vapor lock

The Coast Guard Marine Safety Center modeled and analyzed the lube oil levels in the main engine sump in a static condition. The safety center concluded that with 1,226 gallons of oil in the sump, as was likely on the accident voyage, and a 5-foot trim by the stern (0.4°), the bellmouth opening of the line leading from the sump to the lube oil pumps would break the oil's surface at an 18° port list. At that point, the bellmouth would no longer be submerged in the lube oil and air would enter the pumps, causing a vapor lock.