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I'm dropping a new-to-me sailboat (a thread on this soon!) into the water tomorrow. There is currently only one old zinc on the prop shaft, and no others anywhere else on the boat. The strut was just replaced with a newly crafted stainless steel one, and I never saw the old strut, so I'm unsure if there was a zinc on it or not.
How do I know the appropriate amount of zincs to use, and if one should be added to the strut?
Forgive my typing, (rum)
I have a zinc on the shaft, one on my rudder, and one in my heat exchanger ( I replace every 2 months), it is a small pencil zinc, and dissolves fastly. Without a doubt replace the old zinc with a new one on the shaft.
My 2 cents.
Personally I have 2 on the shaft, and the pencil in the heat exchanger.
I'd also suggest you check it/them regularly, at least initially. I am in a 'hot' marina, and 2 zincs only last 6 months (!!!). I have just installed a galvanic isolator - it'll be interesting to see of it helps.
The only reason to install a zinc on the strut is if there is evidence of corrosion. Otherwise, leave it be. I have seen struts break where they are drilled for zincs. Two on the shaft may not be strictly necessary but is good insurance.
In reference to the comment about being in a "hot marina" and my own experience (using a freshwater zinc in saltwater - not a good idea), I recommend having you or someone else take a look at the zincs periodically while the boat is in the water to see how quickly they are depleting.
I'd point out that if you even suspect your marina as being a "hot" marina, don't dive on the boat in the marina...anchor out someplace away from any stray currents to check the zincs...
So who is the zinc expert here? I have a Helsen 22 and there are NO zinc's on it. I am using 12VDC from the house battery for navigation lights and a British Seagul 40+ outboard. Do I need them?
Since you have an outboard, you may not need them on the boat; but you may have one on the outboard leg.
Any metal that spends its life in sea water needs to be protected by a sacrificial anode - the zinc. That is why they are on the prop shaft and in the heat exchanger.
It appears that the Volvo Penta (MD5A I think) in my boat is salt water cooled and there is no heat exchanger. Does this make sense? Would there be another place to put a zinc on a salt water cooled motor?
If your engine is raw water cooled there will not be a heat exchanger. Most but not all engines with raw water cooling will have a replaceable zinc somewhere on the block. Your manual should show where.
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