@NNF
I have copied items using a similar method, but my method was to use parcel tape over the line that you wish to copy, wax it and then, as others have said, build up the size by laying just a thin squirt of foam at a time along the tape and allow to cure before adding more. 'Misting' it with water helps the cure. If you do it carefully the result should be a strip of foam about sausage(!) size. Whether you are copying the inside or the outside of an object such as your hull use roughly cut cardboard templates which can be bonded on with another strip of foam. When really cured it is easy to cut off spare foam to leave a squared edge if you need it . Then you can draw around to make a plan or even build straight onto this former,
Trevor