The ship requires a large mechanical power for propulsion and a small electrical power for service loads. From the mechanical and electrical power systems point of view, ships are grouped as follows:
Mechanical-drive (conventional) ship
The prime mover directly drives the propeller via mechanical gears and a long shaft running through the center of the ship, and the electrical service loads are powered by the ship service generators. Most merchant cargo ships today fall in this category.
The propellers are driven by large electric motors powered by dedicated propulsion power generators, and the ship service power
is produced by separate ship service generators. Most passenger cruise
ships today fall in this category.
All required power for both the propulsion and the
ship service loads is generated by the main generators with no separate ship service generators. The service load is provided via step-down transformer from the main bus. Integrated power systems are becoming more common in navy ships, where a high load demand during combat can be met by shedding nonessential service loads and diverting more power to the combat weapons. The navies of many countries are moving toward this category.
On many ships today, some auxiliary equipment is either
steam-powered (e.g., space heaters, laundry, and kitchen equipments),
hydraulically powered (e.g., steering systems and submarine diving systems), or compressed-air-powered (e.g., valve actuators and surface-ship turbine engine starters). Converting these remaining nonelectrical equipments to electrical power would make the ship all-electric.
Almost all power systems in ships are 3-phase, 3-wire or 3-phase, 4-wire, grounded
or ungrounded ac systems. The industry standards recommend (but do not require) that the power distribution system coming out of the generator be ungrounded for reliability,
whereas the power distribution system for 120 V service loads be grounded
for personnel safety.