Shaft taper: On both my shafts, old and new, is 1:10 on diameter. 25mm to 20mm in 50 mm length. Keyway 6 mm width.
Threads: Check thread diameter and pitch. Old and new shafts were different. What does your new propeller require?
Propeller: 13x8 inch prop sounds very small compared to the 16x10 recommended to me by Gori. My gearbox ratio is 2,3:1. 3 600 engine rpm gives 1 565 prop rpm, which with 10 inch pitch means a speed of almost 7 knots with 50% propeller slip, (normal calculation).
Your 8 inch pitch by the same token would need 1 956 propeller rpm, gearbox ratio to suit 3600 engine rpm would be 1,84. What is it?
Gori's technical expert advised that a larger propeller rotating slower will give better thrust under adverse conditions, due to the larger blade area.
Food for thought.