Modifying Exhaust to eradicate fumes

[SC500]

I want to minimise the exhaust fumes exposure to the cockpit and bathing platform areas when idling against a pontoon or pottering along at low speed.
I have a princess 430 fitted with Volvo Penta TAMD63P engines. The exhaust exits either side at the stern just above water level, the tube being about 150mm diameter. I am considering fitting stainless steel 90 degree elbows to the exhausts outlets to turn the exhaust gas down and below sea water level, basically an underwater exhaust outlet conversion. I realise this will increase exhaust back pressure but am unsure whether side issues may develop. Is there a danger the engine can draw in water under certain circumstances?
Has anyone already carried out a modification like this with successful results? Will be pleased to hear of the pro’s and con’s if known.
Thanks

 

[Montemar]

Surely it will bubble up so you will still smell it.

 

[Spi D]

I have the same setup. Exhausts just above water but when at speed they're under the surface. Makes no difference to the fumes, though.

 

[SC500]

I believe all new boats now have underwater exhaust outlets, I guess to reduce emmisions by minimising gas to atmosphere. Unsure if systems are direct or whether there are on board valves or electronics built in to control the systems. At speed exhaust is not an issue, the 63P's seem very clean, I want to reduce the gas when river cruising at 6 knots or when mooring.

 

[Bandit]

A lot of the Fairlines and Storebro Bruks ( Royal Cruisers) has fibreglass swan necks exiting underwater which is good for fumes and noise.

It does not eradicate the problem but reduces it.

I would not have thought backpressure is an issue but you can measure it with the right gauges and fittings.

As far as siphoning back into the boat and back flooding the engine there should be a hole at the top of the swan neck to act as a siphon break.

Go to a couple of boatyards and look at 1980's - 90s Fairline 36 turbos, 43 etc.

If you make it out of stainless make sure you bond it to an anode.

 

[SC500]

Thanks guys, your comments and advice are appreciated. I will check other boats out as suggested and hopefully carry out a modification when out of the water in the spring.