Their five decks and open passenger spaces made the ships adaptable, yet their sheltered-water design could not simply be sent to war unchanged. Naval service imposed much greater loads and required worldwide operation instead of short passages in protected water. The conversion strengthened the hull and seaworthiness, added fuel, fresh water, and landing-craft petrol tanks, and rebuilt piping and watertight arrangements. Almost everything above the upper deck was stripped away before troop spaces and unusually heavy gravity davits for assault craft and LCMs were installed.
Chapter · 5:10-9:43
Why cross-Channel ferries suited amphibious conversion
The ferries offered useful internal volume, but worldwide naval service demanded a near-total rebuild above the upper deck.