If you look closely at old photos of cruise ships from the late 1800s or early 1900s, you might see smoke coming from only three of the four funnels up top. This is because ships generally only needed three stacks to adequately vent smoke, heat, and excess steam from the boiler rooms. However, the funnels became symbolic of speed, safety, and power, so engineers often added a fourth purely for aesthetic purposes. Ships like the RMS Titanic, RMS Olympic, and HMHS Britannic are all famous for their iconic four-funnel design, but each of them was built with only three functioning stacks.