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The prop you have now does not have the best "grip" and is "slipping". It turns but does produce a lot of thrust.

Propeller design is complicated and I understand the basics. By increasing the total diameter of the prop, you produce more thrust. The pitch, or angle of the blade, is important for creating thrust but also allowing engine to increase RPM.

Think of rowing a boat with:

toothpicks
oars
snow shovels

You are looking to increase speed over water while at the same engine RPM.

Hope this makes sense.