As a newer model builder, I am only doing modern cargo ships (Great Lakes freighters), and plans for older ships are quite readily available. Depending on what kind of ship your are building will factor into the available of plans. I don't have the resources to draw my own, or redraw, so I am at the mercy of the plans for what ship I want to do. I have a full set of shipyard drawings for one, and as little as general arrangements for others.
I am pretty sure there is a good link on here, if not then the information is online explaining what is generally the bare minimum required as far as drawings are concerned to do a scratch build.