How do I stop running out of supplies when I am sailing?

You can use food items to restore supplies while at sea if you don't want to go back to a port. But you can also increase the number of supplies you take with you on each journey.

To do this view your ship (to see the layout of cabins) and tap on a Storage room, Then select the Storage option. You can change how many of the storage slots are used for supplies. Default is 1.

Each storage slot can hold 1000 supplies, so if you have 1 selected then change to 2 to double the amount of time you can sail.

Since supply and goods share the same storage, you can change the capacity for each item. So you can increase your supplies but it will decrease your storage. To do it, open your storage in the main page and then click modify(at the bottom right of your storage) and then modify to your liking. Hope this helps.