Assuming it's something like a 7.5 by 10. 7.5 would be Diameter and 10 would be pitch. Bigger diameter with same pitch, more bite in the water. Lighter boat bigger diameter and pitch. Assuming they offer those for different size and weight of boats.
WHAT'S THE DIFFERENCE BETWEEN A 7.5 AND A 7.0 AND WHY.( TOHATSU 9.8HP)
Assuming it's something like a 7.5 by 10. 7.5 would be Diameter and 10 would be pitch. Bigger diameter with same pitch, more bite in the water. Lighter boat bigger diameter and pitch. Assuming they offer those for different size and weight of boats.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979
Just a warning be careful changing the size of you prop. You can over do it if you don't know what you are doing.
Since all 9.8 motors call for 8 - 9 inch diameter props I'm guessing that your 7 and 7.5 is pitch. ½" change in pitch will result in about 100 RPM change and I doubt you would even know any difference in operation.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va
You need to check RPM on each and see what makes it run like the Mfg says. Get close to top RPM without going over. Just one thing to consider.