William Bertus Voortmeyer (1891-1952) was a master maritime navigator who developed many early aerial navigation systems, including a numerical signal code by which ships at sea were able to advise pilots of their position by hoisting numerical flags to indicate the distance to a destination. Voortmeyer was born in Rotterdam, the Netherlands, and at age 13 went to sea as a cabin boy on the sailing ship of which his father was captain. Voortmeyer spent the next twenty years at sea, earning a Master Mariners License. During World War I, he was a navigating officer in the transport service. When he left the sea, he held several Civil Aeronautics and Administration (CAA) and California State teaching credentials and was a certified instructor of hulls, an aviation cadet instructor, a pilot, and a Tug Master. He taught classes in marine navigation, writing books on navigation including Guide to Air Pilotage and Meteorology, and Air Navigation. During the late 1920s, he was a navigation adviser on several pioneering flights, including the first successful flight across the Pacific ocean by Ernie Smith, the Dole Flight and the pioneering flight of Charles Kingsford-Smith in 1928. Voortmeyer was also a navigation consultant for Amelia Earhart and for pioneering flights to the Arctic. From 1930 to 1935 he was with the Port of Oakland as a nautical consultant. In 1928, Voortmeyer developed an internationally adopted Numerical Flag Code, a numeral signal code which made it unnecessary for air navigators to master the international semaphore signaling code, by designing pennants with the number from one to ten that ships would hoist when aircraft were flying over head so that the aircraft could confirm their position. During WWII, Captain Voortmeyer was an aviation ground instructor in Navigation at the Pensacola Naval Training Station, Florida. He returned to the sea in command of various transport ships, including the Liberty ship, the Henry Hoyt. In 1946 he was captain of the Benjamin Warner and in 1947 was appointed as a pilot of one of Oakland's fireboats.