Each of the three pieces was clearly three cables long. 
But Simon persisted in assuming that the cuts were made transversely, or across, and that therefore the complete length was nine cables. 
The skipper, however, explained (and the point is quite as veracious as the rest of his yarn) that his cuts were made longitudinally—straight from the tip of the nose to the tip of the tail! 
The complete length was therefore only three cables, the same as each piece. 
Simon was not asked the exact length of the serpent, but how long it must have been. 
It must have been at least three cables long, though it might have been (the skipper's statement apart) anything from that up to nine cables, according to the direction of the cuts.