A security device 10 comprises a plate 20 that extends over a first height and has a top edge 21 and a bottom edge 22. The device also has a bulbous barrier 30 that extends over a second height, which is shorter than the first height, and has a top edge 31 and a bottom edge 32. The plate and the barrier form a slot 50 therebetween. The plate and the barrier are joined together, or integrally formed, at their top edges and a lower portion of the plate, below the bottom edge of the barrier, comprises only a single layer of material. The security device can be fitted to the rail of a ship to make it more difficult for pirates to board the ship.