Edited by board administrator July 14, 2016, 9:25 am
While I have found some information for you, the net effect may be only to deepen the mystery you are trying to solve.
I have found an online publication from 1947, "Troopships of World War II" at http://www.history.army.mil/documents/WWII/wwii_Troopships.pdf. (This is a very large file and takes a long time to load.)
The publication discusses troopship PRESIDENT TYLER in two places. On page 50 is information about a proposal to convert PRESIDENT TYLER to a hospital ship, plans for which were abandoned. On page 239 is information about PRESIDENT TYLER's service as a troopship. A careful reading of this page indicates that PRESIDENT TYLER operated exclusively in the Pacific theater from early 1942 into at least August 1944. This is at variance with your father-in-law's recollection of being on this ship in a convoy to Europe in January-February 1944.
Additionally, this page, listing merchant ships sunk or damaged in World War II, does not list PRESIDENT TYLER as being among the ships damaged during the war: http://www.usmm.org/sunkaz.html#anchor28979. And this page, from the same website, does not list a John Hemphill as having been injured as an Armed Guard sailor: http://www.usmm.org/ag/h.html#5007. (Of course, if the Navy doesn't have a record of his presence or injury aboard PRESIDENT TYLER, why would anyone else?)
With all respect, is it possible that your father-in-law has confused some of the details of his experience? There is not enough information provided to narrow down other possibilities. Does he remember the departure port of the convoy? The approximate departure date?
Your message indicates that your father-in-law must have at least some of his official military service record. But if not, he (or a next of kin such as your wife) could obtain his complete service record. See this page from the website I manage: http://armed-guard.com/searchmil.html, in particular section II.A.1. - Records of Individuals - U.S. Military. You will have to contact the National Personnel Records Center in St. Louis, MO. Provide as much identifying information as possible. There may be a charge for research time, photocopying and mailing, but the Records Center staff will provide an estimate on the cost before beginning work. Whether his complete service record would shed any more light on this matter one cannot say but it may be worth a try.
For your information, PRESIDENT TYLER was constructed by New York Shipbuilding in Camden, New Jersey, completed in December 1920, with an original name of CREOLE STATE. She was renamed several times, becoming PRESIDENT TYLER in 1940. She survived the war and was scrapped in 1957. See http://www.shipbuildinghistory.com/history/shipyards/2large/inactive/newyorkship.htm and scroll to hull number 245.
As an aside, your father-in-law may be interested in becoming a member of the USN Armed Guard World War II Veterans Association. The Association publishes an interesting newsletter/magazine several times a year and, best of all, it's all free! See http://www.armed-guard.com/association.html for information.
I regret not being able to provide more definitive information.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website