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Old 04-21-2016, 12:07 PM   #5
Keysdisease's Avatar
City: South Florida
Country: USA
Join Date: Sep 2014
Posts: 707
Underwater exhaust is common on larger boats and yachts. There is always a slow speed bypass as described in previous posts because without the bypass relief backpressure would become excessive and could damage engine.

Outboard motors with thru-hub exhaust have bypasses too on the back of the leg below the cowling as an example.

I have seen on smaller boats (approx. 40 ft) an exhaust pipe extending just below the hull bottom and turning aft 90* as in pic 1 and 3 below

What this does and what larger boats do with a cowl (pic 2) is create low pressure at speed that sucks exhaust gasses out the bottom. This creates kind of an automatic switch from the low speed bypass to the underwater exhaust path as the vessel increases speed as exhaust gasses will travel the path of least resistance.

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