Monel shaft

[tandm]

Does anyone have any idea on how to tell if boat prop. shaft is monel alloy? (Color or something like that.) Our diver recently mentioned that he is having a hard time keeping zincs on the prop shaft. He believes that this is because the shaft is very hard and made of monel alloy. It would be nice but the PO never mentioned it, so I am not sure.

td
Peregrine #543

[dave davis]

It will be very hard to tell just by looking for a color difference between monel and Stainless. Are you out of the water? If you have a scrap metal dealer in your neighborhood, they usually have a contact instrument that measures conductivity and you can tell the difference between a iron base Stainless(18-8) and Monel (55Ni-45Cu) If you have a small light-weight very strong magnet you will be able to detect a slight magnetic feel in the Stainless and not in the Monel. The hardness and the strength of either one of these alloy can vary all over the lot due to its past fabrication methods (cold rolled) and some Monels can be heat treated just to complicate the subject.
The bottom line is, they are very passive and most galvanic series charts show that Monel is more passive than 18-8 Stainless and therefore more resistant to corrosion. If you have a Martec prop, they come with Monel pins that hold up better than Stainless when used with a bronze prop. These pins are quite soft.
It sounds like the Zincs are doing their job and as long as they continue to corrode, you will not have as much corrosion on your prop and shaft. Some of our electrical garu can tell you how to measure the stray currents in and around our boats. I hang a 4/6 zinc over the side as to protect my prop beside the two on the shaft and the one on the strut. I'm over zinc.
Good Luck...Dave

[jentine]

I would be more concerned with stray electrical currents in the water and/or bad grounds aboard the boat.  If the zincs are being eaten at an alarming rate, don't blame the shaft.  Look for the problem.  Disconnect your shore power cord when leaving the boat and see if that reduces the rate of consumption.
Jim Kane

[tandm]

I do not believe I have a problem with either galvanic corrosion or electrolysis for three reasons. First, the shaft zincs do not appear to be excessively dissipated; they are just working loose and sometimes sliding down to the strut. Second, the zinc on my Max prop. is always in fine shape. I replace it once a year even though it has not reach the 50% point yet .  Third, our port does a current check around every boat in the marina once a year.  My boat has never shown up as having a problem.  Owners of boats that do are strongly encouraged to address the problem, so our marine is not particularly "hot".

I know that some individuals on this list have strong opinions about leaving a boat connected to shore power.  However, for the vast majority of boaters in this area, the dangers and costs of leaving a boat unplug far out way the dangers of leaving a boat connected to AC.  The only boats in our port that do not have a constant supply of AC are the derelicts.  My point being that what may be a reasonable view in one area my not be in another. (Sorry, just couldn't resist getting on the soapbox.)

The reason I posted the question is that I am contemplating the much discussed engine mount/flexible coupling change out this winter.  I also thought that it might be a good time to replace the shaft.  Obviously, I don't need to do this if I have a monel shaft.  I am unfamiliar with any of the many grades of this alloy so I have no idea what it is supposed to look like.  If it is supposed to look like SS then it is definitely not monel.

td
Peregrine #543

[dave davis]

I don't believe you have a problem with the shaft "alloy"  You have a fit F-I-T problem. When you are putting on the new shaft zinc, there should be a gap in the two halves of the zinc that have to deform so the two halves touch each other. This should take a big screw driver and lots of effort. It might even take two separate efforts allowing time for the zinc to creep after the first effort. I have never used a micrometer on my one inch shaft but it sounds like you should check it out. Good luck...Dave