SOLUTION

A submarine is a type of ship that is designed to operate under backwater for lengthy periods.  A diesel submarine is a very good example of a hybrid vehicle. Most diesel submarines have two or more diesel engines. The diesel engines can run propellers or they can run generators that recharge a very large battery. Because of the exhaust gases and smoke and the need for plenty of air for the diesel engines to run, they could not operate while the boat was submerged. Even if some sort of snorkel system made it possible to run the engines, pulling in air and venting smoke such a snorkel was actually employed in some submarines after World War II they still made plenty of noise and black smoke, making detection by surface craft easy.

Only battery power could be used while submerged, and batteries could not be charged while underwater.  The submarine had to remain on the surface while charging, and that could take a while to get a full charge. Once the batteries are fully charged the sub can head underwater.  Being submerged with little or no battery power remaining could be catastrophic. That was only one thing a sub crew had to watch continually and calculate accurately so as not to run out of juice. Battery operation is the only way a diesel sub can actually submerge.

In the fully submerged condition, it is the stored electrical energy of the batteries that meet the power demands of the vessel. As well as providing the propulsive power, the batteries also have to provide power for the operation of sensors, weapons, and auxiliary machinery and those for habitability, ventilation, of the crew.