Running without exhaust bellow - problem?

[HighLineCBR]

I just completed my SEI116 cobra conversion. Essentially converted my 1989 Cobra drive to an aftermarket Alpha One drive.

conversion was easy to do, and a great investment IMO. So far i have 15-20 hours on the new drive and the boat runns better than ever. Happy customer here!

While installing the new drive the SEI instructions state that it is not necessary to install the exhaust bellow back on the drive. Or that i can modify my old OMC exhaust bellow to fit the Alpha drive, but that it is not necessary.

So what i did is cut off about 3 inches from the bellow and attached it only to the transom side. It is NOT attached to the drive. When the drive is all the way down it almost touches the bellow, but there is about a half or quarter inch space between the bellow and drive.

IS it safe to run the boat this way? I mean the exhaust bellow already has slots in it so there is always water there. Its not a question of whether the water will sink my boat, because it will not leak into the boat. But rather if the boat is running will the exhaust fumes and pressure cause any harm if not connected to the drive?

any thoughts would be appreciated.

 

[Brentathon]

Re: Running without exhaust bellow - problem?

I would doubt it.
There's even a service bulletin for Merc that uses an exhaust "tube" in lieu of the bellows, for driveshaft extension models, to prevent the bellows from expanding too much and pushing the driveshaft belows into the shaft.
The "tube is only attached to the gimbal housing.

 

[Philster]

Re: Running without exhaust bellow - problem?

Non-issue. If you are on plane and the exhaust makes some noise that bothers you, then you might want to run it out the drive.

It's got nothing to do with performance or safety. Running hard and up on plane, you might hear it. You might not. It might not bother you.

Running it out the drive ensures it's quiet. That is all.

 

[jtmarten]

Re: Running without exhaust bellow - problem?

I'd replace it with a tube. As said previously, the bellows could wear the drive bellows.

 

[HighLineCBR]

Re: Running without exhaust bellow - problem?

sound doesnt bother me at all. I acutally like the louder rumble.

I was just worried about speeding up the corosion process on the drive bellow if anything.

 

[a70eliminator]

Re: Running without exhaust bellow - problem?

I thought without bellows the exhasut gasses would come out in front of the prop rather than behind it causing cavitation, but evidently not from what you guys are saying.

 

[jtmarten]

Re: Running without exhaust bellow - problem?

I thought without bellows the exhasut gasses would come out in front of the prop rather than behind it causing cavitation, but evidently not from what you guys are saying.

It can, but the exhaust tube will semi-seal the hub when the drive is trimmed down, reducing the cavitation out of the hole.

 

[HighLineCBR]

Re: Running without exhaust bellow - problem?

now that I think of it. when at WOT trying to get on plane I sometimes feel as if the prop is slipping. I gues this could be the cavitation you're talking about caused by the exhaust bellow not being attached to the drive. hmm...

 

[Brentathon]

Re: Running without exhaust bellow - problem?

Cavitation is not the same as ventilation. Cavitation is when the water boils, turning into water vapor bubbles, from severe low-pressure condition.
Ventilation, is when either air or exhaust gases get into the prop blade area.
There is a natural low-pressure area behind the prop's hub, whick will tend to draw the exhaust down through the drive, even if the tube is not clamped/sealed to the drive. Exhaust gasses leaking between tube and drive are nowhere near the blade area, and should not cause ventilation.
The only difference should be noise. I had a tube for some time on my Alpha, and only difference was noise, so I eventually went back to bellows.

 

[Brentathon]

Re: Running without exhaust bellow - problem?

You can also see the low-pressure effect on sterndrives, from when you're idling in neutral, exhaust comes out the 2 holes on the gimbal housing......but when you put it in forward gear, you'll see the exhaust immediately starts coming out the prop's hub, from the low pressure effect when the prop is spinning.