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Your existing Mercury/Mariner 115 weighs 356 lb. Mercury's four stroke 90, 115 and 115 ProXS all weigh 3 lbs more, at 359 lbs. So at least a Mercury 4 stroke has no weight consequences. Nor would you need to move battery to the console, something I would never do on my 115HP powered Montauk.

Since your boat is already Mercury powered, you should seriously consider staying with Merc and greatly reducing installation costs, as everything you have is compatible and can be re-used. Additionally, the Mercs weigh less than Yamahas and have bigger displacement, and probably are faster per HP.

There is an Instagram video showing a Whaler Newport powered with a Merc 115 ProXS literally blowing the doors off a Montauk powered with a 115 Yamaha. If it were me, I would go with the ProXS. Great cowling graphics also, with silver gear case!