Question About Outdrives / Improper Trimming...

[ApS]

I've noticed that some boats produce a loud exhaust sound at certain times—and aware that Mercury has thru-hub exhausts.

(Thank you, Mercury).

While a basic "primer" appears here http://blog.savvyboater.com/thru-hub...why-do-i-care/, it's a rare outboard engine that will produce a noisy exhaust sound.

Interesting that "Forward-Drive" doesn't expose a skier to CO fumes.

Finally, is it improper-trim that causes that monotonous, noisy, "blat" among inboards? Or is it just limited to some outdrives—like Volvo?



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[TiltonBB]

Most I/O exhausts are through the center hub of the propellers or through pipes that route it through the hull. The two primary makers of I/O power are Mercury and Volvo. Regardless of brand, the quiet ones are the ones that have it through the props.

Many I/O boats come with "Captains Call" or have it as an option. This allows the Captain to switch between the two. Switchable exhaust was illegal in NH until a few years ago. You can now legally have exhaust that can be switched but it cannot exceed the decibel limit in either mode.

I have had several boats with Volvo drives and all of them used the propeller hub for exhaust and were very quiet.

[ursa minor]

The Volvo drives definitely have a different tone than a similar boat with a Mercruiser IO. I believe that on the Volvo drive some exhaust exits through the rear of the cavitation plate rather than completely through the propeller hub. On plane, it would be at or slightly above the water surface which may explain the difference in the sound.

[8gv]

Could some of the sound differences be attributed to an exhaust bellows that has come loose from the transom assembly?

[ursa minor]

Originally Posted by 8gv

Could some of the sound differences be attributed to an exhaust bellows that has come loose from the transom assembly?

I think you'd notice a full bilge pretty quickly.

Several of our neighbors have Volvo drives, they definitely sound different.

[8gv]

I don't think so.

When my exhaust bellows came loose from the transom assembly I asked a dealer how urgent an issue it was. He stated that I could wait until the next service, reattach it then, never reattach it or install an "exhaust tube".

Here is a discussion on the matter over at iBoats:

http://forums.iboats.com/forum/engin...ube-vs-bellows

[LIforrelaxin]

Many things can contribute to the sound. First the Bellows can have holes that are more prominent at certain trim levels... Or hey the bellows could be completely gone. Different engines sound differently... and then you have the modified from stock engines that pick up there own sounds... Dependent on the make of boat, and style of the interior some boats have more environmental sounds that aren't even coming from the exhaust.

Bottom line the noise from a boat have to be under certain limits by law. As long as those limits aren't exceeded, there is no problem... Sometimes those limits are run right up too......other times they are quite reasonable below.....

[Dave R]

Originally Posted by ursa minor

The Volvo drives definitely have a different tone than a similar boat with a Mercruiser IO. I believe that on the Volvo drive some exhaust exits through the rear of the cavitation plate rather than completely through the propeller hub. On plane, it would be at or slightly above the water surface which may explain the difference in the sound.

This is exactly what causes the noise APS described.

Some Mercruiser drive and engine combinations require open tubes instead of exhaust bellows, but they are not as loud as standard VP drives, IMO. I'm guessing it's because they blow exhaust directly at the back of the upper gear case which probably muffles the noise quite a bit.

[ursa minor]

Originally Posted by 8gv

I don't think so.

When my exhaust bellows came loose from the transom assembly I asked a dealer how urgent an issue it was. He stated that I could wait until the next service, reattach it then, never reattach it or install an "exhaust tube".

Here is a discussion on the matter over at iBoats:

http://forums.iboats.com/forum/engin...ube-vs-bellows

And we learned something new today! Thanks 8gv. I had been told that the exhaust bellows had a flood / sinking potential if it failed, I'll look into this some more. Good to know the tube is a DIY alternative.

[ApS]

Originally Posted by ursa minor

The Volvo drives definitely have a different tone than a similar boat with a Mercruiser IO. I believe that on the Volvo drive some exhaust exits through the rear of the cavitation plate rather than completely through the propeller hub. On plane, it would be at or slightly above the water surface which may explain the difference in the sound.

