Rudder stock corrosion

[Baggy]

Found corrosion in the foot of my s/s rudder stock
which sits in a s/s shoe

Would fitting an Anode or wire connecting to an Anode stop further corrosion

Must be about 25 years old

Hole's are no more than a few mm deep

 

[savageseadog]

A propeller cap anode would work

 

[Tranona]

That is crevice corrosion as the stock sits in stagnant oxygen starved water. You may well find similar corrosion in the shoe. An anode will not do anything, but cleaning and filling the holes then painting the area of the stock that sits in the shoe might reduce it.

 

[Tranona]

A propeller cap anode would work

Can you explain that please?

 

[macd]

As Tranona wrote, Baggy, it's crevice corrosion, occurring in highly localised cells in the presence of low-oxygen seawater. It's common for rudder shoes to have drainage holes so that muck can get out and water can circulate: does yours have them? If so, are they blocked?

Crevice corrosion can be ameliorated with anodic protection but I suspect the anode must be extremely local to the affected sites, not just attached somewhere on the affected piece of metal. The specifics are beyond my pay grade, but maybe Vyv Cox will pop up with information.

General info on crevice corrosion from Vyv here: https://coxengineering.sharepoint.com/Pages/Crevice.aspx

 

[Baggy]

Crevice corrosion

So the only way to prevent this happening is to allow fresh sea water to
pass over the rudder stock end
The rudder stock, which is heavy, sits enclosed in a quite substantial s/s shoe, maybe drill a draining hole on the bottom plate of the shoe, where the end sits will allow fresh sea water to pass
would this help ?




Oh thanks macd just read your post

 

[Tranona]

Yes. Does it have a bearing in the shoe? and does the stock actually rest on the base of the shoe? Probably not possible to change now but a good design shoe has a top hat Delrin insert in the shoe which provides the bearing surface and stops the stock from touching the shoe. You may be able to adjust the height of the rudder to create a gap at the base. This together with a drain hole will allow water to circulate.

 

[macd]

Baggy: drilling a suitable hole into a big lump of stainless isn't going to be easy. If the shoe can be removed without major surgery, might be best to do it on a pillar drill (slowly, with lots of cutting fluid). But you may find there is already a drain hole, perhaps blocked by antifoul or general crud. Best to have a good root around before taking other action.

 

[Baggy]

The New Shoe

May get the s/s man to drill

 

[Baggy]

Yes. Does it have a bearing in the shoe? [B]No[/B]
and does the stock actually rest on the base of the shoe? [B]Yes[/B]

Probably not possible to change now but a good design shoe has a top hat Delrin insert in the shoe which provides the bearing surface and stops the stock from touching the shoe. You may be able to adjust the height of the rudder to create a gap at the base. This together with a drain hole will allow water to circulate.

 

[Tranona]

Presumably that is a copy of the original and if it worked OK then no real reason to change the design. However, think I would have "improved" it with a deeper socket and a replaceable delrin bush - although having said that I went that route when making a new shoe nearly 30 years ago and the bearing is unworn (spare still a coffee table ornament). I think that is as much to do with there being two other good bearing points for the stock so it runs perfectly true.

Suggest for your shoe you drill a drainage hole and fill the pitting in the stock with epoxy so that the bearing point on the bottom of the stock is epoxy rather than steel.

 

[Baggy]

Thanks Tranoma

will do:encouragement: