Vibration because of long exhaust pipes ?

[HorizonblueDK]

I have made my own thru-hull exhaust. It has been re-designed a few times. The way it is now, is silent outside the boat, at low speed. So I don't annoy any fellow boaters, when warming up the engine(4.3 V6) and leaving the dock. But inside the boat, is sounds like someone is holding a grinder against the hull. This noise appeared after I changed the design into what it is now.

My theory is that the exhaust pipes are vibrating, because the gas exits at the end of the quite long pipes, under the water. There is no vibrating when the boat is running on land.

This is what it was before, there was no grinding noise at that time, instead a lot of exhaust noise behind the boat, because the pipes are only half under water.



As you can see, I have also changed the swimplatform and the Volvo Penta 290 outdrive has been replaced by an SX.

So, what to do now? I would like to keep the thru hull solution, and have it quiet at low speed. That is the reason why I extended the pipes at a downwards angle, so the noise is buried under water. But I don't like the grinding noise.

 

[Don S]

Re: Vibration because of long exhaust pipes ?

Take those deflector plates off and try it. They could be forcing all the exhaust gas bubbles right down to the prop. Without them the gas goes above.

 

[dtj]

Re: Vibration because of long exhaust pipes ?

With the newly designed/fab'd underwater tips you are getting more more back pressure because of the underwater exhaust release which in turn is creating increased resonance at the hull to pipe interface.

It appears that you need a better hull to exhaust pipe interface seal material that can damp the resonance at that joint. What are you using as a seal?................Don

 

[emilsr]

Re: Vibration because of long exhaust pipes ?

I'd say don't give up. Turn downs are pretty common ...it can work. A good set from Gibson is about $400, but if you have the capability to bend the tubing it should be fairly cheap to fab up. I wouldn't even begin to guess why yours is making noise; have never seen (heard) "standard" turn downs do that, but they're round all the way to the exit.

When one of my old neighbors was putting a set on his Fountain, "they" said the exhaust exit needs to be 4-5" below the water line, but not so low that they remain in the water when on plane.

 

[Tail_Gunner]

Re: Vibration because of long exhaust pipes ?

Post a pic of your pipe's going though the transom wall. Are you running hose to the transom or metal piping

 

[HorizonblueDK]

Re: Vibration because of long exhaust pipes ?

With the newly designed/fab'd underwater tips you are getting more more back pressure because of the underwater exhaust release which in turn is creating increased resonance at the hull to pipe interface.

It appears that you need a better hull to exhaust pipe interface seal material that can damp the resonance at that joint. What are you using as a seal?................Don

Between the welded flange and the hull, there is a rubber gasket, cut from a piece of rubber sheet, a bit less than a 1/4 inch thick. The flange is screwed onto the transom with 12 stainless steel woodscrews.

 

[HorizonblueDK]

Re: Vibration because of long exhaust pipes ?

Post a pic of your pipe's going though the transom wall. Are you running hose to the transom or metal piping

Here is a picture inside the boat, the best I have right now, where the exhaust system can be seen. Just under the power steering ram, there is a short exhaust hose.



On this picture, it can bee seen more clearly, how the system is made. I took it before I welded and polished the pipes. They are only spotwelded on the picture.

It is assembled with two pieces of exhaust hose, one at the top and one at the bottom of the vertical pipe, otherwise it would be impossible to install.

I had to fabricate this funny looking neck, to clear the movement of the steering ram.

As you can see, the horizontal pipe is round, diameter 3,5 inch. What may be a little more difficult to see, is the piece at the end, that points 45? down. It is a rectangular tube, measuring 2x4 inch.

Everything is fully welded, so all the gas has to exit at the very tip of the system. The horizontal round pipe is halfway under water.

 

[HorizonblueDK]

Re: Vibration because of long exhaust pipes ?

I know it's not always allowed to live up old threads, but since I started this one only 2 months ago, and I still have the problem, I believe there is room for an exception.......

I have done some thinking on what can be done. For sure the vibrations are caused by the deeply submerged tips. I would still like to keep the exit below the waterline, to keep it quiet in the marina. A straight pipe pointing out from the transom will only be half under water and thus make too much noise.

This solution came to my mind.

Side view

Isometric view

Top view

As it can be seen, the pipes goes both down and out to the sides. Down, because I want to keep it quiet. To the sides, so the sterndrive doesn't collide with them, if turned to one side and fully tilted up.

I'm not going to cut the old tips off and weld these on. Insted I will cut the old tips off, and mount these new tips with an exhaust hose from an OMC Cobra I had laying around.

This exhaust hose will then be outside the boat. It's made of rubber, quite hard, so I don't think the tips will bounce around.

With this solution, the tips are not so deep in the water anymore, and in case there are vibrations, they should be absorbed in the rubberhose. I'm not completely sure about the length, but they are under the swimplatform, so it doesn't matter that they are long.

Any thoughts on this idea?

 

[emilsr]

Re: Vibration because of long exhaust pipes ?

I was wondering if you'd found a solution.

Why wouldn't these work?

Gibson Marine - Performance Marine Mufflers

 

[HorizonblueDK]

Re: Vibration because of long exhaust pipes ?

I was wondering if you'd found a solution.

Why wouldn't these work?

Gibson Marine - Performance Marine Mufflers

Ahh, those. Have actually seen them before. I think they will be submerged too much on my boat. On an ordinary thru-hull setup, the holes in the transom are one feet above the waterline or so. On my boat, the holes in the transom are drilled so the centerline is the waterline. So if I install a pair of these, the exhaust tips are quite deep again.

I think that the vibration in my boat is caused by long pipes that are too deep submerged. A long pipe will vibrate more than a short one. But the shape and angle off course also plays a role.