TOPIC: Horsepower vs RPM opinions

This might be an easy question for some of the techie gurus.
I'm now running a 15', 1976 StarCraft fiberglass with an 85hp 1976 Johnson. Speedo WOT reads 36/37 mph, did not get a chance to check GPS.
The prop is 21 pitch, don't know the diameter but it fits close to the trim tab so I'm guessing it's maxed out.
I have a 1977 140hp Evinrude sitting on a stand that would be an easy plug and play swap since all electrical and mechanical are exactly the same.
Question...is the lower unit gear ratio different between those two motors? Would there be any speed increase using the same prop with the 140.
Tks.

Using the same prop on the 140, that you say is maxed out on pitch, you won't go any faster because the max RPMs will be about the same. However, you'll get up out of the hole a lot faster.

Jan my question was about the lower unit gear ratios. RPM on the tak-o-meter measure engine speed. I was wondering about prop rotation speed. Researching has come up with an answer. The 85hp has a LU ratio of 13:26. Prop spins twice as fast as the motor. The 140hp even though beerier....did I say beerier?? I meant beefier, has the same 13:26 ratio so yes, changing the motor to the 140 would not be a speed gain. Probably would be helpful pulling tubers or skiers or stumps, maybe trolling for the really big ones.

Cal, you got the RPM backwards. Spins at 1/2 engine RPM, not twice.
Yes both run 13:26 ratio and rated power is at 5000 RPM. W.O.T. RPM is 5500.
If your running an old aluminum non cupped prop and go to a newer cupped prop it will be like adding 2" of pitch.

I stand corrected, thanks.
The 140 sure looks a lot cooler though.

The prop spinning twice as fast as the engine didn't sound right to me...100% overdrive? Then again, I know next to nothing about eggbeaters. What I do know is that if you make another mistake like that again, Cal, you're off the forum!

Any idea what the RPMs are? My uncle had a 1968 15' Starcraft runabout and 85 Ev, would run low to mid 40mph all day. Wondering if the 21 may be too much pitch for the 85. I would almost wager a 19p would hit 40+ . That 140 with a 21 should run 50+ on a light 15 due to torque increase over the 85. My old Checkmate 15 with a 1972 Ev 125 ran 58 at 5600 RPMs on top end with a 21 aluminum, probably a bit lighter, and a more speed oriented hull design, but that motor had the torque to spin the prop at top RPM.

Jan, please no. I'll take 50 lashes with a wet noodle instead.
Kerry. good point! One of my old Tomahawks with a 135 would smoke some of the smaller bass boats. If things get slow this summer I can pull a swap. Probably would take less than 3 hours. Would take more time mounting the TNT relay box than putting on the motor.

Any chance of finding or borrowing a 19p to try until/if you decide to swap? Just want to see you get the best from the 85.