Re: 1999 Mariner 125hp 2 stroke autolube
ever see one of those top fuel drag cars with the huge rear (powered) tires?
the size isn't happenstance... they give direct value to 'overall gear ratio', and have greater effect than even the axle ratio.. if chosen too short, too much torque is capable of being applied and the tire can and will break loose upon acceleration... if chosen too tall, it puts too much load on the engine to obtain the optimum RPM band of the engine, and it's slow off the line because of that.
it's my speculation you are seeing something of an anomaly of conventional prop dynamics theory and application.. I'm suggesting you're seeing excessive slip because you're using the higher pitched prop, and the engines torque is 'breaking the bite free', allowing as much as a quarter rotation without resistance. Conventional wisdom would suggest being under-propped allows the prop to spin due to excessive RPM's, and to the point it breaks free.... the same can happen with an engine powerful enough but over-propped...
Imagine an inclined plane (as in a machine screw) without three breaks in it as a three bladed prop has, or four as a four bladed props have.. that screw is driving into water, and pulling the boat along with it (which is what a prop does, it pulls, it doesn't push).. If you used a machine screw as mentioned, you would achieve enough torque to easily tap into whatever- the limitation would be work accomplished (horse power)- if you were to attempt to drive that machine screw into wood, it would lose bite quickly, and slip excessively (as an under propped boat would)...
If you were to use threads consistent with a wood screw, you would translate torque into horse power as the inclined plane bit firmly into the wood- which would increase bite at the cost of torque, which will be sapped commensurate with the work load being accomplished- this would represent a properly propped boat..
If you were to use a nail with an inclined plane designed for holding force once seated, instead of using striking as the driving force, you can spin it stupid fast and use that plane like a screw- but it isn't designed for that, it's designed for holding force alone. If spun fast enough it will in fact drive into the wood, but it's going to slip a lot in the meantime...
in the 'props for boats' world, slipping happens at the absurd bottom of improper pitch selection, AND at the absurd top as well... If you were to use a 14p blade you would see ridiculous slipping.. If you were to attempt a 28p blade, it would slip too after giving up hope of actually biting in... that would determine your range of effective pitch would be between 17p-ish, and 24p-ish... and now if comes down to dialing it in...
based on the power and the load, you've already ruled out 23p+, and likely 16p-, which gives you between 17p and 22p to work with.. based on your experiments, 22p is struggling... the laser 20p in stainless was less effective for your load/power combo than the 21p aluminum- aluminum will behave like a pitch less stainless due to flex, but also, the geometry of that 21p aluminum is skewed due to the same flex as the point of deflection changes......
I still think you will be better suited for a 19p stainless prop for overall performance, or a similar geometry 20p aluminum... for the reasons you've determined advantageous to your goals, those being range and consumption, you'll be better served with a 18p Stainless or a 19p aluminum... you may discover the speed is actually just as great when every rotation matters in terms of bite.
