Thank you for that! This problem is very similat to this:
As you correctly deduced, there are in fact two curves in the final version. The first is the top of the side, the pipe serves as the surface transition from the side to the btm. The second curve represents the keel centerline.
The pipe serves multiple functions. It is constant radius and when mirrored at the bow, the centerline intersection trim forms a transition also.
The final step is to make the side tangen to the front and back vertical pipe sections, in addition to the longitudinal pipe we are working on now. The side surface is then defined on three sides by tangency to pipes. Not simple, and may not be possible.