I am pulling my dinghy up the front of the boat on deck and always rubbing the thing against the railing. I wonder if anyone know of rollers that would come in halves ie possible to mount on railing without taking rails off.

I have not found any clamshell types that would do the trick.
My railing is 1 inch dia (flimsy if you ask me) so the roller should freely roll around it if on bearings even better.

P&S, instead of a roller, what about padding? You could use that foam pipe insulation material and hold it in place with zip ties. If you got a size that was larger than the bowrail it might roll, but at least it would protect the dinghy and bow rail.

Have a machine shop machine some delrin to the right ID and OD. Have it cut in half. Drilled, countersunk and tapped for the mounting screws to screw it back together.

good ideas, both!

As for the delrin wheels, I could fabricate those. Teflon would work well, too...