结构：
时间状语：in 1881 the Serbia
主语部分：核心词 a merchant steamer + 形容词短语后置定语 capable of crossing the Atlantic in 7 days
这部分可以改为定语从句：a merchant steamer who were capable of crossing the Atlantic in 7 days ——有能力七天穿越大西洋的商用轮船.
系动词：was
表语部分：核心词 the first vessel + 不定式后置定语 to be constructed by steel.
这部分可以改为定语从句：the first vessel that/which would be constructed by steel——会用钢铁构建的第一艘船只.