You don't always need to change the pitch when you change the diameter. For your situation, you could just go from an 11/6 to an 11/4. The reason to do this is to decrease the load on the engine allowing it to rev higher. Having higher revs with the same diameter will increase thrust. If you want to maintain the same load but increase thrust you will have to go to a lower pitch prop with a larger diameter. By doing this you will gain thrust by increasing the surface area of the prop blades using the same rpm.