I Don't Feel Safe Around My Dad Anymore

I always thought I was just being stupid but now I'm second-guessing myself.
I'm sorta concerned about the relationship I have with my dad (He is my step-father but he's been around since I was about 2 and I'm now 15).
He's quite sexual about well pretty much everything. I've walked in on him masturbating more than once, he's very open about masturbation and his sex life with my mum, he's even asked me to download porn for him quite a lot, also at times getting changed in front of me, he has on occasion called me hot or attractive and comments on my body. I put all of this under the fact his family growing up was very open about things and he's just trying to build my self esteem or something.
This is when he' in a good mood I guess. When he isn't... run. He's very angry all the time and some stupid things will set him off. He's never hit my mum or I (that I know of) but he will physically shake us, spit in our faces and absolutely go off yelling and screaming, telling my mum and I how pathetic and useless we are etc. He's constantly threatening to leave but never leaves for more than 5 minutes before comign back to start all over again. When he calms down its as if nothing ever happens and you just have to pretend it didn't. My parents go back to 'normal' and are all okay again.. that is until he starts again.
Back to the first issue I was just flicking through my photos on my phone and I found out that he had taken pictures of his "special area" on my phone. And there's no way that he could have accidentally transferred it from a computer or sent it to my phone accidentally or anything like that because it was in the Camera Roll and he took more than one because the first was blurry. I of course deleted it straight away.
I told my mum after it happened on the way home from school and she was just like "why would he do that? that's unbelievable" and things along that line just sort of muttering to herself and then my mum just brushed it off and it hasn't been mentioned since.
I just I don't feel safe in this house anymore. Everything about him just seems sorta wrong but I do love him he is my dad...

I've already posted this in a different forum but I want to ask a range of opinions

He is crossing a line that he knows should not be crossed.
You are now at a point where you need to do something as soon as possible.
This is a dangerous situation to be in and your mother needs to take this seriously. If she does not, then you need to tell another adult quickly.
Your bad feelings are right. Get out. There is no other explanation for his behavior except that he thinks of you in a sexual way.
Im sorry to be so honest with you and I know its hard to hear but it is true.