she was not only the largest ship that had ever been built,

she was not only the largest ship that had ever been built, but was regarded as unsinkable.
翻译为,这船不仅是造船史上建造的最大的一艘船,而且也被认为是不会沉没的,
请问,that had ever been built在这是什么意思,启什么作用,我认为不是过去曾经造吗,怎么翻译在造船史上,

that had ever been built 是定语从句,意为“曾经所建造的（船只）：,修饰先行词 the largest ship
由于表范围,这里意译为”在造船史上“ 是没错的