhaust dropped to 4". In an effort to minimize through hulls, I ran 8" exhaust from the engine room through the transom. The 4" engine exhaust enters the 8" from the end. Both the exhaust gas and exhaust water from the generator separator muffler enter from the side. The 8" pipe bottom is level with the ocean when water and fuel tanks are near empty. The result is one exhaust penetration. When the generator is running you can barely hear it at the stern. One of the big improvements IMO, is that you can see the gentle stream of raw water coming from the generator. It makes it very easy to spot reduced water flow (bad impeller).

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Old 12-28-2018, 05:43 AM   #12
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The generator has a water separating muffler with the gas tied to the engine exhaust as it exited the side, and the water exit below the hull.
That IS interesting. I have 2x Cummins 5.9s each with it's own 4" transom flap exhaust, and one passing extremely close to the generator loop.
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Old 12-28-2018, 06:27 AM   #13
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Any underwater discharge you can join?.


Sink drain or similar?
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Old 12-28-2018, 08:05 AM   #14
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Any underwater discharge you can join?.


Sink drain or similar?

The last thing you want to do is tie an engine exhaust to a sink drain. Some exhaust will vent backward into the sink drain and right into the cabin.
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Old 12-28-2018, 08:16 AM   #15
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Until your next haulout you could just Y the water back into the gas outlet. It should "pour" out the outlet rather than splash as it did before and you can pop in a dedicated thru hull at your next haulout.

The water does not contribute to backpressure after the sep, it drains by gravity and doesn't care where or how it gets overboard, except if you place it too deep below the waterline.

The sizing of the existing gas outlet accounted for the water to begin with, and because its exhausting as a steady stream instead of being pushed out by the exhaust gasses, it should pour instead of splash, and it will never do that "clear the throat" thing you get from just having a lift

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Old 12-28-2018, 08:18 AM   #16
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I was discouraged from commingling the exhaust and or water outlets for a our new gen by non other than the folks at northern lights. Do it our way or no warrantee. Not sure Id really want the incomplete combustion by products or fumes wafting up from a sink drain or creeping past turbos into open exhaust ports.
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Old 12-28-2018, 09:38 AM   #17
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I was discouraged from commingling the exhaust and or water outlets for a our new gen by non other than the folks at northern lights. Do it our way or no warrantee. Not sure Id really want the incomplete combustion by products or fumes wafting up from a sink drain or creeping past turbos into open exhaust ports.
One has to take back pressure into account when combining exhausts into one outlet. This is why I used 8" exhaust pipe when combining 4" engine exhaust with 2" generator exhaust. Part of any good installation warranty certification process generally includes a back pressure test at maximum RPM to verify it complies with manufacturer's standards. When the dealer did my JD warranty certification, we ran the generator when measuring back pressure.

The combining of engine and generator exhausts is fairly common practice with some of the builders of large sport fishing boats in the Carolinas.

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Old 12-28-2018, 09:41 AM   #18
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Ah, I wasn't clear - the GenSep separates the cooling water from the exhaust gas. The exhaust gas would be above the WL, the issue is where the cooling water goes. Per the Centek documentation:

"The water drain is the smallest pipe on the bottom of the Gen-Sep. It can be exhausted above or below the LWL. The water drain line should be as short as possible."
Mine comes out just at the waterline, so there is no splash. The exhaust gas is above.
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Old 12-28-2018, 10:03 AM   #19
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Until your next haulout you could just Y the water back into the gas outlet. It should "pour" out the outlet rather than splash as it did before and you can pop in a dedicated thru hull at your next haulout.

The water does not contribute to backpressure after the sep, it drains by gravity and doesn't care where or how it gets overboard, except if you place it too deep below the waterline.

The sizing of the existing gas outlet accounted for the water to begin with, and because its exhausting as a steady stream instead of being pushed out by the exhaust gasses, it should pour instead of splash, and it will never do that "clear the throat" thing you get from just having a lift.
I would love it if this works as described. In fact, I'll probably try this first and report back on results. If it eliminates 80% of the splashing I'll be thrilled.
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Old 12-28-2018, 10:05 AM   #20
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I have a 12kw Northern Lights generator with Centek Gen-Sep box. The box is downstream from a Northern Lights lift muffler. Water is discharged below the waterline via a dedicated through hull.
The exhaust is quiet. Real quiet. Not sure how it manages to suppress the sound so much but I surmise the exhaust pulses are pretty much eliminated after passing through the NL muffler and then the Gen Sep box.
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