The right stuff needed for “shaft log”

Mahdee had Tobin Bronze (a.k.a. Naval Brass) pipe/tubing used for her “shaft log” (that’s the hollow tube that the propeller goes through; it connects to a bearing on both the inside and outside of the boat) as well as for her “rudder post” (that’s the hollow tube that the rudder stock goes through). I’m having a very hard time finding suitable material for either one of these applications. The parameters are:

Inside and outside diameter must be correct: in the case of the shaft log, that’s 2″ schedule 40 pipe with ID of a sliver over 2″ and OD of 2.375″.

Corrosion resistance to seawater must be there and there are a range of alloys that fit this need.

Final thing, since the shaft log sits right up against some silicon bronze bolts which hold the back end of the boat together, the material chosen must be “less noble” than silicon bronze so that, in the presence of seawater and stray current, the shaft log will not cause degradation of the silicon bronze bolts that are structural to the boat.

This combo of requirements is making it darn near impossible to find schedule 40 pipe for the shaft log. I’ve found copper-nickel, but it is more noble than silicon bronze. I can’t seem to find Tobin Bronze in the right size pipe anywhere. Nor can I find silicon bronze schedule 40 pipe. Onwards, more shopping on the internet for this stuff!

Below is a “galvanic series” chart which shows the various alloys and their position in the series. Those alloys to the left of silicon bronze will cause the silicon bronze to degrade in the presence of seawater and stray electrical current; those alloys to the right will not…instead the shaft log will degrade in the seawater/current situation. The shaft log is much easier to get at and replace than the very long bolts that go around each side of the shaft log.

If the pic below is cut off, you can go here to this link to see the series and more information.

galvanic series

2 Thoughts.

  1. Have you evaluated the the ‘anodic potential difference’ between the bronzes that you are living with and candidates for the new stern tube? things like copper nickel, monel, inconel etc. I don’t have first hand experience in this but do have a engineering background and a steel boat. Easy to know what is perfect and be paralyzed by perfection but MUCH more difficult to know what is ‘good enough.’
    I have some equip and a little time and interest in this so if I can be of help in experimenting/researching etc let me know.
    georgelewisray@gmail.com
    http://picasaweb.google.com/georgelewisray

  2. Hi, George,

    Everything on this boat goes quickly overall. We’re definitely into “good enough” on the project. The decision for the shaft log was made shortly after that post last year–machined and installed almost a year ago. As it turned out the best choice was simply to use red brass. That’s what we did as it is a commonly used alloy for this purpose. Copper nickel, monel, etc are more noble than the silicon bronze drifts and keel bolts adjacent to the shaft log…so that drove us in the “less noble” direction. Thanks for your offer of assistance.

    Brenda and David

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