Okay. Well here is the process I use where you have a subject with no plans available, but where a small model/miniature is on hand as a reference. In this case the yellow submarine, and I used the Ertl plastic kit as a reference, scaled up twice.
Equipment used for the process. A rule, reel of solder or some soft wire that conforms well to a shape, profile gauge, set of calipers (not essential but can be handy for smaller items), pencil and you'll need some paper of course .
First I take some pictures of the model with a rule for measurement reference
Using either the profile gauge or soft wire, I get obtain a reference of the shape at points along the hull.
To get the keel line, I could use the same technique, but this model came supplied in two halves, so this made my life much easier, as I just had to trace a line around the edge as shown
The profiles are transferred to paper as shown here
After a time you finish up with something like this. Not a work of art, but more than adequate to build a hull from. You could if you chose clean these up or even transfer the lines to CAD. I'm more a 'back of the cigarette packet' kind of guy when it comes to drawing.
The completed master next to the original. I made a few tweaks to the larger version to bring it more in line with the movie boat.
I use the calipers for measuring small items like the rudder, periscopes etc.