I am installing a Yanmar 4JH3-TE, 75-hp engine. Would a water-lift muffler increase back pressure in the exhaust system?
Sue & Larry respond:
During the installation of our Yanmar 4JH3-THE, We tried to keep our exhaust system as direct and straightforward as possible to minimize excessive back pressure. However, what's ideal and what's practical aren't always the same. We ran our proposed design for the exhaust system by two separate Yanmar mechanics prior to the installation and got thumbs up from both of them.
Here's how we ran our system: Our exhaust manifold was connected directly to a Vetus LP75 lift muffler via two Trident rubber exhaust elbows (the lift muffler sits directly beside the manifold). From the manifold, the three-inch exhaust hose rises approximately four feet and moves aft about four feet until it reaches a Vetus LT75 gooseneck fitting mounted to the aft bulkhead in our port lazarette locker (the gooseneck is the highest point in our exhaust run, being approximately three inches below our deck height). From the gooseneck, the exhaust run continues for another 12 feet at a slight downhill angle until it meets the transom. At our transom fitting, we included a rubber flapper to discourage water from finding its way up the hose.
All in all, we used 21 feet of exhaust hose, but I feel that only the run from the lift muffler to the gooseneck is of great consequence from a back-pressure standpoint. By the way, I really like the addition of the Vetus gooseneck fitting. We could have saved a little money by just looping the hose up high, then turning it aft, but the LT75 Vetus incorporates an expanded chamber (similar to a catch basin) below the highest point in the fitting. If seawater were to find a way up the exhaust hose from the transom, there would first have to be enough volume of water to fill this expanded chamber area before the water would run down into the lift muffler.
You could eliminate the lift muffler by running an insulated dry exhaust riser, but I'm not convinced that is necessary. Our last boat, Safari, had a Perkins M 80T (turbo), and we ran a Vetus lift muffler for 3,000 hours without incident and without the engine ever missing a beat. On Serengeti, we've got less than 10 hours so far, but I can tell you that the engine runs smoothly, accelerates cleanly, and is very quiet. I guess time will tell about its reliability, but we've got a good feeling. Good luck to you.