In The Cay by Theodore Taylor, what is the Germans' motivation for blowing up the oil tankers?

At the beginning of The Cay, Phillip is on the island of Curacao, a Dutch island off Venezuela. In early 1942, Germans blow up the oil refinery on Aruba, the neighboring island. Then they blow up six tankers that bring oil to the refinery on Curacao (page 10). The oil is used to make kerosene, diesel, and gasoline. German subs really did destroy the Lagos oil refinery in Aruba in 1942 because this oil refinery was used to fuel Allied ships. By blowing it up, the Germans hoped to disrupt Allied shipping in the Caribbean. The Germans also actually blew up oil tankers in the region (see the link below) with the same goal of disrupting Allied shipping in the area. Later, Phillip is on a British tanker that is sadly torpedoed by a German submarine. 
https://willemsubmerged.wordpress.com/2009/01/13/pedernales-the-phoenix-of-aruba/