30 mph from a '99 115 merc.

[gobeernet]

There is no mention of how much furniture and other weight you have on your boat. When our boat was new, we could get 32-33 mph with our 125 HP. Now that it is a little older we only get 27 MPH. But also note we have a lot of furniture on our boat.

When you add the center log you are also adding more weight so your gains might not be as great as you think they are. With that said, I would still add the third log. The ride is so much better and safer. You may not get all of your money back when selling, but I think you will get most of it back.

You never did mention how big you 15 pitch prop is.

Boat is currently a fishing layout. L shaped bench across the back and up left side + Helm + 4 fishing chairs. All the seats are shifted up a bit to leave a fishing are in the rear.
I will probably have 2-4 people most times. Maybe 6 occasionally.

Prop is a Ballistic 13 5/8" x 15.

[BobG]

The most I've seen out of mine, is 24 MPH GPS.

But that's at 5,000 feet elevation, where (hmm, 3% loss per 1,000 feet of elevation = 0.97 ^ 5 = 0.858 * 115 = 98.75 HP) I'm not seeing 115 HP at all.