It's not the difference in the sound: it's a "loud wailing howl". From behind, the boats making this caterwauling can be heard for miles! Is it the helmsman's inappropriate trimming at fault? Why can't this design be annoying to passengers and crew?

After finding it's the Volvo that's responsible for this exhaust noise, I checked other boating websites, and found that the "loud wailing howl" occurs at moderate speeds—particularly at 3000-RPM. How could this bawling noise have been missed for the "Trojan Horse" bill—now a NH boating law? Was it the condition that the exhaust be tested at a dock?

Any citations been written for the Volvo outdrive exhaust noise?

I don't see how the NHMP could miss the opportunity to "write" for this infraction.



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[VitaBene]

Originally Posted by ApS

It's not the difference in the sound: it's a "loud wailing howl". From behind, the boats making this caterwauling can be heard for miles! Is it the helmsman's inappropriate trimming at fault? Why can't this design be annoying to passengers and crew?

After finding it's the Volvo that's responsible for this exhaust noise, I checked other boating websites, and found that the "loud wailing howl" occurs at moderate speeds—particularly at 3000-RPM. How could this bawling noise have been missed for the "Trojan Horse" bill—now a NH boating law? Was it the condition that the exhaust be tested at a dock?

Any citations been written for the Volvo outdrive exhaust noise?

I don't see how the NHMP could miss the opportunity to "write" for this infraction.



.

You started the thread with a legitimate question, but can't help yourself. This has nothing to do with switchable exhaust.

[ApS]

Originally Posted by VitaBene

You started the thread with a legitimate question, but can't help yourself. This has nothing to do with switchable exhaust.

The geniuses who pushed for the RSA for the benefit of those who revel in loud exhaust noise first required that boats' exhaust noise be measured at a dock.

While "fly-by" noise-metering is still on the books, unsafe measuring speeds naturally evolved into an increase in excessive exhaust-pipe noise—lakeside.

Perhaps it was a coincidence that an NHMP presence appeared shortly after my last post. He shut off his engines and drifted within a major passageway. After ten minutes, he left. (But that's the best way to determine a "moving" noise infraction). And, no, he wasn't having lunch!

A few days prior, an NHMP had stopped a nearby renter/neighbor's boat—just why—is unknown. Noise wasn't a factor that I had noticed of that visitor. After about 30 minutes, the two boats parted: the renter's boat never appeared on the water again. So, it appears, the NHMP is still watching for infractions—but why not more frequent "shut-off-and-drift" surveilling?

As to "legitimate question", that question still doesn't have an answer why some pedestrian boats (including the occasional Cobalt) are so loud unless "faulty trim" is the answer.

[ursa minor]

It's not faulty trim or anything else (switchable exhaust). It's a design difference between the way Volvo and Mercruiser deal with disposing of the engine exhaust behind the boat, nothing more, nothing less.

Volvo has used the cavitation plate exhaust method in one way or another since they invented the stern drive in the late 1950's / early 1960's, it's nothing new. More likely what you're noticing is that there are more boats equipped with Volvo drives now than was the case 20 years ago. I get that the drone can carry especially on calm nights, that doesn't mean it exceeds any noise limits or that it's any louder than an older direct drive inboard for example. I'm pretty sure Volvo designs their products to meet whatever standards apply in this case.

[MAXUM]

The exhaust bellows at least on Merc stern drives actually contain water to create backpressure for the engine. These at least on GEN II + do not even need the bellows and no the boat will not sink if they are removed. If the bellows that contain the shift cables and speedo tube break that is a different story.

I had a hell of a time with my exhaust bellow on my Alpha one, kept popping off. Talked to the guys at the marina and put on a very cool fix for that. The bellows tube was removed and in it's place was a boot like rubber piece that attaches to the transom side, the end is open. When the drive is lowered into an operating position the part of the stern drive where the other side of the bellows would attach to slips inside the boot and seals it up allowing the exhaust to make it's way out the hub. Briillant design and works great. It does not operate any louder than having the whole thing sealed up.

See the pic below