I posted about this over on Trek Movies I-X, but realized it's probably a topic of interest here and not all of you here may read that forum. Sorry for the double-post.
Anyway, in short, I discovered yesterday that those shuttles seen flying in and around Spacedock in Star Treks III and IV were actually designed to be tugs.
I learned this speaking to Bill George of ILM, who designed and built the thing for ST III. When I mentioned that I'd never seen much about this craft, he said, "It's a tug. That's the tractor beam emitter," and pointed to the array of quarter-circle ribs on the back of the ship above the nacelles. He went on to explain his idea that these ships would act as tugboats in spacedock, which is why you see them moving around the Enterprise...ready to guide her if necessary.
So, there ya go...right from the designer's mouth, a bit of trivia I suspect none of us knew.
You can see the tractor beam detail in the photo in row3, columns 2 and 3, and row 4, column 3 of this page.