It is all a function of load. Load produces heat. And heat is what drives the turbo. C Lectric pretty much nailed it.
BUT....to answer your question....as it relates to MY boat. It happens about 1500RPM. You hear them starting to "sing". And then the props and engines start to dig in. It is a wonderful noise. I honestly believe that in most of these applications, the RPM will be similar.
All above is correct as I understand it.
Observing my boost gauges, full rpm in neutral no load - produces 0 boost. I start seeing boost pressure at about 1400 rpm and that coincides with the bow starting to rise as the boat is moving above displacement speed. At my cruising speed of 2400 rpm I get about 10 psi boost and 15 kts on full plane.
Boost goes up to about 12-15 psi as the boat is getting over the hump but settles down to 10 psi on plane.
"Observing my boost gauges, full rpm in neutral no load - produces 0 boost."
I would expect 1-2 pounds of boost at no load full rpm - please check your data sheet.
He's not reading a "data sheet". He has basically the same boat as I had with the same engines equipped with the same boost gauges. While I agree with you, I do see my needle lift ever so slightly off of the post, it is difficult to read when the gauge looks like this.