Too much zinc?

fstbttms · Mar 15, 2014

rpell46 said:
It was suggested that I add a collar zinc between the strut and prop on my 30 Cat.....in addition to the usual shaft zinc to better protect the prop. I sail the New York, Jamaica Bay and Sheepshead Bay areas.

I've heard of problems that can arise by too many zincs and can use some experienced opinions.

Thanks and Happy Sailing...

It is possible to "over-zinc", but two shaft zincs ain't even gonna be close to doing that. In fact, using two shaft zincs is very common. But my professional opinion (based on 19+ years in the hull cleaning business) is; if it ain't broke, don't fix it. Unless you have a compelling reason to double-up on zincs- don't. And some old salt walking the docks with lots of sage advice to give doesn't equate to a compelling reason, in my book.

Charles Erwin · Mar 15, 2014

Do you remember anything about what was involved in your conversation prompting "someone to suggest" additional zinc is needed?

Do you have accelerated zinc wasting? Does your zinc become coated with a whitish deposit? How many zincs are presently mounted on the shaft/propeller? Do you have zinc mounted elsewhere - other than the shaft or propeller. Are the underwater metals in your boat bonded?

If your present zinc compliment wastes 50% over 6-12 months then there are no obvious symptoms that suggest adding more zinc will be helpful. If the zinc wasting is much more rapid additional zinc might be appropriate.

In order to give you a useful answer one needs to know the present rate of zinc depletion. If wasting is too fast or too slow the folks here can comment on the advice you have already received - but it is not possible to confirm your prior advice unless we have a run down on your actual present zinc consumption rate.

In any case - more zinc is no threat to a fibreglass hull.

Charles

ggrizzard · Mar 16, 2014

But the extra zinc outboard may cause vibration.

weinie · Mar 16, 2014

Maybe the extra zinc helps keep the Coney Island whitefish from fouling your prop?

Charles Erwin · Mar 16, 2014

The probability is that your propeller is made from an alloy called manganese bronze. This alloy is not corrosion resistant - in fact it is not bronze at all. Manganese bronze propellers need active fresh zincs in order to avoid pitting and "dezincification."

Adding an additional shaft zinc between changes is perfectly in order. And - it is up to you to be sure that your shaft/propeller mounted zincs are maintained fresh and active in order to protect a manganese bronze propeller. Try not to let your shaft mounted zincs erode more than 50% between changes.

Charles

fstbttms · Mar 16, 2014

rpell46 said:
Thank you. I do keep after the zinc, but am still somewhat concerned, as I mentioned. I am leaning towards that extra zinc at the prop.

Thanks again.

The position of the zinc on the shaft is inconsequential.

BillEarl · Mar 17, 2014

Can someone explain how you can have too much zinc? Admittedly, you don't want to overweight the prop shaft or cause unneeded vibration. But is there some reason that too much will affect the electrolysis/corrosion problems?

patrick33308 · Mar 17, 2014

JMHO, if you are in a marina with other boats that are plugged in to 110AC, U should check your zink often as the current in the water will eat the zink quickly. The more boats near you, the more often you need to check. I have had two zinks on the shaft with no problem or vibration, currently have one zink on the shaft behind the prop and a 2nd zink inside the boat near where the shaft comes out of the transmission so the I wont lose the shaft if it comes away from the tranny. And I have no vibration or issues.
I'm in Tampa and have a diver check and clean the bottom every 4 months or so and do the zink when needed.

Maine Sail · Mar 17, 2014

BillEarl said:
Can someone explain how you can have too much zinc? Admittedly, you don't want to overweight the prop shaft or cause unneeded vibration. But is there some reason that too much will affect the electrolysis/corrosion problems?

Too much can lead to the growth of a self protective coating/crust on the zincs themselves that leads to them becoming nearly ineffective.

On wood boats over protection can actually damage the hull and on aluminum sail drives, hulls or out drives it can lead to alkali corrosion or cathodic disbondment of the paint to the hull, out drive or sail drive leg...

Silver/Silver Chloride Reference Cell Measurements:

Fiberglass = -550 to -900 mV
Aluminum = -900 to -1100 mV
Wood = -550 to -600 mV
Steel = -800 to -1050 mV

Maine Sail · Mar 17, 2014

patrick33308 said:
JMHO, if you are in a marina with other boats that are plugged in to 110AC, U should check your zink often as the current in the water will eat the zink quickly. The more boats near you, the more often you need to check. I have had two zinks on the shaft with no problem or vibration, currently have one zink on the shaft behind the prop and a 2nd zink inside the boat near where the shaft comes out of the transmission so the I wont lose the shaft if it comes away from the tranny. And I have no vibration or issues.
I'm in Tampa and have a diver check and clean the bottom every 4 months or so and do the zink when needed.

The zinc inside the boat is doing nothing for you to protect from galvanic corrosion. It needs to be immersed in an electrolyte (salt water) to work. If your boat sinks then it would work fine...

The inside zinc will work fine to keep the shaft in the boat but not for helping with corrosion.

patrick33308 · Mar 17, 2014

Maine Sail said:
The zinc inside the boat is doing nothing for you to protect from galvanic corrosion. It needs to be immersed in an electrolyte (salt water) to work. If your boat sinks then it would work fine... The inside zinc will work fine to keep the shaft in the boat but not for helping with corrosion.

Appreciate your post. As I mentioned the inside zink on the shaft behind the tranny is there to keep the shaft in the boat in the event it comes lose from the tranny. The outside zink does the corrosion work and gets checked every 4 months or so.

bobbyfingers2001 · Mar 17, 2014

Coney Island Whitefish

weinie said:
Maybe the extra zinc helps keep the Coney Island whitefish from fouling your prop?

That is too funny, I haven't heard that expression in years... I actually grew up in Coney Island and used to take my friends fishing on my boat, I used to tell them to look for the Coney Island Whitefish as we were motoring out the Cropsey Channel...

weinie · Mar 17, 2014

bobbyfingers2001 said:
That is too funny, I haven't heard that expression in years... I actually grew up in Coney Island and used to take my friends fishing on my boat, I used to tell them to look for the Coney Island Whitefish as we were motoring out the Cropsey Channel...

Is this before or after you took girls to watch the submarine races?

patrick33308 · Mar 17, 2014

You should know to stay away from Breezy Point, seems that the bars there have beer taps that aren't grounded, lot of current gets into the water and will eat up a zink real quick.

harleyjef · Mar 17, 2014

No Zincs for freshwater?

Maine Sail said:
The zinc inside the boat is doing nothing for you to protect from galvanic corrosion. It needs to be immersed in an electrolyte (salt water) to work. If your boat sinks then it would work fine... The inside zinc will work fine to keep the shaft in the boat but not for helping with corrosion.

Ok, this may be a "stupid question" but hey, I'm a fairly new boat owner. (I'm new...not the boat...C-22 hull # 7480) But, if I am reading this right, zincs are not needed in fresh water? The reason I ask is, after 4 years of keeping my 22' on a trailer, I may have the opportunity to keep her in a slip in Lake Michigan. No zincs needed?