The Swimmer Delivery Vehicle MK 9 was designed for maximum power and shallow-water landings. Its crew could sit upright for surface travel or lie down for subsurface electronic navigation. Its "pancake" design, made mostly of fiberglass, reinforced plastic, and non-ferrous metals, minimized magnetic signature. It was unique among SEAL submersibles because it could be armed with two heavyweight torpedoes, or carry two Limpet Assemble Modular large anti-ship limpet mines in place of the torpedoes. Satchel charges and smaller limpet mines could also be carried in the payload bay behind the divers' feet if they lay prone. Its large flat construction gave a rough ride at slow speeds in choppy seas, so production was limited.