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.

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.

I suspect a loose prop.

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.

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

Is Navigator still in business? If yes, why not call them and get some REAL info?

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?

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 RPM’s 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 don’t think it was coming from the running gear.

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

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?

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
Cheers

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.

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?

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.

thank you

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

We have the same exhaust config. with our AT 34. So far no issues in the brief time we have owned it.

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.

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.

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.

(In Taiwan inspecting new builds)

All the Navigators with underwater exhaust had the bypass installed, mainly for idle and low speed relief. It’s not a complicated system and worked well.

The curious thing is when one installation in a series has an issue when the rest don’t, It may be as simple as addressing the wedge in front of the outlet on the bottom of the boat, it was their to detach the water flow as it passes over the exit hole and allow the exhaust gasses to exit. It’s possible that it could have been left off or not built up tall enough. Another question would be does it happen on one side or both? Does the boat have Volvo 63L or 63P engines?

I recal one issue with a 61 Navigator operating in the Delta in NoCal. The shallow water was an issue, as an identical 61 was tested with the same engines and same underwater exhaust design and had no issues. The 61 in the Delta would live in skinny water for its owner and ended up being converted to side exhaust.And it was interesting to note that there was no issue observed on the 61 during its delivery trip from Newport Beach to the Deltaas it was in deep water, obviously. The Delta water depth was only 5 - 10 feet below the props at that particular location.

My experience is that the usual purchase surveys would not include exhaust back pressure measurements at a range of speeds as a standard package. This would be a special request at an additional charge, maybe to address a known issue .