Paint bubbling around anodes?

[Paul C]

15 minutes ago, Bee said:

 Anodes should be placedat regular spacings all along the hull but then you might not get in a lock. 

 

The better narrowboats have regularly-spaced recesses for anodes, so its possible to fit them close enough. I've always thought that just the front and rear, on a decent length narrowboat, isn't enough.

 

Obviously for a widebeam the extra few inches it adds to the width isn't the critical issue of the narrowboat, but without a suitable recess its still vulnerable to collision damage.

[Alan de Enfield]

Just now, Tracy D'arth said:

are you talking magnesium in fresh water Alan or zinc in salt water? 

 

Both.

I have normally replaced Mags on my NBs roughly  every 18 months, I found that it you have bolts welded onto the hull it is simple to bolt the mag anodes on instead of welding them on, and so simple you can do it by feel without having to lift the boat out of the water.

 

The Zincs are inspected annually on my cruiser at lift-out time, these last ones have been on two years now but will be replaced  this winter.

The cruiser is GRP so the anodes dont get as much 'hammer' as on a steel boat as whilst I have 13 thru hull fittings that they are protecting (they are all bonded inside the hull) there is very little metal.

The shaft anodes seem to stay pristine the longest.

 

 

 

[system 4-50]

On 29/07/2022 at 22:45, system 4-50 said:

 

This anode was about 6 years old.

[cuthound]

38 minutes ago, Paul C said:

 

The better narrowboats have regularly-spaced recesses for anodes, so its possible to fit them close enough. I've always thought that just the front and rear, on a decent length narrowboat, isn't enough.

 

Obviously for a widebeam the extra few inches it adds to the width isn't the critical issue of the narrowboat, but without a suitable recess its still vulnerable to collision damage.

 

I have often wondered why narrowboats don't use a long strip of magnesium, like a rubbing strake, bolted to the lower part of the hull.

 

Not only would this give better anodic performance, but would act as an underwater rubbing strake in locks and when mooring.

[Iain_S]

If anodes are welded on, I would think it unlikely that the paint/blacking in the near vicinity is to the same standard as the rest of the hull. The welding will remove some paint (has to, to get clean metal!) and it would be very difficult, if not impossible, to prepare the area of hull below the anode to a reasonable standard for painting. I guess that once "bubbling" starts, it can continue under the paint/blacking.

 

Is there anything in the water that makes it slightly more conductive than normal fresh water?

[blackrose]

15 hours ago, Bee said:

. Anodes are a bit like God, I don't believe in God but I don't want to risk going to hell so I don't argue with the church, I don't believe in anodes nut I've got some on the boat.

 

So in other words you don't believe the electrochemical process of galvanic corrosion exists? 

15 hours ago, Paul C said:

 

The better narrowboats have regularly-spaced recesses for anodes, so its possible to fit them close enough. I've always thought that just the front and rear, on a decent length narrowboat, isn't enough.

 

Obviously for a widebeam the extra few inches it adds to the width isn't the critical issue of the narrowboat, but without a suitable recess its still vulnerable to collision damage.

 

I have a couple of slim anodes down each side of the boat which are protected by the baseplate overhang. 

[blackrose]

14 hours ago, cuthound said:

 

I have often wondered why narrowboats don't use a long strip of magnesium, like a rubbing strake, bolted to the lower part of the hull.

 

Not only would this give better anodic performance, but would act as an underwater rubbing strake in locks and when mooring.

 

It might result is overprotection? You can have too many anodes. 

[blackrose]

9 hours ago, Iain_S said:

If anodes are welded on, I would think it unlikely that the paint/blacking in the near vicinity is to the same standard as the rest of the hull. The welding will remove some paint (has to, to get clean metal!) and it would be very difficult, if not impossible, to prepare the area of hull below the anode to a reasonable standard for painting. I guess that once "bubbling" starts, it can continue under the paint/blacking.

 

Is there anything in the water that makes it slightly more conductive than normal fresh water?

 

You can paint the area under the anode before it's welded on it not. It really makes little difference as the anode will protect the steel behind. The paint will have to be removed in the tab area to weld the anode on but the anode tab can always be painted over afterwards. Again it makes no difference. None of this is relevant to the paint bubbling 1ft away from the anode.

 

Yes impurities in the water can make it a more active electrolyte in which galvanic action can occur. 

[cuthound]

52 minutes ago, blackrose said:

 

It might result is overprotection? You can have too many anodes. 

 

True, but other than paint blistering there seems to be little downside to having to many anodes. The metal under the blistered paint always seems to be uncorroded.