how can I model this so that both the sides and the middle profile align with the rest of the hull? I used sweep to achive this, first selected the middle line, then the arc as a profile. moving the points of the middle line, gets me either alignment of the sides or of the center profile, not both. the main hull is a loft with only 3 curves.

Normally you have a native cool tool Transform : Orient / Line-line and select few regulates following that you want! :)

I am sorry, but that is not the problem, the curves do not follow a smooth path, there is a dent in the skin, either at the bottom, or left and right, depending on the vertical curve.

Maybe you could draw those top curves in to use as construction geometry instead of looking for sweep to adhere to the unknown

I would use blend to create the top curves from the surface to the point, then have 4 tangent curves to work from

Cant test without a file…

You want or don't want this dent ? :)

ok, I will rephrase my question: how can I finish this lofted open shape, so that the end is smooth and round and has no visible division line, so that it looks like one continuous smooth object?

thanks
wim

Maybe Network ?