When running a large, dual fuel, main propulsion diesel engine on heavy fuel, which of the following precautions should be observed when switching from heavy fuel oil to diesel oil?

Correct Answer: Option B — The temperature of the fuel from the preheater should be gradually reduced after switching over the three-way valve.

Rapidly dropping heavy fuel temperature causes thermal shock, viscosity spikes, and fuel system binding. Marine engineering best practices and 46 CFR Subchapter F standards require gradual cooling to prevent plunger scuffing and maintain stable viscosity during fuel changeovers.