I recently saw a video clip showing the installation and use of sacrificial anodes on a propeller shaft, are these legit and do they actually work?
TIA!
TIA!
Sacraficial Anodes
- Thread starter AnyS100
- Start date
Completely legit and commonly used on any inboard powered boat. The sacrificial anode, often zinc but sometimes aluminum or magnesium depending on where the boat is sailed, becomes the least noble metal in an electrolytic circuit that includes all of the underwater metal it’s connected to. This allows the anode to protect other more expensive parts from galvanic corrosion.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,435
Welcome to the SBO Community
“getting the NY40 ready for local racing and snowbirding down to the Caribbean”
40 years of boat saltwater sailing, and your just learning about zinc anode and galvanic corrosion of under water metals?
Yes they work. In the Caribbean waters running without a zinc you may find your prop sitting on the sandy bottom with your anchor.
“getting the NY40 ready for local racing and snowbirding down to the Caribbean”
40 years of boat saltwater sailing, and your just learning about zinc anode and galvanic corrosion of under water metals?
Yes they work. In the Caribbean waters running without a zinc you may find your prop sitting on the sandy bottom with your anchor.
Yes, they definitely work and are essential to protect your prop and other underwater metals from galvanic corrosion. Use magnesium in fresh water, zinc in salt water.
This old photo shows my prop shaft at autumn haul-out a few years ago. The left anode has seen two seasons and will be replaced before launch, while the right anode has seen one season and will be replaced after one more season. There is always one new anode on the shaft when the boat splashes in the spring.
You need at least one shaft anode to protect the prop. I doubled up on anodes because we went through a period of stray current activity (apparently resolved for now) in my harbor. I’ve just continued the practice because I’m not seeing any downside to it.
This old photo shows my prop shaft at autumn haul-out a few years ago. The left anode has seen two seasons and will be replaced before launch, while the right anode has seen one season and will be replaced after one more season. There is always one new anode on the shaft when the boat splashes in the spring.
You need at least one shaft anode to protect the prop. I doubled up on anodes because we went through a period of stray current activity (apparently resolved for now) in my harbor. I’ve just continued the practice because I’m not seeing any downside to it.
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I run 2 Magnesium shaft anodes on my boat, plus a cone anode on my new MaxProp.
I sail in the fresh waters of Lake Michigan, and I replace 1 of the 2 anodes each year (So I get about 2 years out an anode).
I do not have a galvanic isolator…and may be looking at one. My friends don’t go through anodes as fast as I do…
Greg
I sail in the fresh waters of Lake Michigan, and I replace 1 of the 2 anodes each year (So I get about 2 years out an anode).
I do not have a galvanic isolator…and may be looking at one. My friends don’t go through anodes as fast as I do…
Greg
@AnyS100 , how many years have you been without a zinc ?
The next time your boat is on the hard, clean up the prop and have a close look at the blades for any sign of corrosion. If it's heavy, maybe a cause for concern and have a pro look at it. This is what the zinc protects against. Hopefully you're still safe.
I sail on Georgian Bay, so no salt water. I have magnesium sacrificial anode on my shaft. Rather than completely replacing it every season, I take it off and turn it on a small lathe to remove the corroded crust practically to the bare metal, typically about 2 mm. I make sure that hole and shaft are also bare metal, before reassembly. This allowed me to use the anode for two to three seasons.
Any comments on that?
PS: if you do turning, you have to make sure that the magnesium filings do not spark and catch fire,
Any comments on that?
PS: if you do turning, you have to make sure that the magnesium filings do not spark and catch fire,
The turning approach should be fine, though probably not strictly necessary. As far as I know magnesium doesn’t form a passive layer the way some metals do, at least in fresh water, so the corroded layer you’re shaving off probably wasn’t hurting anything.
The effectiveness of an anode is directly related to its surface area. If you turn the anode down too much it will have a smaller surface area, but the 2mm at a time that you mentioned probably won’t hurt. Making the anode especially smooth also reduces surface area though, so leaving it with a rough surface might be favored there.
True, it is not like aluminum or stainless steel, so no passive layer. On the other hand, even though the corrosion process of the anode is rather slow and the corrosion products on the surface are porous, there is still some covering of the metal, reducing its active surface and also hindering diffusion.
I did not try to make the surface really smooth, but I will keep your suggestion to keep it rough in mind when I do it this coming spring. I can make it with tiny groves. This may balance the slight reduction of the surface area.
Magnesium anodes are not pure magnesium. Turning them is not a bad idea to remove oxide. I'm not so sure about putting grooves in, they could potentially fill with oxide and reduce function faster than at first blush. But hey, you can try that and see how it goes.
I usually just replace all my anodes, but I do have one that is a large one and it takes years to need changing. I typically just wire brush it clean in place. But I'm in the ocean so not directly related... I used to have two others that were large and had not been changed in about 20 years, according the the previous owner. Two years ago, they were looking pretty much gone and I looked everywhere for a similar anode to replace them but there was nothing even remotely close. So I simply removed them and redesigned the mounting system to use available anodes. These new anodes were a lot smaller and this year when going through my anodes I could see they had worked.
I guess I'm saying all this to support your idea of reusing anodes...
dj
I usually just replace all my anodes, but I do have one that is a large one and it takes years to need changing. I typically just wire brush it clean in place. But I'm in the ocean so not directly related... I used to have two others that were large and had not been changed in about 20 years, according the the previous owner. Two years ago, they were looking pretty much gone and I looked everywhere for a similar anode to replace them but there was nothing even remotely close. So I simply removed them and redesigned the mounting system to use available anodes. These new anodes were a lot smaller and this year when going through my anodes I could see they had worked.
I guess I'm saying all this to support your idea of reusing anodes...
dj