The mid-eighteen hundreds saw steam overtaking sail as the propulsion of ships. Steam required coal and a new breed of warship had to constantly refuel. A large deep water harbour midway between Portsmouth and Plymouth, requiring no tortuous navigation for ships, was a tremendous advantage and so Portland became a coaling depot for the Navy and then a Naval Base. The breakwaters were built with coal in mind.

The Royal Navy was to remain at Portland for a century and a half and during this period the harbour saw the gathering of great fleets and became an important location for underwater weapons development, especially the torpedo. It saw action in World War 2 and later became a base for naval helicopters.