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Old 11-07-2011, 13:41   #1
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Lag Screws in a Lead Keel ?

I am replacing the mast step in my Pearson. The original was a steel plate in the bilges which rotted the bottom of the Al mast. The keel is solid lead encapsulated in glass. The original step was attached with steel lag screws through the fiberglass floor of the bilge and into the lead keel.

I cut off the bottom 3" of the mast and have built a new step with UHMWPE and Al which I planned to attach to the keel with bronze lag screws. Then remembered it would probably be a good idea to check the relative galvanic activity of the bronze and lead before permanent installation.

Turns out the lead is way down the scale from bronze so now I have to decide if that will be a problem or not. Since the lead is more active than the bronze I wouldn't think the screws would go away but what about the lead eating away from around the screw? I plan to bed everything well to keep it dry and isolate the screws from the lead but will that be enough? Iron and lead are next to each other on the scale so maybe I should go back to iron screw and replace the rusted ones every few years?
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Old 11-07-2011, 16:20   #2
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Re: Lag Screws in a Lead Keel ?

Sine the force on the step is nearly all compressive, it seems as though your plan is sound.

I suspect you are using bronze rather then stainless to minimize corrosion, not sure if you have priced them yet, but FWIW, I suspect stainless would be perfectly acceptable. Even if the lead did corrode away, there will not be a large shear load on the fasteners... unless you have MUCH bigger problems to worry about.
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Old 11-07-2011, 16:57   #3
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Re: Lag Screws in a Lead Keel ?

There is so much mass in the lead compared to stainless lag bolts that I can't see it being a problem. Just use a bedding compound, as you stated, and it should be fine. After all, we use stainless screws in aluminum masts and they are fine as long as they are isolated from each other.
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Old 11-07-2011, 17:05   #4
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Re: Lag Screws in a Lead Keel ?

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Originally Posted by s/v 'Faith' View Post
Sine the force on the step is nearly all compressive, it seems as though your plan is sound.

I suspect you are using bronze rather then stainless to minimize corrosion, not sure if you have priced them yet, but FWIW, I suspect stainless would be perfectly acceptable. Even if the lead did corrode away, there will not be a large shear load on the fasteners... unless you have MUCH bigger problems to worry about.
I am working on assumption that the load will be pretty much compression. And yes, bronze instead of SS to eliminate the worry about crevice corrosion in case their is leakage around the screw.

Looking at the galvanic table SS and bronze are side by side so no advantage in going to SS in that regard.

The only reasonable metal near lead on the table is plain old steel.
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Old 11-07-2011, 17:09   #5
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Re: Lag Screws in a Lead Keel ?

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Originally Posted by DeepFrz View Post
There is so much mass in the lead compared to stainless lag bolts that I can't see it being a problem. Just use a bedding compound, as you stated, and it should be fine. After all, we use stainless screws in aluminum masts and they are fine as long as they are isolated from each other.
Had not thought about the mass issue but would not the galvanic corrosion happen where the two metals are in contact?

I do have plenty of SS screws in the mast. Looks like they were coated when originally installed and some of them actually will come out without use of a sledge hammer or dynamite.

I will probably stick with the bronze lag screws, especially since I already have them. Just wanted to check to see if anyone had any horror stories to scare me off.
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