I would have a lot less problem if they had published the power curve that way. However they claim a power output and ship the engine with a standard prop. Then for various boats they give you a choice of about 5 other props. The engine can not meet its max rpm that produces its rated hp with the lowest pitch prop they sell for it.
unless the engine is in neutral.
It's great to claim a certain HP rating in the lab but if it can't meet it under real world conditions doing actual work then the manufacture needs to de-rate the engine regardless of what the federal government says. It would be easy enough and honest enough for the manufacture to state the federal government in bench lab tests rates this engine at 6 hp at the shaft at 6000 rpm. However under actual work conditions due to differences in how 4 stroke engines produce torque this engine has a comparative 2 stroke 4 hp rating.
How hard is that.. truth in advertising
It wouldn't be that hard for the manufacture to publish data on each engine with each prop against a water speed vs engine rpm chart so that people would know that with this engine with the lowest pitch prop if your hull speed is such and such this is how the engine would perform. All it would take is a circular test tank kinda like a lazy river that you can control and measure water flow in and with a set up like that they could even incorporate thrust test equipment into the engine mount they used. Each new model gets thrown on and tested from 0 knots on up with each prop sold for it by the manufacture and the data compiled. I think a world wide manufacture such as tohatsu could easily afford something like this. If I was a manufacture making engines I would require that kind of data just for my internal use as a matter of fact. To my mind the fact that you can not get these kind of performance curves/data is a deliberate choice on tohatsu's and other manufactures' parts. The cost of creating this kind of setup and using it is relatively cheap.
As to the bollard pull..... I agree it is not 100% valid as it doesn't correctly let the prop move through the water. However when done across different engines it does give an apples to apples comparison that i think is very relevant for displacement speed boats. We are talking a difference of 4 to 6.5 knots speed through the water on average from being tied to the dock. Yes this does have a measurable affect on engine rpm...around 500 rpm at 5.5 knots for this engine. This still lacks about 1500 rpm or so of meeting rated rpm.
Maybe if the boat was moving through the water at 20 knots it would make up that other 1500 rpm. How many boats or dingys of any size does this engine push to 20 knots of speed? The engine being able to meet rated rpm at 20 knots of speed is just a wild ass guess of mine but the thought counts. What speed through the water does this engine prop combination have to be moving to allow the engine to get to max rpm? It starts to feel like a chicken and the egg question.
If I had the equipment or could think of a way to arrange it I would rig some kind of scale or meter that would allow me to measure thrust at the engine mount with the engine on the boat in the water as it is moving. Unless someone can give me a simple way to test for this I am thinking it is beyond my capabilities.