Sentence:  His hon. and gallant friend raised the question as to the two classes of destroyers, one for ocean work and the other for work in narrow waters. It was not true to say that the new twenty-five-and-a-half “<WSD>knotters</WSD>” had proved inferior to the old thirty “<WSD>knotters</WSD>.” In anything like dirty weather the twenty-five-and-a-half “<WSD>knotters</WSD>” had shown their superiority. As he had stated to the Committee, they had failed to produce a better design which would combine greater speed with greater stability at a limited cost. 
Meaning of the word marked with <WSD> tags:  A vessel capable of reaching a specified speed in knots.