A 3S battery under load produces about 11.1 volts. The no load speed at 11.1 volts and Kv=1800 would be 19,980 rpm. The loaded speed should be about 80% of the no load speed... about 16,000 rpm. Therefore, select a prop that requires a little less than 400 watts to turn 16,000 rpm. APC lists power and thrust data for all of their props at different rpm. You can see it at https://www.apcprop.com/technical-in...formance-data/ The data for the 8x5 prop shows it peaks at 390 watts at 16,000 rpm. It is recommended that the ESC not be loaded to more than about 80% of its rating. If you keep the 40 amp ESC, then select a prop that will pull closer to 320 watts at 16,000 to limit the wattage to what your ESC can handle long term. The 8x4E prop data shows a peak near 350 watts at 16,000 rpm. You can check other propeller diameter and pitch combinations. However, I would start with a 7x6 or 8x4 prop and run it with a watt meter to make sure the maximum wattage does not exceed the rating of the motor or 80 % of the ESC. If you don't yet have a watt meter, I suggest you get one. It will more than pay for itself with saved motors and ESC's. Here is what I use: https://www.motionrc.com/products/gt...yzer-gtp180awm Its the best $25 investment you can make for electric RC.