Complex sentence: The third deck over the ships' machinery and magazine was armored with two layers of special treatment steel (STS) totaling in thickness; the steering gear was protected by two layers of STS that totaled on the flat and on the slope.
Split into simpler sentences: The third deck over the ships' machinery and magazine was armored with two layers of special treatment steel (STS) totaling in thickness. The steering gear, however, was protected by two layers of STS that totaled on the flat and on the slope.