To tell the difference between a 5.5 and 7.5 horse prop, I lay them on a bench and look across the blades. The 7.5 prop blades here on the right are noticeably higher than a 5.5 blade. This can save a lot of confusion when you can’t read the numbers up high in the hub.
Also, the 7.5 prop usually has ‘7 1/2’ cast into the front of one of the blades. The pitch difference between the two is 1 1/4" which is roughly 15%.
