Sentence:  ... there was a stretch of twelve miles of channel running in a north-easterly direction which the ship could not possibly negotiate under sail unless a change of wind should occur — of which there seemed to be absolutely no prospect. The only alternative, therefore, would be to <WSD>kedge</WSD> those twelve miles; truly a most formidable undertaking for four persons — one of them being a girl — to attempt. 
Meaning of the word marked with <WSD> tags:  To move with the help of a kedge, as described above.