Following assignments to a number of ships and stations ashore, Knapp was an executive officer on a gunboat at the outbreak of the Spanish–American War.

[2] Outstanding service in a variety of important billets afloat and ashore was rewarded on 3 August 1908 when Knapp assumed command of the protected cruiser Charleston (C-22).

"Meritorious service" in this post, laboring to protect Allied shipping from German U-boats and to make the Caribbean Sea secure from enemy aggression, earned Rear Admiral Knapp the Navy Cross.

Even after Vice Admiral Knapp was placed on the retired list effective 27 June 1920, the Navy utilized his singular abilities.

[3] His remains were cremated and the ashes were scattered at sea after a funeral service at his home in Hartford attended by his Naval Academy classmate Rear Admiral W. L.