The H.M.S. Challenger was actually built as a warship.  It was a steam corvette in the Royal Navy outfitted with 17 guns and capable of over 1200 horse-power.  It was three-masted, square-rigged, and built of wood with an overall length of approximately 200 feet.  It became a research vessel after a request from a professor to the Royal Society of London.

In 1870, Professor Wyville Thomson requested the Royal Society to ask the government for use of one of its ships for an extended research cruise.  The government agreed and the H.M.S. Challenger was manned and modified to carryout oceanic resarch rather than to carryout missions.  The Challenger essentially became a sailing ship rather than relying on the steam engine because of all the stops the Challenger would make when collecting data.  The steam engine was reserved for dredging operations from the depths of the ocean floor.  Fifteen of the guns, along with ammunition, were removed.  They were replaced with laboratories and workrooms, storage for the trawls, dredges, and specimens.