Presume it ill-Advised to plug up the above water exhaust port so that all exhaust passes through the prop? (Wifey/Admiral is super sensitive to the exhaust smell which is drawn into the cockpit from the hull speed turbulence or if apparent wind is coming from stern... so ideally if it can be driven further away underwater that would be ideal for our 1200-1800rpm hull cruising speeds... which is 85% of use) 2009 75hp, 200 hr service performed 6 months ago and tuned for the xd100
I have run a 75hp a lot of hours at those speeds also. A couple of things you could try. XD100 oil has a particular odor, try switching your engine setting to TCW3 and run XD50 as long as you are not running in very cold weather conditions. Also the amount of oil burned is dependent mostly on engine load. 1800 rpm could be a high load on your engine so you may benefit from switching to a prop with 2" less pitch. It may take a bit higher rpm to cruise at the same speed but with less load you will burn less oil and possibly less fuel. Optimum wide open throttle on that engine is 5200 rpm, I propped mine to turn 5400 to gain better low speed power and handling.
I just saw your earlier post on prop selection. I ran a Solas pontoon boat prop in a 9 pitch. Similar to a Hydrus. A pontoon type alloy prop has big ears and very little cupping which brings the rpm up as compared to a stainless cupped prop.