Apologies for the generalization and mention of an auto-motive typical statement in the marine sector (must have been thinking about my drive to work!! )
But, back to sizing. Exhaust flow is determined from the amount of HP the engine makes and the more HP, the more exhaust flow; therefore it will take
larger piping and/or less bends, shorter lengths etc., to meet back pressure requirements. Also, a very simple to understand concept (even though most installers seem to forget) is the “bend” equation part of planning an exhaust. Figure a smooth radius 90 degree bend is equal to about 6-10 FT of DRY piping and
15 FT of wet piping. Another good rule of thumb to remember is for every 50 PRODUCED horsepower, the engine makes about 100 CFM of exhaust gases, but this does not include the water and / or steam that becomes part of the mix when water is introduced.
That’s why dry piping can be smaller than wet piping, WATER and steam add to the total flow in a substantial manner; therefore once water is introduced, the piping
MUST BE LARGE
Also take into consideration the type of muffler,water lock and the overall length of the system, the amount of both dry and wet bends, the amount of water that you inject into the system (some of it, if not all of it), the angle of the discharge AFTER you inject water and how the system exits the vessel. All of these affect the total system back pressure. I am using Volvo engines as that is what most of my experience is with and I am using 3″ Hg or 41″ of water column as the maximum back pressure limit:
Bottom line engine manufacturers usually know "how to" and what size there exhaust systems need to be as in there supplied exhaust outlet size if you want a long running compliant generator (and with in warranty requirements)follow that size is my opinion
Cheers Steve, NZ Chief Eng 1