If you have no plans for your boat, you really only have two options (I think)
1. You need photos to show plan, elevation, bow and stern. You would then need to label each picture with reference points from which you could take measurements and develop a lines diagram. You would have to find a reference point for size, such as a doorway and work the dimensions from there.
2. Design your own. This really isnt as daunting as it sounds. If you have a good range of photos of the boat and the general specs such as length, beam, draft you can work from that.
Take a series of relevent dimensions from the pictures and using the scale proportions of the length of the ship in the photo to the real length you will be able to locate key features on the ship. EG in photo, measure distance from bow to front of superstructure, scale that dimension to full size, then scale again to the size you want to build at.
If you decide to go this way, when you are working the design up, I have found that if it looks right then its ok. Also, who is going to criticise the model if there are no kits or plans available?
If you decide to use delftship, I would recommend you design at full size dimensions, and when you are happy with the design, scale it down using the scale function. This way, if there are any lines that are not perfectly fair (and in my experience there will be unless you buy the pro version with auto fairing), when you scale down, they are much less obvious.
Hope this helps.
Ian