Friday, November 18, 2005
Making the Rudder Tubes
The carpal tunnel operation I had is healing well, and I'm now able to do light work. One of the jobs I'd been saving is making the rudder tubes.
A couple of books talked about making up an integral bearing and rudder tube, using graphite powder mixed in epoxy as the bearing surface. So, I thought I'd give it a try. After two coats of graphite/epoxy and one layer of 200gsm cloth tape, I had to cut lengthwise with a knife to rotate the tube. Then, there'll be a further 5 layers of cloth tape. I hope I can get it off the shaft!
What about corrosion?? There's about 0.5v difference beteen graphite and stainless in the galvanic table, with graphite being the cathode. However, electrolytic corrosion should not occur because the graphite/epoxy mix has high electrical resistance. There's slight risk of localised pitting to the rudder shaft, caused by contact of individual graphite particles. But, the surface area of the graphite exposed to seawater is very low compared to the surface area of the stainless. So, I'd expect any pitting to be minimal.
"The Boat Owners Guide to Corrosion" is an excellent book for anyone interested in controlling corrosion.
Getting the tube off the shaft was almost impossible to do, without damaging the tube. With 20/20 hindsight, it would have been a great idea to turn the shaft in a lathe to get it absolutely circular - there was a slightly raised section along the pipe's seam.
After a few more experiments on scrap tube, I decided to ditch the idea, made up the rudder tube on poly pipe, and ordered Maritex bearings to suit.