Your boat was built with an engine with stbd side exhaust. Your YM engine has port side exhaust. At shutdown, the
water lift
muffler is essentially a reservoir that collects the water coming back from the exhaust hose, the exhaust elbow, and also the elevated siphon break. When your boat is sailing on stbd tack and heels 70 degrees in a big gust, where does the water go?
That’s why we want to keep the whole exhaust system parallel with the ships centerline — within reason. So, I’d like to see at least your
muffler moved to port side or even close to the centerline and most of rear hose too. It doesn’t need to be a solid mounting BTW. It could float on lines or bungees supported largely by the hose.
The anti-siphon is an easy fix for you. Getting geometry of exhaust system right is the tougher problem.
No way you need or want 2” hose for this engine. The Yanmar U mixer/riser is used on many Yanmar engines over the years, even a 34hp (3HM35) model, and while 2” hose is used for bigger engines, using the internal threads of the mixer for a SS hose tail is best for smaller engines. Those threads are 1.25” BSPT. 2” hose is also much stiffer and awkward and it will contain more water to flow back at shutdown due to slower velocity. My friend’s
Catalina 320 has a 29hp engine and uses a quite long 40mm (1-5/8”) hose. Greater back pressure is created by the sharp 90 degree elbow in their custom muffler than from this hose.
I do worry some about extending height of your mixing elbow higher than needed as these engines do shake. If extending more than 5” you might consider adding a brace. My 2GM20F has a cast iron manifold to take the stress and I hope the YM does too.