Sentence:  What impressed me particularly was the mental and muscular superiority of these two officers.  Despite their age—the mate sixty-nine and the second mate at least fifty—their minds and their bodies had acted with the swiftness and accuracy of steel springs.  They were potent.  They were iron.  They were perceivers, <WSD>willers</WSD>, and doers. 
Meaning of the word marked with <WSD> tags:  One who wills; who causes by an act of will or willpower.