The two values listed for propeller sizes are Diameter x Pitch. Diameter is simply the measurement from tip to tip, or double the distance from the center hub on props with an odd number of blades. The second number, pitch, refers to how aggressively the angle of the blade is. Generally, more aggressive angle or higher pitch results in higher torque at high throttle for speed. A lower angle of pitch yields increased low end torque for quick starts and towing.
Of the many factors involved, the most important is selecting a diameter x pitch propeller that keeps the engine turning at an acceptable RPM at wide open throttle. Check with the particular engine manufacturer to find the ideal RPM for your engine model at WOT. Then verify that the prop selection by flooring it and checking the engine tachometer. Sounds like fun, but please note, do not try this at home. Should the RPM be too high, increase the pitch - meaning the angle of the blades; if RPM is too low, decrease the pitch. Tuning the propeller in this way will help achieve the highest performance, safety and fuel economy. Since not all boats are created equal, slight differences will apply to different boats and applications. Also consider how the boat is used. Less pitch achieves better low end torque for towing skiers or for work boat applications versus higher pitch for speed and high performance.
Imagine a simple push of the button at the helm station that brings the bow down resulting in increased speed and fuel efficiency. Trim tabs are amazingly effective, and empower the boat operator to bring a boat on plane at slower speeds or when a vessel is laden with extra passengers. By lowering the bow, visibility increases, pounding over waves diminishes and and wake lessens as the boat no longer plows through water. The principle is simple. Piston driven flaps located at the bottom of the transom deflect water. Tuning them underway accelerates how quickly a boat planes without adjusting throttle and allows boats improved fuel economy by diminishing water displacement. Use trim tabs on any boat to correct list, eliminate porpoising, offset prop torque, improve handling, reduce hull stress, reduce engine laboring and eliminate squatting.