The Collaborative International Dictionary of English v.0.48:

Dismast \Dis*mast"\, v. t. [imp. & p. p. Dismasted; p. pr. &
   vb. n. Dismasting.] [Pref. dis- + mast: cf. F.
   d['e]m[^a]ter.]
   To deprive of a mast of masts; to break and carry away the
   masts from; as, a storm dismasted the ship.
   [1913 Webster]