This is a pretty simple tutorial showing how to create various bathtub and sink designs within seconds. Loft with the “Loose” option is used so that the control points of the resulting loft surface will match the control points of the profile curves. Turning on the History recording in Rhino will let you adjust the general shape of the shape with ease.
You can also add extra curves at the bottom where the drain for the drain pipe is located, in order to make it history enabled and be able to change its location, size or shape at any time as long as the History is still enabled.
One thing you can do with that loose loft trick is to Smooth the lowest/innermost curve in X/Y so that it approaches circular - sometimes I smooth the next curve or two as well, but progressively less.
Yeah, I was going to add some extra curves at the bottom, but I forgot to do so while recording the video. This is why, I only wrote that as a text description in my original post above, because it’s a really convenient way to make the drain for the drain pipe that way. I would use Circle > Deformable with 20 control points (same count as the rest profile curves) instead of Smooth, because that would guarantee creating almost a perfect circle shape.
One other thing to point out that can be useful is that if you draw a profile curve as a degree 3 curve and place a lofting curve at every control point of that curve, the loft will follow the shape of the curve.