pict 1 : to be clear ... a boat with few lines ... and 4 networks to do the external hull

the 2 construction lines at the back can be easily modified ( edit point etc ... )

pict 2 : in yellow .... i copy the lines and do a small scale ( scale 3d - boundary box - type .98 ) 98 percents

i need to align the group of yellow lines on the x axe ( in right view ) so later the bastingage will be horizontal

pict 3 : now i have an external and an internal hull - they need to be joined

pict 4 : the most important !! i draw 4 segments to join the hulls ( i zoom very close for each segment ) to prevent gaps

i select the border of each network patch and do JOIN to be sure to obtain a closed curved ( without that ... no solid later )

when i see Closed curved , i can click separate

pict 5 : the 2 hulls are joined correctly ... select all the faces and join ... now it's a solid wich can support holes and various details