My understanding is that the Viper is a bow lifting prop. Lots of dealers seem to install them because they will probably give the most speed, but I suspect they're not very good for trying to hang on plane at minimum speed to keep from going airborne when you're offshore in a nasty chop! Although your motor is relatively light for a modern clean-tech engine, it's still heavier than what the boat was designed for, so you probably need a stern lifting prop. The BRP Rebel is a good stern lifting prop with lot's of blade area, much like the Merc Mirage, but the 15" pitch model is about 15.5" in diameter, so too large for your motor. My dealer said that the Rebel is primarily used on the 3.3/3.4L big block V-6 motors. I'd suggest contacting Ken at
PropGods with info on your boat and the results of the props you've tried and see what he recommends. BRP's range of props is fairly limited, so Merc, Solas (who I think makes the BRP props), PowerTec or Michigan may have a better prop for your boat. If you're looking at one with a 2-piece hub, the BRP, Michigan and PowerTec designs appear to be much more robust than the Merc design that has a reputation for failures of the plastic hub.