mm with 7 decimals is maybe not the best for a boat ?

Hi,

I think, Pilou have a good way with Networks.

But if you don't want have a bad continuity with the main piece, you must use Blend function to generate the curves used by network.
I attach my file with colors to understand what I used,
blend between the 2 red curves
blend between the 2 orange curves
blend between the 2 cyan curves

Then, use network with the curves you obtain by blend function (in my file, I assign those curves as Blue).

Don't know if this is the best way but it seems to be ok.

I used the Sweep tool.
The green curves were used for the profiles and the red curves were used for the rails. The blue curve was used for the scaling rail.
If you show points on the curves, it's quite easy to alter the shape as required.

Martin.