When our 31 Bert was hauled recently, the port prop was found to be severely damaged by electrolysis. The starboard prop was fine. The props are 2 years old and are 4 blade nibral. The bonding system is in excellent shape and there were 2 zincs on each shaft plus zincs on each rudder and a large zinc attached to the transom below thw water line.
Any ideas?
Prop damage
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Sandy,
The only electrical connection that props and shafts have within the bonding system is through the shaft/transmission couplings, into the oil filled transmission and then through a bonding wire from the engine to the bonding strap. There are plenty of areas in this line that continuity can be lost. Also the couplings are iron and don't conduct as effectively as copper or even steel.
Check the couplings for rust, especially on the inner faces. Run an ohm meter between the bonding strap & the shaft. Should be near zero ohms...
The best protection & direct contact with with the bonding strap is by using shaft brushes. The "brush" is a small block of bronze that ride, under tension from a stainless steel strap, against the face of the shaft. The brush is wired directly to the bonding strap.
Br,
Patrick
The only electrical connection that props and shafts have within the bonding system is through the shaft/transmission couplings, into the oil filled transmission and then through a bonding wire from the engine to the bonding strap. There are plenty of areas in this line that continuity can be lost. Also the couplings are iron and don't conduct as effectively as copper or even steel.
Check the couplings for rust, especially on the inner faces. Run an ohm meter between the bonding strap & the shaft. Should be near zero ohms...
The best protection & direct contact with with the bonding strap is by using shaft brushes. The "brush" is a small block of bronze that ride, under tension from a stainless steel strap, against the face of the shaft. The brush is wired directly to the bonding strap.
Br,
Patrick
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Pete Fallon
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Prop Damage
Sandygold,
I had a similar problem a few years ago on my 31. Turned out to be the marinas under the dock 110v wiring in contact with a piece of rebar on a piling feeding current into the water, only affected the prop nearest to the dock wiring. I've surveyed other boats that had similar problems, one bad grounding wire on the refridge, another had a bad air conditioner pump wiring and another had damaged thru-hulls from a chaffed shore power cord laying in the water two boats away. If you know anone with a corrosion meter(Guest makes one, availible at West Marine) borrow one, most good surveyors have them. If the boat is still out of the water make sure to check all your thru-hulls carefully and make sure all bonding drops are still intact.
Pete Fallon AMS SAMS
I had a similar problem a few years ago on my 31. Turned out to be the marinas under the dock 110v wiring in contact with a piece of rebar on a piling feeding current into the water, only affected the prop nearest to the dock wiring. I've surveyed other boats that had similar problems, one bad grounding wire on the refridge, another had a bad air conditioner pump wiring and another had damaged thru-hulls from a chaffed shore power cord laying in the water two boats away. If you know anone with a corrosion meter(Guest makes one, availible at West Marine) borrow one, most good surveyors have them. If the boat is still out of the water make sure to check all your thru-hulls carefully and make sure all bonding drops are still intact.
Pete Fallon AMS SAMS
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CaptPatrick
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Mikey,
They're made by several companies & while all do the job, design is somewhat different from one company to the next. You should be able to order, (if not in stock), from most majpr marine supply houses or marine electrical component company.
Also simple to make, but the ss spring strap should be about 1/8" thick x 1" & semi-hard in temper so that a constant pressure can be maintained on the bronze brush. The bronze brush can be made from any suitable piece of bronze & doesn't necessarily have to be concave like the image shows. On old bronze through hull fitting, cut & halved would make a fine brush.
The connecting wire from the bonding strap to the brush should pig tailed & have sufficient slack so that the wire doesn't work harden.
Addendum: Since I'll need a shaft brushes for Patrick Hancock's B31, I went ahead and made up a pair for him... Found materials within the shop, (all original B31 elements from boats worked on), studs tapped into blocks, (cut from old 1 1/4" shaft strut tube), TIG welded. I.D. of strut tubes without cutlas bearings, 1 13/8". Will make up the spring straps another day... Gotta' get back to fiberglassing!
Total time, including scouting up materials: 35 minutes
Br,
Patrick
They're made by several companies & while all do the job, design is somewhat different from one company to the next. You should be able to order, (if not in stock), from most majpr marine supply houses or marine electrical component company.
Also simple to make, but the ss spring strap should be about 1/8" thick x 1" & semi-hard in temper so that a constant pressure can be maintained on the bronze brush. The bronze brush can be made from any suitable piece of bronze & doesn't necessarily have to be concave like the image shows. On old bronze through hull fitting, cut & halved would make a fine brush.
The connecting wire from the bonding strap to the brush should pig tailed & have sufficient slack so that the wire doesn't work harden.
