Here's an awesome article supporting the use of these magnificent knives at sea-
http://www.melfisher.org/pdf/Navajas...2_Galleons.pdf
Gilkerson has mentioned the use of jack knives of this sort used at sea. It's great to find confirmation like this, though. Once again, like the Spanish/Portuguese cup-hilts, these little boogers were probably the side-arms of soldier/sailors guarding the Treasure Galleons. Some of the examples in Neuman's book and online are truly vicious weapons (one I just saw had an overall length with blade extended of 24" (a 13" blade!!), more than your typical jack-knife.