The APC 18x6W produces about 17,8 pounds of thrust at sea level on a standard day on the ground. So as you get moving say 30 mph you are down to 12 pounds of thrust and 60 mph you around 4 pounds of thrust if the engine stays at the same 8000 rpm. With this prop I think you will see around 10,500-11,000 in the air at full power and producing around 8-9 pounds of thrust.
So for max climb this prop may not be the best and you need to try other props smaller and more pitch maybe to find what you like the best. APC makes a 17x8 stnadard and 17x8N that are good props to look at. The 17x8N APC prop makes less thrust at 8,000 rpm on the ground but at 60 mph it is make around 7 pounds of thrust at the same 8,000 rpm. With this prop depending on the plane you can fly as fast as the 18x6W but at a reduced power setting.