Argh. How can I trim this surface?

In the attached image, I am trying to trim the transom surface (the black surface on the left) to the six surface edges that intersect it (two each hull sides, chine flats, and hull bottoms) and the arc drawn from the top corner of one hull side to the other (on the bottom in the image as the hull is upside-down).

How I got here: I modeled the hull (sides, chine flats, and bottom) from an existing table of offsets. The table stopped one station short of the transom, so I drew a separate surface for the transom with the correct camber and proper rake, then positioned it the correct distance aft of the last given station. I couldn't get the "extend trimmed surface" command to extend the sides/chine flats/bottom to the new surface, so I deleted the surfaces, used Extend curve to extend the sheer/chine/keel curves to the new surface, and then re-created the surfaces. All of which goes to say, I'm reasonably sure the fore-and-aft surfaces actually meet the transom surface.

Finally, I drew an arc from the top, aft corner of one hull side to the other, with the midpoint at the midpoint of the top edge of the transom surface. The transom surface shape is now fully defined, but oversize.

I tried Trim, selecting all the surfaces and the arc as cutting surfaces, and the transom surface as the surface to be cut, but nothing happens. Now I'm stuck. No idea how to trim the transom surface properly.

Thanks in advance for any help you can offer.

 

If it is drawn as a solid you can draw another solid to coincide with the overlap and subtract one solid from the other.

 

I don't have your drafting program so take it with a grain of salt.

 

Thanks, but they're surfaces, not solids. There's a way to trim one surface using an intersecting surface but I can't seem to get it to work.

 

That(surface) I do not know how to do.

 

I can see in your picture that the surfaces do not reach the level you intend to use for triming them. There is no intersection between transom and surfaces. Try to extend surfaces beyond the transom surface and try again

 

That did it - thanks!