Ashley, I’m curious as to why you are advising against filling a prop shaft tube with grease. I make my own shaft assemblies, and always install a capped grease tube in the main shaft tube, which is filled with waterproof outboard motor grease. Each end of the prop shaft is fitted with an o-ring, which helps prevent water intrusion, and keeps the bearings lubricated. I understand that there will be some power loss due to the slight friction caused by the grease, but unless you are running fast electrics, this power loss is almost negligible. The only disadvantage I have found to my method is that some grease can be extruded from the shaft, inside the hull, and centrifugal force can fling it around somewhat. To overcome this, I usually fit a small, curved, removable brass plate over the inside bearing.
I have used the above system in all my models, and have never had a water intrusion (via the prop shaft) problem. If I was at home, I would attach a photo illustrating my comments, but I am in Sydney at present, on our son’s PC, so I don’t have access to my photo library.
Peter.