Download and install OpenCPN for android here
Download and unzip chart files to some spot on your phone. Raster – pictures of traditional charts, Vector – electronic version. If either of the links break, search “chart downloader noaa rnc” or “chart downloader noaa enc.”
Open the app, go to settings (wrench in top left) > Charts > Chart Files. Add Directory. Point it to the location you unzipped the chart files.
Click the button on the bottom, “Force Full Database Rebuild” and Bob’s your uncle.
Other reference for TRANSAS on Android, but since that article 4/2017, TRANSAS android app has been up and down. The accompanying chart store has been hit or miss. While their raster charts are superior to NOAA, it just hasn’t been worth the hassle or expense (about $15 per area) they charge for the charts.
PSA: NOAA charts are free. Enter OpenCPN. When I was initially trying out this app, it was wonky. As of the current version (4.1.1028) has been worth the switch.
OpenCPN website
There isn’t an official OpenCPN app for iPhone, looks like SEAiq is the equivalent version. But as I recall, TRANSAS iSailor app, was geared towards iPhones, and that would be your best bet.
Another offline vector map app, OSMAND, has a marine version, but it’s junk. The street maps are pretty darn good, and editable by anyone.
