S/Steel Prop shaft corrosion

[DaveAlpine]

I have recently hauled out after a usual 18 months afloat to find the prop anode completely gone and serious pitting on the stainless steel shaft which requires the shaft to be renewed.

The boat is GRP and I have changed nothing electrical on the boat since the last haul-out.

There is a second anode attached to the hull forward of the shaft to which earth wires are connected - this anode shows no significant "wear down" since the last time the boat was hauled out. (There is no SSB fitted to the boat.)

Can anyone suggest where to start checking systems for stray current that might be the cause of this pitting?
The prop is a "common or garden" bronze type prop - why was this not attacked first?
What material should anodes be made of Zinc, Aluminium or Magnesium?

My final dilemma is removing the shaft from the coupling. It appears to be held only with a single allen screw and the shaft/coupling is "keyed" - it has so far refused to move on my initial attempts - any suggestions to solve this one?

Thanks

Dave

 

[oldjohnnyb]

It is possible that the pitting on the shaft is not due to galvanic action.
Stainless steel will pit if stagnant water reduces the presence of oxygen.
Oxygen is needed to enable stainless steel to remain corrosion free.

 

[Guest]

Dave, firstly try fitting a larger anode close to the prop shaft and make sure that it connected to all the other anodes and engine etc. Make sure that you have a good connection across your coupling especially if you have a flexable joint.
As for removing the coupling from the shaft, there are 2 ways I can suggest. 1) coupling should have a slot in it, try opening it with chisel/screw driver first, so that you can "spring" open the bore of the coupling. 2) Last resort try a little heat on the coupling, but be careful if you have oil/diesel in bilges. Its always good to have a fire extingsher handy for safety's sake.
hope this helps?

 

[AndrewB]

Yes, I was going to suggest the same - specially if the pitting is under the cutlass bearing. Is the bearing clogged with mud, or the water intakes blocked? I once lost a propellor to this.

The shaft seems to practically weld itself to the coupling with time, and separating them is often difficult. Your arrangement is usual. My method is to separate the coupler from the gearbox, then I can just fit in a three-legged puller to push the shaft backwards and break that initial grip. I don't recommend heat - if it doesn't work it'll make things worse.

 

[miket]

To remove coupling without puller, release bolts attaching to g'box flange.
push shaft/ collar back. Insert spacer (large socket works well) and re attach collar to g'box collar. Tighten bolts a little at time.

This worked for me. Came off with an almighty bang.
If it is still reluctant, apply a little heat (with warnings already mentioned) whilst it is still under tension.

If all else fails, cut through the shaft in front of stern gland. Well iof its shot anyway..........!

 

[longjohnsilver]

If boat not in use then it's recommended to turn shaft fairly regularly to ensure no corrosion due to lack of oxygen.

I changed my shaft earlier this year and the only way to release it wasa with lots of heat, we used an industrial blow torch until it was red hot and wet sponge on shaft to keep it cool. Still took a good 20 minutes or more. Essential to have a bucket of water and fire extinguishers just in case.

If your other anode shows no signs of corrosion then it would appear not to be doing its job, use a muti meter to check continuity of connections etc.

Good luck.