Are bull mastiffs good guard dogs?

Yeh they are very good guard dogs. My mum has 3 of them:P I remember this one time when my mum walked one of them around the woods.she tripped over fell to the floor and there was these 4 guys wanted to help get her up but my mums dog would not let them any where near her. dog was barking and showing his teeth even though he wouldnt bite its usally not a Bullmastiff way of doing things as they will jump on them and use there body wieght to bring them to the ground. Id say they are very good guard dogs always feel safe walking one of them around the park even though im a martial artist so i dont really have anyone to fear lol. but also remember they are big Softies:)