Addendum: Since I'll need a shaft brushes for Patrick Hancock's B31, I went ahead and made up a pair for him... Found materials within the shop, (all original B31 elements from boats worked on), studs tapped into blocks, (cut from old 1 1/4" shaft strut tube), TIG welded. I.D. of strut tubes without cutlas bearings, 1 13/8". Will make up the spring straps another day... Gotta' get back to fiberglassing!
Total time, including scouting up materials: 35 minutes
Br,
Patrick
I've checked my shaft's connection to the bonding system a few times, and I have always found zero resistance. And so I decided to forego installing the shaft brushes, and also to forego the shaft zincs, which in the past have caused me more trouble than not. Something in your post makes me think twice about the wisdom of that however:
Do you think that the connectivity between the prop/shaft and the bonding system, although verified repeatedly on my boat, could actually be intermittent?
I have positive connections across the shaft couplings via a braided copper wire. But I've never cracked open a gear box... so the guts of that magical metal box are pretty much a mystery to me... so I don't know how much actual metal-to-metal contact between the shaft and the engine block is in there.
If the gears only allow contact through separated metal plates swimming in oil, any ideas where I am getting such good conductivity from?
CaptPatrick wrote:The only electrical connection that props and shafts have within the bonding system is through the shaft/transmission couplings, into the oil filled transmission and then through a bonding wire from the engine to the bonding strap. There are plenty of areas in this line that continuity can be lost.
Do you think that the connectivity between the prop/shaft and the bonding system, although verified repeatedly on my boat, could actually be intermittent?
I have positive connections across the shaft couplings via a braided copper wire. But I've never cracked open a gear box... so the guts of that magical metal box are pretty much a mystery to me... so I don't know how much actual metal-to-metal contact between the shaft and the engine block is in there.
If the gears only allow contact through separated metal plates swimming in oil, any ideas where I am getting such good conductivity from?
Sean,
The majority of boats don't use shaft brushes and too many zincs can cause a problem also. That is why you should have the boat from time to time checked with a Kapoc type corrosion meter which reads galvanic action.
It also has to do with the boats around you and the marina your in and whether you are using a galvanic isolator.
The oil in the gear box may act as an insulator and is not the best connection as a direct shaft brush. The gear bearings are pressed onto the shafts and then ride on tapered roller or roller bearings which run on a cushion of oil for lube. The race or outer part of a roller brg are pressed into pockets in the case of the gear. This is how the connection works.
But this only needs to be done if you have a problem.
If you aren't experiencing problems now, don't bother.
The majority of boats don't use shaft brushes and too many zincs can cause a problem also. That is why you should have the boat from time to time checked with a Kapoc type corrosion meter which reads galvanic action.
It also has to do with the boats around you and the marina your in and whether you are using a galvanic isolator.
The oil in the gear box may act as an insulator and is not the best connection as a direct shaft brush. The gear bearings are pressed onto the shafts and then ride on tapered roller or roller bearings which run on a cushion of oil for lube. The race or outer part of a roller brg are pressed into pockets in the case of the gear. This is how the connection works.
But this only needs to be done if you have a problem.
If you aren't experiencing problems now, don't bother.
Pat,
I wouldn't recomend using anything but a motor type of brush as the contact between the shaft due to the conductivity and the slow wear.
Using an actual piece of bronze will wear down much quicker and the casting leaves very tiny pockets of air which will reduce conductivity on the face.
Copper is also better as the arm than stainless.
I wouldn't recomend using anything but a motor type of brush as the contact between the shaft due to the conductivity and the slow wear.
Using an actual piece of bronze will wear down much quicker and the casting leaves very tiny pockets of air which will reduce conductivity on the face.
Copper is also better as the arm than stainless.
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CaptPatrick
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Bruce,
Graphite brushes would be the ideal, but I think that the conductivity of the bronze, even though castings do have some porosity, would still be highly conductive. If the shaft is clean and smooth in the contact area, there will be more of a burnishing effect on the bronze which will smooth over any minor surface porosity.
While maybe the bronze will wear somewhat quicker, it could still be hundreds of hours before the possibility of needing to change the brushes out. The stainless strap would only be used as a tensioning device, not as a conductor, & with the bonding wire attached directly to the stud of the brush.
Sean,
I'm sure there is a lot of conductivity through the gear box and between clean iron couplings & under good conditions you'd get a zero ohm reading between the bonding strap & the shaft. The use of shaft brushes reduces any possible interruption of electrical continuity through the drive train, shortens the path between the shafts & bonding strap, & allows for very easy inspection.
Br,
Patrick
Graphite brushes would be the ideal, but I think that the conductivity of the bronze, even though castings do have some porosity, would still be highly conductive. If the shaft is clean and smooth in the contact area, there will be more of a burnishing effect on the bronze which will smooth over any minor surface porosity.
While maybe the bronze will wear somewhat quicker, it could still be hundreds of hours before the possibility of needing to change the brushes out. The stainless strap would only be used as a tensioning device, not as a conductor, & with the bonding wire attached directly to the stud of the brush.
