There was no stealth in resupply of Ft. Sumter. Rather, Lincoln had corresponded with the governor of SC, to notify him, and Lincoln had out of good will, limited supplies to non-lethal items.
Based on that, SC fired on the ship and the fort.
Not true. Lincoln leased a merchant cargo ship, NOT a US Navy ship to attenpt to bring supplies to Ft. Sumter. If this was an open attempt to resupply the fort, why would he use a merchant ship? The use of the merchant cargo ship was a stealth attempt to resupply the fort.
The South Carolinans suspected the ruse, fired on the ship and forced it back to sea. The tactic is the same one employed by the British in WW1 with the Lusitania sinking. The Lusitania was a passenger that was carrying munitions in violation of existing treaties at the time and the Germans sank her.
What Linclon tried to do was use an average looking commercial merchant ship to carry supplies to the Union soldiers at Ft. Sumter.
Since SC had already joined the Confederacy, they were trying to get the soldiers to leave the fort and previously requested them to do so; orderly and peacefully. And, since they refused the request they were surrounded and prevented from being re-supplied by Lincoln.