OK, here's an interesting one.

See the forward-most curves? There at the forward end of the pair a longer line, one to port, one to starboard. If you sweep a ruled surface across the starboard pair, then the port pair, each will take a long time (depending on your machine) to generate.

However, my fix was to delete the upper part at the "end" snap point, since i know that is where it is supposed to bend. Actually, there are two cooincident/colinear lines there on each side, but even without that set, the sweep still takes a while.

Now, here is something i discovered serendipitously, a benefit of not just using only one side and mirroring. If you run the ruled surface tool across both lines (after you've removed the troublesome upper piece, or, instead, just use the send set of curves farther aft in the model) at the bottom of the model and which is also where the centerline is, the surfacing tool generates a surface all the way up to the extents of the curves. NICE! It saves some steps. or so it seems.

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Correction: The actual error message is "Invalid solid created during healing!", not anything about ACIS errors specifically stated.

Here is where it doesn't have an advantage -- yet. Add a cover (not the cover tool) by creating a ruled surface between the forward and after surfaces' edges. Now, try to stitch into a solid. You should get ACIS or other errors. I need to figure out how to defeat that error.