In 1935 William L. Crothers started employment in the Design Division of the Philadelphia Navy Yard (later named the Philadelphia Naval Shipyard). In the same year he joined the Philadelphia Ship Model Society. These two maritime organizations formed his life for the next 37 years until he retired from the shipyard in 1972. His membership in the Philadelphia Ship Model Society, however continued.

In 1941 William, along with a friend, started to produce plans for the use of ship model makers, using the logo SEA GULL PLANS. These plans, which are still available, were of American naval and merchant vessels of both the age of sail and modern steel construction. His interest in flag hoist signals and house flags originated as part of the plan production process. To develop a complete picture of any vessel seemed to demand inclusion of her flags and signals.

After the death of his associate in 1974, William continued production of their plans but slowly found himself drawn to the subject of flag hoist signaling in detail. Over the years he and his wife visited many museums along the east coast ferreting out the far-flung information that was tucked away in each facility. There was much to be found and, as is usually the case, the subject kept growing as new information was found and collected.

Between 1990 and 1996, he partially shelved the project. In that period he gathered together all the information accumulated over the course of fifty years and wrote a book titled "The American-Built Clipper Ship, 1850-1856" which was composed to a great degree of data used in the drawing of SEA GULL PLANS. Upon completion of the book, he turned again to the flag hoist signal project and completed the subject of house flags to the fullest extent that he was able. In 2001, the volumes were presented to Mystic Seaport Museum Library in order to insure that they would safely survive for posterity as a convenient source of reference on the subject.