Loud Exhaust - GT-40

[SanJoseSteve]

I just bought a 1999 Air Nautique, DD, GT 40 (310HP). I need some advice.

It is very loud in the back. Only in the back seat and only at speeds between 12 MPH and 30 MPH. I have been in a lot of ski boats and never heard anything this loud. When I say loud I mean that after 10 minutes your ears are ringing.

It is a kind of sonic bass exhaust noise. Definitely from the back and not from or around the engine cover.

I had the boat checked at the dealer. They said nothing was wrong.

The boat only has 68 hours on it, so it is practically new. They checked the impeller pump. It is OK. The boat ran at a consistent 160 degrees. Plenty of power, tops out at 43.4 MPH. They checked for overheating. There was no indication of the engine having ever been overheated or the muffler. I was told by Sam @ Centek Marine Muffler that the 99 Air Naut. was one of the quieter boats for that year. He also told me that the boat already has a centek muffler as OEM equipment. Any suggestions? Do you know of any muffler system that will fix the problem? Have you ever heard about this problem before?

[skinautique]

RE: Loud Exhaust - GT-40

there was a rubber type of matting that layed between the hull and the muffler. see if that is still there. it was glued to the hull. Also, does this sound ever go away at those speeds?

[SGY]

RE: Loud Exhaust - GT-40

I remember a resonance at low speeds that was quite loud on my 99 SNOB. But it would go away at around 18-20mph. Did your dealer take the boat out on the water?

[SkiTexas]

RE: Loud Exhaust - GT-40

This sounds very familiar to the issue I had with my 1999 AN. It turns out the "hull hugger" muffler was missing a black dense foam pad. There is suppose to be one between the muffler and the hull. Mine was resting directly against the hull creating resonance at certain RPMs. Additionally, the muffler was missing a baffle arrangement inside. I contacted the company printed on a label attached to my muffler, Centek, I believe. They promptly mailed me a replacement muffler, no questions asked. The company did say they find it highly unusual that mine was missing the entire baffle. I mailed the orginal muffler back to them for inspection. I never did hear back from them.

One other thing I found on mine, the large 4" rubber "tail" pipe was slightly pinched, half way between the muffler and the exhaust flange at the transom. I didn't see this until I detached the fuel tank and slid it forward. The problem was the rudder tail pipe section was a bit to long, causing the weakest point to buckle and bend it.

Another thought is prop-wash, bad cutlass bearing (a 1999 with very low hours), shaft alignment, etc. Boats are like a big guitare, they will resonate if things aren't just right IMO.

Hope this helps,
Scott