The Collaborative International Dictionary of English v.0.48:

Boomkin \Boom"kin\, n. (Naut.)
   Same as Bumkin.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bumkin \Bum"kin\, n. [Boom a beam + -kin. See Bumpkin.]
   (Naut.)
   A projecting beam or boom; as:
   (a) One projecting from each bow of a vessel, to haul the
       fore tack to, called a tack bumpkin.
   (b) One from each quarter, for the main-brace blocks, and
       called brace bumpkin.
   (c) A small outrigger over the stern of a boat, to extend the
       mizzen. [Written also boomkin.]
       [1913 Webster]