Pirates?

Here's my proposal (I've heard of a ship that was well-prepared doing something similar, though I do claim to have thought of it separately). Ships have something of a 'lip' around them. The rig I heard was simply spoils of barbed wire, but I'd have thought a system of perspex extending several metres all round the ship could be produced relatively cheaply with little maintenance required. Barbed wire (electrified if you're willing to pay for it) would crown the edge. It would be removable fairly quickly by undoing bolts and folding inwards, but that's fairly impracticle for a hijacker. Obviously, that doesn't stop them shooting RPGs at you, but it's better than nothing. The main point is most pirates simply can't be arsed with a ship that's obviously got some level of preparation for them and defences to penetrate, and will move on away from you.