Complex sentence: It was designed to run at a consistent depth of, and was fitted with an indicator mast that just broke the surface of the water; at night the mast had a small light fitted which was only visible from the rear.
Split into simpler sentences: It was designed to run at a consistent depth of, and was fitted with an indicator mast that just broke the surface of the water. At night the mast had a small light, only visible from the rear.