Aluminium Rudder Stock?

[Dockhead]

We just dropped the rudder out of the Discovery 67 we bought last month, and were surprised to find the rudder stock heavily pitted, with what looks like heavy galvanic corrosion.

I was even more surprised to see that the rudder stock is made of alumnium tubing. I've never seen that material used before for this application. My Moody's rudder stock is a massive piece of stainless steel bar, bonded to the hull anode. Never had any slightest bit of corrosion on that one.

I'm not quite sure what to do about this. Alu is barely more noble than zinc, so I'm not sure whether bonding the rudder stock to the hull anode will help much. Maybe paint it? According to the design drawings, the rudder stock was supposed to be heavily anodised, but it doesn't look like it.

Anyone run into someething like this before?

[tkeithlu]

That's horrible. Aluminum tubing for the rudder stock? And then I see that you've got a spade rudder with only half a skeg What happens when you hit one of those Baltic rocks? Sort of like a soda straw between a thumb and four fingers? Do at least tell me that it is very heavy aluminum tubing.

This is not just a question of strength. Aluminum makes lousy springs. Its elasticity is minimal and its plasticity is considerable. That means that a blow that would cause stainless steel to bend and then return to being straight will bend and not return to being straight. It's a bad choice for that task at any given strength.

Definitely bond it. Then have a serious conversation with Discovery Yachts.

[Matt Johnson]

It's typically not tube, but a solid shaft if done in aluminum. Lots of aluminum expedition yachts with aluminum stock - not an issues if large diameter.

Yes, it should be bonded.

[goboatingnow]

Stainless underwater is worse then aluminium , crevice corrosion and oxygen deprivation have serious effects in stainless , aluminium ductility is better too.

Most rudder stocks in GRP these days are solid aluminium

Note European yachts will NOT bond underwater fittings together in the main. If it’s not bonded don’t bond it. Bonding together in grp has more drawbacks then advantages imho. ( after 30!years and 7 boats )

Not to mention bonding aluminium to zinc isn’t good idea to begin with !!!

[goboatingnow]

If there was pitting I look at several issues. Firstly make sure there no stray electrical connections especially shore power connection to the rudder stock. On a modern European vessel fitted with RCDs mains earth should not be bonded to anything on board

Secondly examine the anti fouling and ensure that the copper antifouling never touch’s the rudder shaft. Aluminium compatible anti fouling should only be used around that area and upto 2” away. Yards and owners slap it on regardless

Also examine the bottom bearing and the bearing housing for corrosion it is typically aluminium these days and the bearing is usually plastic. But lots of types exists

The key is to ensure the shaft is isolated and the anti fouling is kept away.

Aluminium is used far more then stainless these days in rudder stock

( if you want expert advice email Jefa Rudder in Denmark , probably the worlds leaders in rudder construction , nice people to talk to )

[HeywoodJ]

Our 70mm OD aluminum rudder stock (don't even know if it is tube or solid) was installed in 1999 and is still going strong. Very minor amount of pitting over all that time, not enough to cause any concern.

[Shrew]

To be clear there is a difference between Metal Tube (hollow) and round bar stock (solid). Regardless of whether it is is Steel, Stainless, Aluminum, or Titanium, I cannot see anyone using tubing of any kind. It's why it's referred too as as 'Rudder Stock" and not "Rudder Tube".

As for Aluminum, after seeing what galvanic corresion has done to aluminum trim tabs and an aluminum outdrive on a power boat we owned for 10 years, I wouldn't want a rudder stock made of aluminum. My shaft and rudder stock are both solid SS.

[Dockhead]

Originally Posted by Matt Johnson

It's typically not tube, but a solid shaft if done in aluminum. Lots of aluminum expedition yachts with aluminum stock - not an issues if large diameter.

Yes, it should be bonded.

I was wrong -- it's not tubing, but rather a solid billet piece, 148mm in diameter, then tapered.

[goboatingnow]

Originally Posted by Dockhead

I was wrong -- it's not tubing, but rather a solid billet piece, 148mm in diameter, then tapered.

Jeepers that’s huge

[Dockhead]

Originally Posted by goboatingnow

If there was pitting I look at several issues. Firstly make sure there no stray electrical connections especially shore power connection to the rudder stock. On a modern European vessel fitted with RCDs mains earth should not be bonded to anything on board

Secondly examine the anti fouling and ensure that the copper antifouling never touch’s the rudder shaft. Aluminium compatible anti fouling should only be used around that area and upto 2” away. Yards and owners slap it on regardless

Also examine the bottom bearing and the bearing housing for corrosion it is typically aluminium these days and the bearing is usually plastic. But lots of types exists

The key is to ensure the shaft is isolated and the anti fouling is kept away.

Aluminium is used far more then stainless these days in rudder stock

( if you want expert advice email Jefa Rudder in Denmark , probably the worlds leaders in rudder construction , nice people to talk to )

As it happens, Jefa designed this rudder. So I guess it must not be such a stupid design.


Wouldn't you expect that such a rudder stock should be bonded to a hull anode?

[goboatingnow]

Originally Posted by Dockhead

As it happens, Jefa designed this rudder. So I guess it must not be such a stupid design.


Wouldn't you expect that such a rudder stock should be bonded to a hull anode?

If it’s Jefa it’s the best.

No I would not expect it to be bonded. Bonding aluminium to zinc isn’t a great idea. Bonding everything together in a GRP boat isn’t a good idea anyway and is not traditionally done with European yachts.

If in doubt email Jefa Rudder but I know what they will say.