It turns out that Sheldon is the leader of a band of modern day pirates who are using Jack and his family to smuggle a cache of money and jewels. They order the Robinsons to turn over the yacht and leave in the lifeboat.
It turns out that Sheldon, the head of a contemporary pirate band, has commandeered Jack and his family to smuggle a collection of cash and precious items.  They instruct the Robinsons to relinquish control of their yacht and embark on a lifeboat.