Effectual means good at achieving the results you want. The most effectual method for avoiding cavities? Brush, floss, lay off the candy, and see your dentist every 6 months.
Wait, aren't effectual and effective basically the same adjectives with the same meaning? Well, generally speaking, yes. But if you want to get really nit-picky, you might note that effectual actually describes something that produces the desired result, but in a big way. Effective simply gets the job done. No one is going to call you out for using either word interchangeably though, as long as it's done effectually.