Re: Is this my Hub?
Looks like just the cap is missing. You should be able to get a replacement at anyplace that sells trailer parts. Just measure the inside diameter of where it fits. It goes back on with some taps of a hammer, prefereably a non-metallic one.
That said, it looks to me like some maintenance is needed on your rig. At a minimum, I'd pull those wheels off, clean them up and paint them. I'd also inspect and pack your wheel bearings, replacing any bearings that are bad. Way too much rust there...enough that you may have problems getting the lug nuts off without busting something.
That much rust on your wheels and lugs isn't a good thing. Left that way to get worse, and you may be looking at much larger problems down the road.
While you're at it, give your trailer a good inspection as well. Looks to me like you're in salt water with it. Check for rust on the trailer and for weak spots caused by it. Again, a good cleanup and repainting is probably in order.
Funky trailers wreck boats way too often. Regular maintenance is needed, especially if salt water is involved.
