Erosion of P Bracket

mhouse

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Hi.
Each winter the stainless P bracket of my Dehler shows erosion to the flat side of the P bracket.
Image0039.jpg
. The Bracket is not connectet to anything internally and the cutlass bearing is phenolic . The shaft anode erodes a fair bit each year . I do use mains power with a galvanic isolator but I unplug when the boat is left . A solar panel is left charging the batterys.

Any ideas how to stop this other than bolt an anode onto the bracket. I do hang an anode over the side when moored which is disolving fast this year so much so that the 8mm stainless wire eroded and parted.
Thanks
 

vyv_cox

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I'm struggling to see what you mean by 'erosion', Is that the metallic part on the bracket? Just looks like poor paint aghesion to me but maybe I'm not looking at what you mean.

Stainless steel is pretty much immune from corrosion as a P-bracket. It could pit in seawater but I see no evidence of that. Your anode is not protecting the P-bracket, it isn't connected to it electrically.
 

mhouse

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Sorry the picture was taken after all winter ashore. When the boat is lifted out the patches are really clean with deep pits and almost a crystaline look .
 

vyv_cox

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It sounds like the stainless steel that has been used is not 316 but something lesser, perhaps a 304 or maybe even a 400 series. Your 'crystalline' appearance could be etching, which is not typical of a 300 series stainless. Anodes are not the best option to prevent pitting and I suggest a good epoxy primer and two-pack epoxy paint might be the better option. Before priming make sure the P-bracket is perfectly clean, abraded to about 120 grit and finally degreased.

What is the wire of your overside anode connected to? Something odd seems to be happening if its stainless is disappearing so quickly.
 

VicS

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Just a thought. The result of turbulence caused by the shaft anode.
I dont know if that sort of thing happens with stainless steel . It certainly does with copper and copper alloys. Vyv?

Presumably there is not the space to fit a shaft anode ( even the collar type) between the "P" bracket and the prop.


BUT there is something horrible going on if 8mm stainless wire on the hanging electrode failed.

Just what are you connecting the wire to.
 

mhouse

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The wire is connecter to the pushpit which is bonded into the battery negative. The keel is also connected via one of the keel bolts. I have found normall I get a small amount of wastage of this anode but it does stop the keel rusting. The 1st year I had the boat I did not use the anode and had some interesting rusticles on the keel.

Has any one any details of the voltage etc I can expect between the hanging anode and the negitive pole ?

thanks
 

VicS

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Has any one any details of the voltage etc I can expect between the hanging anode and the negitive pole ?
The connection to the anode should be a very low resistance. There should therefore be no significant voltage between it and the pushpit rail, the battery terminal or the keel.

It won't protect anything not connected to it and it needs to be reasonably close to be effective

Checking the correct level of cathodic protection can be done with a reference electrode such as Coreng's Galvatest electrode http://www.galvatest.com/
 

SHUG

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Could it be that the stainless is being electroplated with zinc to give the crystalline appearance?? (Over to VicS or Vyv!!)
 

lydiamight

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As the owner of a Dehler 36CWS with exacatly the same problem we found a solution by getting the yard to weld a small piece of stainless stud on to one side of the P bracket.
We then slid over a small "button" type anode and fixed it by screwing a nut onto the stud. . It has definitely helped and the anode needs replacing every year so it must doing something.
 

VicS

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the anode needs replacing every year so it must doing something
Not necessarily.

A piece of zinc bolted to a chunk of stainless steel and immersed in seawater is going to corrode whatever.

Dissimilar metals, galvanic corrosion and all that stuff!
 

mhouse

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Thanks. I guess the plate sides of the P bracket must be made of a deifferent grade of stainless as the tube part thats welded onto the bottom never erodes.Its interesting that another Dehler has the same problem if its the same 36 that my neighbour owned the age is the same so it could be a faulty batch.

Ive hope solved the anode problem.
The negative from the battery charger was loose and touching the case so the battery switch was by passed and with the solar panel etc was making the hull positive etc.
The new anode is still there so far.
 

jtwebb

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I had the same problem and it appears the stainless steel is not good quality, mine corroded down the leading and trailing edge. I did get it welded up and had studs welded on each side to take small Plastimo Anodes part no 38221. They slowly go and the P bracket is protected. What is the objection to having anodes?
 

rudolph_hart

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I'm on my 3rd Dehler, and here are my experiences with P-brackets & anodes:

1. 1987 D34. S/Steel P-bracket was epoxy painted. Shaft anode 3/4 gone each year.

2. 1991 D34Top. S/Steel P-bracket unpainted. Looked a bit like your photo. Painted it with epoxy tar. Shaft anode 3/4 gone each year.

3. 1994 D35CWS. Haven't stripped P-bracket back yet, as antifouling isn't peeling. Anode consumption erratic. Standard shaft anode gone in a year. Large 'bullet-shaped' Duff shaft anode (2x size of std ones) also gone in a season.So fitted 2x standard ones, and the one nearest prop (3-blade folder) 3/4 gone, the other 1/4 gone in a year.

One year, also had a pan-scourer-sized 'clump' of what I believe to be electrolytic deposition around cooling water intake (scary, but not occurred since!)

Thread Drift All 3 P-Brackets seemed identical, and none of them had the 'protuberances' on the starboard side of yours. What are they for?
 

vyv_cox

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I had the same problem and it appears the stainless steel is not good quality, mine corroded down the leading and trailing edge. I did get it welded up and had studs welded on each side to take small Plastimo Anodes part no 38221. They slowly go and the P bracket is protected. What is the objection to having anodes?

As I said in post #4, anodes are not the best way to protect against pitting corrosion because the reaction is so localised. They work very well for general corrosion, steel in seawater for example, and for galvanic corrosion, e.g. a prop on a shaft. They will give some protection from pitting but a coating would be better.
 

Heckler

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As I said in post #4, anodes are not the best way to protect against pitting corrosion because the reaction is so localised. They work very well for general corrosion, steel in seawater for example, and for galvanic corrosion, e.g. a prop on a shaft. They will give some protection from pitting but a coating would be better.
Why is the pushpit connected to an anode?
Stu
 
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