http://us.yanmar.com/media/ext/uploaded/397ab248-e8da-4a50-af5e-8523552b8641/3JH5E_Brochure.pdf

http://www.yanmarhelp.com/i_exhaust.htm

" The minimum inside diameter at any point of the exhaust system must be the same as the outside diameter of the mixing elbow.
Yanmar recommends increasing the size of the hose by 25mm / 1 inch where it exits the waterlock or muffler. This size increase isn't necessary on low hp engines but is recommended on 88hp and over or any vessel with a long exhaust run, e.g. a mid mounted engine.
The exhaust gas backpressure measurement is more important on turbocharged engines and must be measured immediately after the turbocharger. Yanmar tolerances for exhaust gas backpressure are higher than other manufacturers so they should be religiously adhered to. Exceeding the backpressure results in a loss of horsepower and an increase in exhaust manifold and cylinder head temperature which can lead to premature engine failure."