Crops/whips come in different sizes. Dressage whip is really long and you can touch the horse on butt or behind the leg while not moving hands. Jumpers are using much shorter crops. Personally I prefer the dressage one, because it's long enough so I don't have to move my hands while using it.
BTW, using the whip on shoulder is meaningless. You use it behind the leg to "back up" the leg cue. I tap the butt too sometime, but some horses may throw a buck when you do it.