Question

Why did lincoln send a ship to fort sumter in april 1861?union troops had been attacked by the confederacy. union troops had run out of ammunition. union troops desperately needed food and water. union troops were about to surrender to the confederacy?

Answer 1

The union troops desperately needed food and water. 

Answer 2

President Lincoln had vowed to the South that "the government will not assail you." He was simply sending food and water to the troops at Fort Sumter/

The short answer is the troops desperately needed food and supplies.