Perhaps the Navy has commissioned the Collins Family to supply some ships and Nathan is in charge of overseeing the activity. This would make sense to have one of their own, an officer, on top of things.

I would have to agree with this - if memory serves, wasn't there a scene in Jeremiah's room, where he says to Nathan, that if the Navy wants the alterations to the keel then, that's what they'll have? I believe, it was just after Nathan had met Millicent, and Jeremiah informed him that she could affod to be sea sick!  

^..^

Barnster