I know there are a number of climbers here and at least one Boy Scout, not to mention sailors, and I bet you guys know a lot about knots, so I thought I'd ask.

I have a length of rope that I want to shorten. Both ends are in use. There is not constant tension on the line. I don't care whether it jams or not. I'd rather not have a free loop occur in the shortening knot.

The best I've found so far is to double the middle of the rope and tie an overhand knot, but that fails my last requirement. For now, I've done that and then tied the loop off, but it's rather bulky. I'd rather have something like a sheepshank, but it'll require contant tension (and might need a free end to tie -- not sure about that).

Any ideas?