Guard Dogs

My Charlie is a great dog, and I've loved Bostons since I first saw one, but I've never known them to be much in the way of guard/alert dogs. Does anyone out there have one that barks at strangers or keeps abreast of any strange movement in the yard? I'm thinking of getting another dog with this in mind, but I don't really want anything other than a Boston. Is it the breed or just the few I've come across?