Belt size and how many pulleys

[welderskelter]

I built my mill and I am putting a larger engine on it. Not sure if I need an extra pulley for the extra power or not. I am going to a 20 hp. engine. Just used a single on the 17 hp. Harold

[millwright]

My WM with a 25 hp has a double belt, and at times it will slip if tension is not really spot on. I would go with 2. 

[donbj]

As millwright says, go with a double. My 24hp Woodmizer can do the same under heavier load if tension isn't up there