The Collaborative International Dictionary of English v.0.48:

Rumbowline \Rum*bow"line\, n. (Naut.)
   Same as Rombowline.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rombowline \Rom*bow"line\, n. [Etymol. uncertain.] (Naut.)
   Old, condemned canvas, rope, etc., unfit for use except in
   chafing gear. [Written also rumbowline.]
   [1913 Webster] Romeine