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Old 04-27-2019, 08:04 PM   #1
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48 navigator with under hull exhaust

I am buying a 48 Navigator with under hull exhaust when the boat gets to cruising speeds it jerks every 2 to 4 seconds even in calmer seas. I was told it is because the air expelling from the exhaust system gets trapped momentarily
beneath the hull and it creates the change in sound and a little jerk or leap through the water. Anybody care to respond on this issue. My last boat was a 62 Mckinna with under hull exhausts and never noticed this movement The engines are running smoothly and shafts and wheels are in alignment and perfect pitch.
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Old 04-28-2019, 03:25 AM   #2
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I don't have any experience with the exhaust you've described, but I'm curious who told you that, and if that behavior was observed during the survey.
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Old 04-28-2019, 07:54 AM   #3
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I suspect a loose prop.
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Old 04-28-2019, 08:42 AM   #4
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Under hull exhaust can do strange things. Is there a smaller diameter relief tube? The McKinna's I have been on had the relief tubes going to the transom to vent exhaust at lower power settings and that feature worked.
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Old 04-28-2019, 02:45 PM   #5
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Props were checked during survey and where tight, my old Mckinna had relief ports on each side above the water line. The new Navigator also has the same configuration. The hull surveyor said the movement and sound was caused by that. While the engine surveyor didn’t feel or hear anything while he was in the engine room while we, on the fly bridge, felt and heard the movement and sound. I am not buying that the exhaust is trapped otherwise other people would be talking about it
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Old 04-28-2019, 02:52 PM   #6
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Is Navigator still in business? If yes, why not call them and get some REAL info?
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Old 04-28-2019, 03:01 PM   #7
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Under water exhaust ?

Navigator is no longer in business. My hope is to find an owner of a 48 who can tell me if he experiences the same thing then it is a design issue if not I have to track it down. Any 48 navigator owners out there?
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Old 04-28-2019, 03:41 PM   #8
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We recently purchased a 2008 Navigator 46 that also has primary underwater exhaust. We brought it down to her home port on Easter Sunday which was our first good 2 to 3 hour run. At higher RPMís I felt the same burping shudder (best description I can come up with) from the fly bridge and the captain I hired to help me move it said that was normal for a lot of underwater exhaust boats. Just had the props re-pitched, balanced and prop-speed coated so went into the engine room while underway and all was smooth and vibration free down there so I donít think it was coming from the running gear.
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Old 04-28-2019, 05:10 PM   #9
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under hull exhaust noise and motion

Thanks for the response for Navigator owners, At least I am not alone or crazy in what I experienced. Perhaps we will hear from others. Enjoy your new boat fair winds
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Old 04-28-2019, 06:44 PM   #10
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Sorry.......without trying to hijack the thread whats the point of an underwater exhaust - with the complication of a diverter for low revs and at least 2 huge holes in the hull to worry about whats the advantage?
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Old 04-28-2019, 07:58 PM   #11
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Quieter and at one point the US was considering a law to do just that to all new boats.....more importantly for those of us who like clean boats and no backwash of burnt diesel fuel. It is more pleasant sitting in the cockpit with white clothes on and not inhaling the station wagon/original SUV backwash of exhaust.
But thank you for your in put....very helpful
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Old 04-28-2019, 08:02 PM   #12
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I ran patrol boats modified for covert operations. The underwater exhaust along with thick soundproofing hid the sound of the engines. I assume that's the same reason for yachts.
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Old 04-29-2019, 05:47 AM   #13
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You Are right. Now did you experience any motion, as described in this thread of conversations, different then boats that used a more common exhaust port?
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Old 04-29-2019, 09:07 AM   #14
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I was on a navigator 48 for many trips a few years back, didn’t notice anything like that, and I notice things like that.
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Old 04-29-2019, 11:27 AM   #15
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thank you
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Old 04-29-2019, 11:31 AM   #16
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Like Ski said,

Under hull exhaust can do strange things.

I know just enough about these to be dangerous, but...

Typically there is a cowl over the underwater exhaust which at speed creates a venturi effect that generates low pressure in the cowl that sucks the exhaust out the bottom. The transition from the above water bypass at low speed to underwater at higher speed is automatic in the sense that exhaust gas will choose the path of least resistance, lower pressure, out the bottom................in theory

There are a few things that can happen in the "strange things" category:

Because of this observation: "creates the change in sound and a little jerk or leap through the water" it may be exhaust interfering with the propellers, causing them to aerate in a cycle as large "bubbles" of exhaust are generated and released. This would create the change in sound as well as the jerk.

Another effect sometimes seen (heard) from underwater exhaust is a rumble as the exhaust gasses bounce off the bottom of the hull aft of the cowl. This too may create the change in sound and a "leap" as the wetted surface is changed to the point where the boat "jerks."

Some of the "fixes" for these problems is a

"fence" that trails aft of the cowl to direct exhaust gases away from the propellers

or

vortex generators that create a vortex which "breaks up" the bubbles into smaller bubbles that have less effect on the bottom.......maybe

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Old 05-03-2019, 02:14 PM   #17
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We have the same exhaust config. with our AT 34. So far no issues in the brief time we have owned it.
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Old 05-03-2019, 04:32 PM   #18
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My last three boats (including my current boat) had underwater exhaust and none experienced the problem you described. Sorry I can’t help but I do love the quietness and clean exhaust.

Good luck.
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Old 05-03-2019, 07:31 PM   #19
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I would venture to say that there is some back pressure building up at certain speeds/engine rpms/exhaust flow. The sheeting of the water as it passes over the exhaust exit on the bottom of the hull could use some relief, a frp Wedge in front of the opening may be needed, or made a little taller if it exists.

Check the props and rudders at haul out. If the exhaust is being pulled into them, you will see some visible wear. The underwater exhaust outlets had a certain angle outboard that should have let the chine carry it away from the props or rudder when underway. If you are in the cockpit or the aft bilge when the shudder or noise is apparent , you should be able to pinpoint the location, try changing the rpm to see if it goes away.
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Old 05-04-2019, 07:30 AM   #20
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Underwater exhaust systems can be tricky, many builders come up with seat of the pants designs, which often results in strange anomalies, from the "burping" you describe to prop ventilation, which may also be occurring.

Your pre-purchase analysis should include an engine survey, which in turn should include a measurement of exhaust back pressure thought the full RPM range (I've pounded this drum before, with few exceptions, pre-purchase engine surveys on diesel inboard vessels should include checking exhaust back pressure). That may provide an indication of the source of the surging; and/or of the back pressure is too high.

As Ski and others have noted, underwater exhaust systems must include a relief port so exhaust gasses can escape on start up, idling and low speed running. Also, most UW exhausts include a deflector just fwd of the underwater discharge, which deflects water flow, thereby reducing pressure, making it easier for exhaust gasses to exit. If that's absent it can lead to this sort of problem.

Based on your description alone, it sounds like the relief port is either not installed, or not working properly, or the props are being ventilated by exhaust gasses. Some builders use "prop fences", longitudinal vertical fairings along the bottom designed to keep exhaust gasses from migrating into the props. Are those present?

Like a lot of other things, the Germans were among the first to successfully use UW exhaust on their Snell boats to reduce noise when patrolling in enemy waters. It took a lot of trial and error to achieve what they were looking for, but they eventually did, naval architect Dave Gerr wrote an article about it in ProBoat several years ago.

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