This thread has no moral.
This thread has no end.
This thread only goes to show the folly of men.
This has been beat to death at this point and we are back to repeating posts. I can tell you are a bright, articulate guy and I do not detect any malice in your posts. In fact, I think you are a nice guy. You feel you got less than you paid for and that it isn't fair. And I agree. It's not fair that you bought a 6 hp engine to replace a 4hp and it pushes your boat slightly slower. In a perfect world that wouldn't happen. But we all know that this is not a perfect world and that we are not guaranteed a perfect result...At least not in outboards.
So, what are we left with? The engine is not defective. The engine develops 6 hp at it's rated RPM. All outboards are rated the same way. And, all outboards 6 hp and below in the U.S. are four-stroke. And, your screwed. It's no ones fault, but there you are. Due to your application you aren't going to get that perfect result. I don't want to wade back through this mess, but what are we talking here? Something around a .2 difference at hull speed if I remember correctly. Let it go. You got a free prop out of it. I offered you another free one to try. That's 10% of the cost of the engine there alone
Outboards are prop shaft rated due to Federal law. This was done back in the late 80's to standardize outboard hp ratings. At the time, U.S. produced outboards were crankshaft rated and the imports were all prop shaft rated. The Feds felt that it was too confusing to the public to try to determine how much power loss occurred from the transfer of generated hp at the crank to usable hp at the prop shaft so....No more useless crankshaft ratings. This brings up another point which is when someone begins to babble about the torque of their outboard. That's a dead giveaway that they don't have a clue as to what they are talking about as torque by itself is meaningless when determining what an outboard does in the water. I do think that all manufacturers should furnish static measurements of prop shaft hp produced at multiple set RPM's as that could be useful in diagnostic testing.