Initially OMC mounted the Ring so as to force some water into the vortex behind the prop, then in the 80s or 90s the Ring was flipped around on some props to make the low pressure area behind the prop larger. I don't think it makes any difference, as some props like OMC/BRP RAKER just have a straight tube. We lost the ring on our 35hp, and I can't recall any difference in performance or the props grip on the water. Perhaps if the exhaust tube was quite short and didn't extend much if any distance behind the blade root, it might allow exhaust to get into the blades, then it might be important
