Have a boat I acquired a couple of years ago with 30 year old Mercury 150hp 2-strokes knowing that I would probably have to repower the boat within about five years. When putting new motors on an older boat, of course weight is a big concern - modern 150hp four strokes weigh much more than the older 150hp 2 strokes do, and would exceed the max weight on the boat's transom. So looking at smaller motors, like 115hp (much less costly too!!). This is a deep-vee hull center console, and I rarely run above 35mph on any boat even when it's able to go much faster, so maximum horsepower isn't a big concern to me.

I guess my question is "how much of a power difference will I notice switching from OLD 150hp 2-strokes to brand-new 115hp 4-strokes?"

I believe the 90 would be best for you

WHAT boat?
Start there.
Where are you using it?
How much load do you carry in passengers?
I don't like the weight of the four stroke on the shallow sport, but not going to give up the holeshot, so propped right, it'll sling you off the back, which when you're in the gunk is priority #1.

23' Ranger center console deep-vee twin engine. Typically it's just me and my daughter, but sometimes up to four people. Use is open water on Lewisville, Texoma & Ray Roberts, with some offshore use anticipated. Total weight of the boat & motors as currently equipped is about 3,100#

I would think a 23' Ranger CC could handle a 150 Mercury 4 stroke. I run the 150 Mercury 4 stroke on my 19' Mako CC. I would think a 90 HP would be under powered. Any idea on what does the Ranger's hull weighs? Looking at Boast Trader, every 23' Ranger CC I see is running a 250 HP motor. Seems to me that a 150 four stroke should work. You will just be under powered.

You got plenty of boat to handle whatever you want to hang on it. NOT ONCE did anyone complain later about putting the rated motor on their boat. Especially if you're planning on running blue water in it, underpowered in six foot seas is absolutely zero fun.

My 2004 18' Lund has a rating for 150. It went from 2 stroke to 4 stroke with no problem.

Well, I feel stupid, but I think that I found my answer in an old sales brochure that I found online. The base engine for this boat back when they were being sold was a single 225hp Merc, with a max rating of 400hp for the boat. So power from twin 115's should probably be adequate, but after looking at some engine specs, I'd feel better about a pair of 140's on the back for the same weight as the 115's.