The answer to this question can be found in chapter 2. The ship that Phillip and his mother board in Curacao that will take them back to the United States and supposed safety is the S.S. Hato. The ship itself is a small Dutch freighter, and we are told that it has an elevated bow and stern. The bridge is in the middle of the vessel and located between two of the well decks. The ship also has a single long stack. The S.S. Hato is a familiar vessel to Phillip because he has seen this ship quite often in the St. Anna Bay because it usually runs a trade route between Willemstad, Aruba, and Panama. As Phillip is getting situated in his cabin, Phillip's father assures him that the Germans would never waste a torpedo on such an "old tub" of a ship. Unfortunately, that prediction ends up being quite wrong. Two days after leaving Panama, the ship is torpedoed.

Phillip and his mother were on a ship called the S. S. Hato. In later chapters, sometimes he simply calls it "the Hato." ("S. S." means "Steam Ship," and "Hato" is the name of an airport on Curacao; it's also a last name. So the name of the vessel basically means "Steam Ship Hato.")

As he's narrating the story, Phillip describes this ship as a small freighter that often emits thick, black smoke, and it's a ship that Phillip is particularly familiar with, since he often sees it in the bay. It seems to be well-equipped with life boats, as he and his mother are familiar with how to follow a drill and make their way onto a lifeboat.

Before he and his mother board the ship, Phillip's whole family notices that only some of the ships that leave the islands actually make it to the mainland. Others are torpedoed during the journey. So it's actually a terrifying decision between making the journey on the S. S. Hato, or staying home in Curacao and risking being attacked there, on land. 

How did Phillip and his mother react when their boat was torpedoed in The Cay by Theodore Taylor?

When the ship, the Hato, was torpedoed in The Cay by Theodore Taylor, Phillip was thrown from his bunk onto the floor. His mother reacted very calmly, and Phillip noticed this was unlike her usual demeanor at home. She quietly spoke to Phillip as she got herself ready, reminding him to take his sweater and his leather jacket. Phillip noticed that her hands were not shaking as she helped him into his life jacket and then told him to remember what he had learned during the abandon ship drills the ships' officers had held each day. Phillip, too, was reasonably calm, listening to his mother and following her instructions. Once they got to the lifeboats, as they were getting ready to be lowered into one, the ship lurched, and they fell into the water. Phillip tried to get to his mother but was hit in the head, and they were separated.