Before doing that, I had succeeded with replacing my granddad's Merchant Marine medals and decorations using the instructions at http://www.usmm.org/contact.html. It was easier than the records request, and the response letter helped there too.
Regarding SS Sacajawea, she is listed at usmm.org as 0612 - Sacajawea - WSAT AP USAT; giving her hull number and specs as US Navy commissioned, War Shipping Administration and US Army transport.
Sacajawea shows in one of the convoys; intercoastal in Papua, New Guinea in July, 1944 at http://www.convoyweb.org.uk/bg/index.html?bg.php?convoy=106!~bgmain . The locations mentioned there are visible at http://www.geographic.org/geographic_names/name.php?uni=-3036275&fid=4838&c=papua_new_guinea; terminating NW across the border (near Jayapura). The WW II Operation Reckless aka Hollandia took place there 3 months earlier.
My searches were aided by retrieving official logs and Navy Armed Guard logs for any ships he sailed on in the 1940s. The "Z number" I mentioned appeared on only 2 of the first 24 crew manifests I located, but having it helped with the Coast Guard's attention. Some details on crew manifests are at the end of the article at http://www.genealogyandfamilyhistory.com/researching-merchant-mariners/
If your efforts continue, they will eventually lead to using the National Archives' resources. One introduction is here: https://www.archives.gov/research/military/logbooks/merchant-vessels.html Good luck on your searches!