Help me out here....

Baker wrote:


There is plenty of good metal left in the strut...again the reason for this haul.

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I certainly don't understand all the potential issues at work here, but I believe the reason that bronze turns pink in these situations is because the copper is being leached out of the bronze.* This weakens the bronze.* What I don't know is how deep the leaching goes or how fast.

So unless you know or have been assured of this already, it might be worth trying to find out not how much metal is left but how strong*that metal really is.*Is there a risk that the strut has been weakened--- even though it looks as substantial as it ever has--- to the point where the stress of the loads and vibration it's subjected to could cause it to fail?

I have no idea if there is any sort of test for this, like magnaflux (which I assume doesn't work on bronze), acoustic frequency*testing or whatever.

Marin, we installed a zinc on the strut just for added protection and "in the mean time". In order to do this, we had to drill through the strut. My fear obviously is that the drill would go through the strut like butter. That wasn't the case. It took us 20 plus minutes while taking turns on the drill with a good cobalt bit. Not only that, the 5/16s hole in the strut allowed a good sample view of the condition of the interior metal....it was shiny pretty bronze all the way thru except on the surface.

Sunchaser, will certainly do although the people that run the yard generally don't do the work on my boat....just the haul and paint if needed. My reasons for posting this was the inconsistencies of what could be going on...that is all. I was confused as to what could be going on because it didn't add up. I was just throwing it out to y'all to see if I was missing something....that is all. I am well aware that it can't be solved over the internet. I just had an issue and figured I would put it out there....instead of talking about anchors and dinghies I figured I would get some help with some problem solving....and obviously learn about problems I didn't even know I had....

Baker wrote:

Not only that, the 5/16s hole in the strut allowed a good sample view of the condition of the interior metal....it was shiny pretty bronze all the way thru except on the surface.

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Well, you can't get better evidence of the metal's condition than that.* So you can cross that problem you didn't know you had off the list

*

The zinc is leached out of the copper. It is the remaining copper that gives the pink colour, not the other way around. I'm also going to guess that your bronze is less bronze than brass like. Good bronzes have a very small % of zinc, maybe 5%, to none. Good bronzes replace the zinc with tin and other metal which are much closer on the galvanic scale to copper.
Manganese 'bronze' is about 20-40% zinc, is really a brass, and it is likely what you have for the strut. Some brasses can be good if used properly and protected. Witness many manganese bronze propellors which last for years IF they are protected.

Nonetheless the metals should be zinced. There is lots of metal in the strut so if it looks like mostly surface then you have years to go, zinced.
I see you have installed a prop shaft zinc - good. You might even go fo a second one. Just keep it at least two to three shaft diameters from the cutlass bearing.

I'll wager that you lost electrical continuity between the various parts. Even if the bolts were tight eventually seawater can creep into the joints and create enough oxidation to cut the continuity through the joint. The joints should be disassembled, cleaned and redone. I may be off base here but I think you could use a sealer or caulk to keep out the water for a long time. The bolting pressure will squeeze the bulk out leaving a metal to metal contact but close off the remaining gaps, and they willl be there, to water entry. Confirm it with an ohmmeter.
All the connections internal should be cleaned and checked. If any are the least bit dirty or oxidized they may lose continuity and thus protection

The voltages here are quite low so it doesn't take much resistance to upset things. An ohmmeter, at any connection, should show less than 1 ohm.

You might also do some quick checks to ensure you don't have some DC electrical leakage from something like a faulty bilge pump or switch. They can leak into standing bilge water if the insulation or seals are compromised in any way. From your description this may not be the case but be sure.

If any bolts from the forward parts of skeg and strut come through the hull and you have access to them you may be able to check the zinc protection levels over the year. Silver-silver chloride half cells are available over the net and with a dmm can be used to monitor. I think I posted a link which included how to make one by going to a jewellers store for a few bucks.

This subject can be huge and the variables widely variable so if you have any further questions and it appears that you still have a problem get someone to give you a hand and repost.

Your transom zinc should be checked also for CLEAN mounting and continuity to the internal bonding. I had a fellow do mine for a couple haulouts and one of the zincs didn't waste away. Next H.O. ohmed it - isolated - so I pulled it and cleaned the mount with the grinder and a disc. I've also found that sometime I have to grind the zincs, the contact area, as any zinc oxide will interfere with a good connection.

Hopefully a full cleanup of all bolted connections and wire connections will take care of it. But you can do a lot to check in the meantime, before next H.O., to see if you have the problem under control.

Good luck any good boating.
Clark


EDIT:

*** There are microohmeters but the last time I checked they were pricey enough that even though I drooled , I decided I would have to do without.* Too close to retirement even though I could have used it a bit at work, I couldn't justify it.** Take a look if any are interested as a few years have now passed and some amazing tools and meters are 'affordable' compared to just a few years ago.

-- Edited by C lectric on Friday 25th of February 2011 11:30:16 PM

John,
The reason I asked the question earlier about the prop shaft being grounded is it clearly has a zinc in the photos. If this is a new zinc and the shaft was not zinced prior, I would say your problem is solved. I would also check the prop shaft to make sure the ss shaft is connected to the grounding system and a bonding wire has not corroded off on the tranny for example, as is pretty common. The electric current will take the shortest path to ground and you want to eliminate the 'battery effect' of the SS reacting with the bronze as there is no question that these are dissimilar metals with a known propensity to interact with each other. I believe you want the shaft tied into the bonding system as close to the coupling as practical to provide the shortest ground path; on my boat for example, there is 10ga bonding wire coming off the back of the transmission close to the shaft coupling even though the engine is of course bonded directly to the earth side of the 12v system as well as the bonding straps.
Marc

Marc

Your*response raises the eternal debate, does the shaft need a grounding brush or contact strap following the coupling? This strap or brush*would be in addition to the transmission and engine grounding wires and shaft zincs you mentioned.

sunchaser wrote:

...does the shaft need a grounding brush or contact strap following the coupling?

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On the advice of our electrical shop we added shaft wipers to our boat.** They did not say they were essential, only that they could provide another layer of protection*if we wanted it.* We also run a pair of shaft zincs on each shaft.* I don't know if the wipers contribute anything significant to the boat's protection but they can't hurt and they were very inexpensive to install.

-- Edited by Marin on Friday 25th of February 2011 02:03:00 PM

John,* Donald Cave in Bermuda has your same boat.* SOMERS is his member name.

Maybe you can PM him and compare notes on your boats.

Just a thought.


JohnP

I use a shaft brush that rides the shaft.

I keep three 2" zincs on the shaft (haul every 2 years) but since these sometime can be thrown or loosen, I finally added the brush. My understanding is to NOT depend on any connection through the coupling which depends on the gear. You do not want any electrical current, though small, running through the bearings to the gear case. Even if you don't damage the bearing there is always a chance that when not running, the gears and bearings could float in a bath of oil effectively isolating the shaft and prop. from the zincs.

Just my understanding.