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Old 10-07-2017, 09:13   #1
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Some questions about raw water and exhaust

Hey all, I'm in the early phases of a refit on my new to me nauticat 38 , and I had a couple of questions I would love to get some feedback:

The nauticat design seems to want me to install a lift muffler on my wet exhaust system, but the last 15 feet before getting to the ejection point in the stern is going uphill! This seems like a poor design, and I'm really looking for suggestions, but the Engine compartment under the pilothouse is too far below the Stern to do it any other way , unless I eject it from the side of the boat which presents obvious problems with heel... Am I missing something? Is this relatively okay to have as a design as long as the exit point is well below the top of the loop from the lift muffler?

Also,I know I need An anti-siphon device on the raw water as it exits the engine before it goes into the exhaust, but the engine is placed in the middle of the engine room with nothing around, and I can't see any place to anchor this vented loop without running it clear over to the side of the boat and back, which gets in the way and is a lot more expensive. Any suggestions for having one float in the middle of the compartment? (Above the engine is the access hatches)

Thanks for the help in advance. I'm sure I will be a leech of information for the next six months to aClick image for larger version

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Old 11-07-2017, 03:52   #2
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Re: Some questions about raw water and exhaust

I am no expert in exhaust design but I have always understood you need to go much higher than the outlet level before dropping back down to the water trap (lift muffler?).

In you sketch, a following sea could quickly fill your exhaust hose and it wouldn't take much more force to push it over the u bend (under the pilot house bench). Think following sea and pitching etc. Once the water trap is full, it will go into your engine!

The exhaust should get as high as possible (use a swan neck etc) just before exits the transom. But if you don't want to have the exhaust rise up beside your bed, then try to get the U bend at least as high as the aft deck level.
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Old 11-07-2017, 04:05   #3
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Re: Some questions about raw water and exhaust

Anti siphon device: You can install the model without a valve in it. It just has a small continuously "pissing" hose to outside your hull. As long as the engine is running, you get a small stream of water "pissing" from the hull.

Anyway on my Yanmar 3JH4E the antisiphon device is just "in the air", held up by the 2 rubber hoses going to/from it.
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Old 11-07-2017, 05:02   #4
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Re: Some questions about raw water and exhaust

Good Old Boat - Cool and Quiet and trouble-free article

Exhaust System Fundamentals ~ by Dave Gerr - Part 1 = 10 pages Part 2 = 12 pages Dave Gerr is director of the Westlawn Institute of Marine Technology, a practicing Yacht Designer, and noted author of: "The... Cruisers & Sailing Photo Gallery

TRANSVERSE EXHAUST SYSTEM - Cruisers & Sailing Photo Gallery

Boat Building Standards | Boat Exhaust Systems
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Old 11-07-2017, 06:21   #5
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Re: Some questions about raw water and exhaust

Havent had time to look all the expert info contained in the previous thread, but just for further input,

1) Starting at the outlet end (through hull fitting point)the hose needs to rise up as high as practical(recommended MIN approx 45 cm) (items like the Vetus Goose neck are also designed to do this) to keep the hose from letting water enter while healing, if not using a Goose neck then the hose is usually just secured up under the transom,
2) From there the hose will run down hill to the muffler (if fitted) or direct to the water trap/water lock which is mounted below the engine manifold height, one point here is that I Often see is under size water locks fitted and very long exhaust hose (like yours) the issue here is that when you stop the engine the water in the exhaust hose runs back down to the water lock both from the exhaust out let end rise and from the injection elbow end, obviously if the water lock is not of suitable size then the water can fill the water lock and run up to the manifold and flood the engine.This is what happens also when you turn over a non starting engine to long, the system fills with water and then floods the engine.So remember if your engine wont start in a reasonable time period close off the sea cock for raw water supply and be ready to open again once firing,
3) The pressure line sea water should exit the heat exchanger and rise as high as feasible(must be MIN 40 CM above water level) with the air vent/vented loop at the top point before running down to the exhaust riser and out the exhaust.

This is the set up for an engine mounted below the water line or close(slightly differs for engine above water line) Actually the on line Vetus site has very nice clear concise images of both exhaust set ups and worth a look,

There images are worth a 1000 words!
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Old 11-07-2017, 10:14   #6
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Re: Some questions about raw water and exhaust

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Maybe this picture more accurately illustrates the problem. I think I have a very well-designed system up to the point where it makes its last run to exit the Stern. but because it's going to slowly go uphill, and there's no other way I can see to make it work without having a exhaust line on the sole of my aft cabin. So my question is this: is the constant presence of water illustrated in the picture okay? Or should I do something else because this set up will cause problems? What are the problems with this set up? Does anyone have an idea of a better solution?
Thanks again guys for your help on this. There's not a whole Lot of literature out there to help me with this issue.
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Old 11-07-2017, 10:38   #7
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Re: Some questions about raw water and exhaust

I didn't see that you said what year your Nauticat 38 is or what engine that it has in it. Is it the stock engine that came with the boat or have you re-powered?
The older Nauticat 38's that I am familiar with came with a Ford diesel. I have a 1983 Nauticat 36 but it has a Perkins with a heat riser exhaust system so I can't really help you with uyour questions.
Instead of asking a group of people who are not familiar with your vessel, why not contact the mother yard. They are very helpful and speak good English.

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Old 11-07-2017, 10:45   #8
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Re: Some questions about raw water and exhaust

My NautiCat38 isn't even a NautiCat. It was built in '72 by a different company that sold the plan to NautiCat, who I believe, started production in '76.
It has a rebuilt Perkins Sabre 115t with 0 hours on it, that was mounted but none of the connections made (Fuel line, Battery, Raw Water, Exhaust, Controls) and now I'm working to get it hooked up and the boat back in the water with all the parts purchased by the previous owner that passed away, and left no real notes. It's been fun! Like a Lego kit with pieces missing and no instructions.
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Old 11-07-2017, 11:20   #9
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Re: Some questions about raw water and exhaust

Damn, Gord May, you got there before me! A traverse exhaust obviates the need for an anti-siphon device, and, if constructed so the exhaust slopes down from the "top centerline", you get little chance of downflooding and far less back pressure, as you essentially have twice the area of exhaust. In addition, if sailing for extended periods, you can put an inline seacock just below the top "T-fitting" and can keep the exhaust self-draining at all angles of heel. Of course, you must remember to open it before starting the motor, but it's a small price to pay for a greatly simplified and arguably less restrictive exhaust system, in my view.

This is the picture that matters. That tranverse pipe doesn't have to be flat; it can slope down from the T-fitting. I am actually building a box under the bottom pilothouse stairs to give me even more height to make this a straight slalom of the exhaust out the boat. I would LOVE to bail on the anti-siphon loop by making siphoning a thing of the past. I've discussed my plans with Dave Gerr himself, by the way: https://www.cruisersforum.com/attach...xhaust-med.jpg
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Old 11-07-2017, 11:40   #10
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Re: Some questions about raw water and exhaust

This does seem like a great solution to the problem, thank you so much guys. Cutting 4" holes in the side of the boat is a little scary. How far above the water line is it recommended?
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