Action of 21 May 1918 The Action of 21 May 1918 was a naval engagement of World War I fought between an American armed yacht and a German submarine in the Atlantic Ocean off Spain . Background . In May 1918 , the Great War had been raging for four years and the Germans were making every attempt possible to sink enemy shipping which fueled the war in Europe . On 24 May 1918 , the fight was still at hand when — under Lieutenant Commander M . B . McCord — sighted a distinctive oil slick while escorting the slow British steamer " Danse " north from La Pallice to Quiberon Bay . Unknown at the time , a German submarine — commanded by Lieutenant Commander Wilhelm Kisewetter — was nearby . " Danse " was about behind the main convoy of allied merchant ships , making about with " Christabel " off her port bow . The North Atlantic was smooth , the weather was clear and there was no wind . Action . Once the allied convoy was within of He de Yeu , a well - defined oil slick was sighted between the American warship and the British steamer , off " Danse " ′ s port bow . " Christabel " cruised over to the slick for better observation but saw nothing to further indicate a German submarine ' s presence . The convoy continued for a little while when at 17 : 20 the wake from was spotted by the officer - of - the - deck and a lookout , about off the port quarter . " Christabel " was — at this time — about from the port bow of " Danse " . " Christabel " headed for the wake , making all possible speed , which was around , whereupon the wake disappeared and a number of oil slicks were seen . The U - boat had apparently submerged . The American commanding officer ordered his ship to follow this oil for as long as possible and at 17 : 24 — believing that his ship was just ahead of the submarine — " Christabel " ′ s crew dropped a depth charge , but nothing resulted although the charge exploded . The action was over for now and the allied vessels continued northward . At 19 : 00 , the convoy changed course , following the contour of the Spanish coast , making about for almost two hours when " Christabel " encountered the German U - boat once again . This time at 20 : 52 , " Christabel " was astern , making about to catch up with the convoy . The German submarine was sighted by lookouts who witnessed a periscope roughly off the starboard beam . Her commander was quickly notified , and " Christabel " turned toward the U - boat when the periscope disappeared under the water . At 20 : 55 , a depth charge was dropped which detonated 10 seconds afterward . A second charge was dropped a few moments later . No secondary explosion was heard after the explosion of the first charge but after the sound of the second depth charge a third , " very violent " , explosion was heard which threw up a large water column close to " Christabel ′ s stern . An " enormous " amount of debris from the damaged submarine was seen , mixed in with the water column of the third explosion . " Christabel " was then ordered to turn and cruised in the vicinity of " UC - 56 " ′ s position when she was engaged . The crew of the American armed yacht noticed a quantity of thick , black oil and splintered pieces of wood . There were also very large oil bubbles rising to the surface , no doubt belonging to SM " UC - 56 " . Sometime during the dropping of the depth charges , a number of other charges — which were prepared and live aboard " Christabel " — were shaken lose and Ensign Daniel Augustus Joseph Sullivan reacted quickly by jumping on top of them and securing the charges before they could detonate . Sullivan would later be awarded the Medal of Honor for " extraordinary heroism " in this action . Aftermath . Nothing further was heard of this submarine before it surfaced after the engagement ; it was not capable of submerging again due to battle damage . On 24 May 1918 , the U - boat arrived at Santander , Spain after a dangerous three - day voyage in a severely damaged condition . The crew of " UC - 56 " were interned , the Germans reported to the Spanish authorities that their submarine had been seriously damaged by USS " Christabel " and that they had had no choice but to take refuge in a neutral port . It was originally thought that the yacht sank the German submarine so a traditional white star was painted on " Christabel " ′ s smoke stack which represented a U - boat kill . Although the American ship did not actually sink the German vessel , " Christabel " was still responsible for protecting her convoy and inflicting serious damage on an enemy submarine which resulted in internment . No Allied vessels were damaged as the German submarine was spotted and attacked before it could line up for an attack . No German casualties were reported .