I apologize for the mix up, but as you likely know, prop pitch gauges are calibrated in degrees, and actual measured blade pitch changes across the blade from root to tip, with greater pitch at the root, and less at the tip. The pitch stamped into the blade is the theoretical distance the prop would travel in one revolution at 100% efficiency.
If your boat was reaching near hull speed before, it cant go any faster. It just wont. Adding 2 inches additional pitch is the equivalent of asking the boat to go 13% faster.
But your losing an inch in diameter so one of two things will occur. Either the smaller prop will slip enough (loss of prop efficiency), or it wont, in which case the engine will be incapable of turning it at full engine RPM, overloading the engine.
But as we dont know how the boat performed previously, or how well the two propellers equate to one another in quality/efficiency, its hard to say how the new prop will perform without physical testing.
Further, Marine Diesels, unlike marine gas engines, are governed. The max engine speed is locked in and even without any prop at all, diesels wont exceed the maximum no load speed. From that point, the only thing that would pull the speed down, with the throttle fully forward, would be too much prop. The goal then, is to find a prop that allows the engine to reach as close to maximum speed with the throttle fully advanced, as possible. Anything that pulls the engine down is hard on the engine. Less prop wont hurt the engine unless you go way under, you just wont get the full rated HP available.
If I were looking to increase low speed thrust, I would go with a larger prop and less pitch, not smaller prop with more pitch. It would sacrifice maximum forward speed as a trade off, but the prop would be much more efficient, and the engine would run with less load.
However, larger props run into clearance and sound/hull resonance issues. This is where the three and four blade prop comes into play. A really good prop, like a three blade Campbell, could actually allow you to decrease pitch further, possibly to 13 or 14, yet still maintain current forward speed (through greater propeller efficiency) while increasing low speed thrust.