Will a 115 hp work for a boat rated at 85hp?

First, check regulations in your area- there are a couple of states where exceeding the reccomended hp is actually illegal (Florida is one).

Weight should not be an issue, most outboard manufacturers use hp groupings; ie an 85 hp Mercury 2 stroke outboard is identical to a 115 hp, the increase in hp is achieved by changing timing and fuel delivery. Even if your 115 hp is a bit heavier than an 85, you can simply compensate by carrying less fuel and cargo.

Another factor to consider is the USCG rating process of recreational boats. They still use a formula that was developed over 50 years ago, and is hokey at best. The formula builds in a safety redundancy factor of 25-30% , so even your increase in hp (and maybe weight) would still be safe. As to the actual increase of power: If you can achieve the same performance results at a lower rpm with a larger outboard, you will end up with a huge savings in fuel used. And remember, there is no law that says you have to run wide open throttle

I say get on with your project, and happy boating.

Legally you can not alter the data plate or exceed the HP rating of the boat. Now if you could find some 85 HP decals, you would be all set.

If you exceed the maximum horsepower you're placing you and your family at risk. And if your involved in an accident your insurance may not cover because of the engine. A boat's transom is designed for a maximum thrust and if you exceed that the transom could fail.

NOT a good idea. Your insurance wont allow it.