Exhaust noise SX drive

[huskaduska]

First post, please excuse me if this is already answered in another thread.

I very recently aquired a 18ft bowrider with a VP 3.0GL and SX-MLT drive and the exhaust noise is annoying. As soon as we're going any quicker than crawl speel so that the transom (partly) clears the water it sounds like the exhaust is coming straight out from the back of the boat. It kind of resonates and gets amplified in the wake "hole" just behind the boat. I noticed almost no exhaust comes through the prop and I checked and found the bellow good by tilting up and jumping in the water (engine off). The drive was inspected/replaced last season so I believe it's good. What else?

I started investigating drawings and videos to look for other ways the exhaust exits and as far as I can tell there is a passage through the anode on the transom shield?


So if this anode is worn and has a larger hole now could that be the source of the noise? I also noticed another type of anode with a spacer and plate which seems to restrict the exhaust. What's the difference between those anodes -- zinc vs. aluminum? And then one of the types needs the spacer nuts and the plate?



I guess this exhaust port (if it really is a port) should not be blocked off (so that it can drain).

 

[kenny nunez]

The reason for the exhaust relief was to help larger boats to get on plane. If you do restrict the relief it may also affect your planing speed. Just try it out.

 

[huskaduska]

The reason for the exhaust relief was to help larger boats to get on plane.

Thanks for that info, so the idea is to not choke the engine to gain some acceleration? This is a small boat and getting on plane shouldn't be an issue. But does it still need some relief for drainage or to reduce exhaust pressure or is that not strictly necessary? I thought that's what the perforated bellow is for. However, I'm almost certain the bellow is not perforated on this boat. Could I break something without relief? All the exhaust will then go through the prop which would of course silence the boat significantly.

So maybe I need to choose some combination of parts: either open relief and non-perforated bellow or restricted relief and perforated bellow. Hard to tell what Volvo intended with that (apparently) optional anode+plate.

 

[kenny nunez]

The perforated bellows might be the best for noise reduction with restrictor under the zink plate.

 

[huskaduska]

According to VP Sweden the block off plate is for sound regulation:





I also found a reference to another kit which blocks off the rectangular ports on the side of the drive.


With both these options installed the drive should be a lot quieter.

 

[Scott Danforth]

you know on a volvo the exhaust comes out the anti vent plate and is above water as soon as you are on plane.

as apposed to say a Mercruiser Alpha 1 drive where the exhaust goes thru the prop.

my guess is that with the idle relief port blocked, it will be slower to plane, however you will still have the same exhaust noise.

 

[huskaduska]

I just spoke with a repair shop and realized just like you say that this drive does not even have prop exhaust...

 

[huskaduska]

Well this is confusing. Judging by this it looks like it does have through prop?

 

[Scott Danforth]

Well this is confusing. Judging by this it looks like it does have through prop?

you are confusing OMC Cobra and Volvo SX

if you have a volvo SX you have exhaust that exits from the anti-ventilation plate, you do not have and OMC Cobra that exits from the prop

 

[ripjmk]

I believe the SX-MLT drive is functionally similar to the SX-M drive that I have, and that drive exhausts through the prop. Although the kit the OP refers to indicates it is for diesel engines only.

 

[huskaduska]

I believe the SX-MLT drive is functionally similar to the SX-M drive that I have, and that drive exhausts through the prop. Although the kit the OP refers to indicates it is for diesel engines only.

Thanks, I will try to look for prop exhaust later today when I'm taking it out. According to the accessories catalog where this image is snipped from, it's for petrol engines.