Sean,
I'm sure there is a lot of conductivity through the gear box and between clean iron couplings & under good conditions you'd get a zero ohm reading between the bonding strap & the shaft. The use of shaft brushes reduces any possible interruption of electrical continuity through the drive train, shortens the path between the shafts & bonding strap, & allows for very easy inspection.
Br,
Patrick
All very excellent information, on a subject in which I'm very interested. Thank you.
Your discussions about the graphite brushes being the ideal / best made me think of something else: I've been running GFO packing in my original-style packing glands. A few years ago I postitively wired the bronze packing glands to the bonding system. I wonder if the GFO packing in them - which I'm pretty sure is heavily laced with graphite - is also acting as a shaft brush. If so, that is a nice little benefit of the GFO.
Bruce: I had a slight stray current problem once, until I realized that my galvanic isolator was cooked, probably from my two weeks in the Bahamas hooked to very questionable shore power. You may remember that you helped me on that issue with some good advice (much appreciated) a few years ago. Since then I've tried to educate myself on how to test the bonding system.
The checks for conductivity / resistance between the different submerged metal components and the bonding system is pretty straightforward. Once the bonding system is established as good... then checking the actual protection that the zincs are giving to the boat... gets into area that is a little voodoo.
The procedure I arrived at was mostly based on Nigel Calder's book, and I copied it below from one of my old posts.
My question is: is this the type of procedure you were referring to when you decribed a "Kapoc type" corrosion meter? I did a search on that and came up empty.
If there is some other testing I can do then I want to do it. My particular marina is famous for stray currents. So far I've been able to thwart them, even while hooked to shore power full time.
........................................................
To check out your boat and it's zincs, you use a Silver/Silver-Chloride Reference Rlectrode and a good-quality digital multimeter. You dangle the electrode over the side and read DC voltages.
When conducting this test, components of (and connected to) the boat's bonding system should have a DC voltage between -900 mV and -1100 mV (relative to a silver/silver-chloride electrode) to properly protect underwater metals from galvanic corrosion.
Readings less than -900 mV indicate the cathodic protection system (zincs) is weak and/or failing.
Readings higher than -1150 mV indicate overprotection (over-zinked), a situation that could damage underwater steel, aluminum alloys and and wooden thru-hull backer plates.
...........................
I've always though that it was cool that when conducting this test, different metals (bronze, stainless, nibral and aluminum) all give you slightly different voltage readings. If you compare the differences in the measured voltages to the "corrosion voltage potentials" for each metal from Calder's book, the voltage differences are about the same.
One of those instances when theory and reality coincide. Very cool.
Your discussions about the graphite brushes being the ideal / best made me think of something else: I've been running GFO packing in my original-style packing glands. A few years ago I postitively wired the bronze packing glands to the bonding system. I wonder if the GFO packing in them - which I'm pretty sure is heavily laced with graphite - is also acting as a shaft brush. If so, that is a nice little benefit of the GFO.
Bruce: I had a slight stray current problem once, until I realized that my galvanic isolator was cooked, probably from my two weeks in the Bahamas hooked to very questionable shore power. You may remember that you helped me on that issue with some good advice (much appreciated) a few years ago. Since then I've tried to educate myself on how to test the bonding system.
The checks for conductivity / resistance between the different submerged metal components and the bonding system is pretty straightforward. Once the bonding system is established as good... then checking the actual protection that the zincs are giving to the boat... gets into area that is a little voodoo.
The procedure I arrived at was mostly based on Nigel Calder's book, and I copied it below from one of my old posts.
My question is: is this the type of procedure you were referring to when you decribed a "Kapoc type" corrosion meter? I did a search on that and came up empty.
If there is some other testing I can do then I want to do it. My particular marina is famous for stray currents. So far I've been able to thwart them, even while hooked to shore power full time.
........................................................
To check out your boat and it's zincs, you use a Silver/Silver-Chloride Reference Rlectrode and a good-quality digital multimeter. You dangle the electrode over the side and read DC voltages.
When conducting this test, components of (and connected to) the boat's bonding system should have a DC voltage between -900 mV and -1100 mV (relative to a silver/silver-chloride electrode) to properly protect underwater metals from galvanic corrosion.
Readings less than -900 mV indicate the cathodic protection system (zincs) is weak and/or failing.
Readings higher than -1150 mV indicate overprotection (over-zinked), a situation that could damage underwater steel, aluminum alloys and and wooden thru-hull backer plates.
...........................
I've always though that it was cool that when conducting this test, different metals (bronze, stainless, nibral and aluminum) all give you slightly different voltage readings. If you compare the differences in the measured voltages to the "corrosion voltage potentials" for each metal from Calder's book, the voltage differences are about the same.
One of those instances when theory and reality coincide. Very cool.
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