Anodes

[Gibbo]

Question ? Where is the best place to fit anodes at the stern of the boat, A - close to the stern tube/prop shaft to give best protection to them but interferes with the smooth water flow to the prop, or B - at the forward end of the swim so as not to disturb water flow to the prop but would this position, being further from the stern tube/prop shaft give less protection.

 

From a corrosion point of view, the closer to the prop the better. The anodes protect the steel hull from galvanic corrosion caused by the prop. Unless you have an aluminium prop in which case they protect the prop.

 

I have seen anodes fitted at the front of the swim and seen much worse hull corrosion near the prop when this is done.

 

I'll let someone else answer the question of water flow as I know slightly less than nothing about it.

 

Gibbo

[Timleech]

From a corrosion point of view, the closer to the prop the better. The anodes protect the steel hull from galvanic corrosion caused by the prop. Unless you have an aluminium prop in which case they protect the prop.

 

I have seen anodes fitted at the front of the swim and seen much worse hull corrosion near the prop when this is done.

 

I'll let someone else answer the question of water flow as I know slightly less than nothing about it.

 

Gibbo

 

That's the usual approach, though there is another argument along the lines that the anodes have a limited 'range', so the further forward you can get them within the physical protection of the counter, the greater the area of the hull that will be protected.

The anode manufacturers would probably like you to do both

Bigger boats/barges often have them on the rudder itself.

 

As for water flow, don't worry about it, I don't think you'll notice any difference.

 

Tim

[John Orentas]

From pure observation (no science involved) the influence of a sacrificial anode extents no more than a 600mm radius around it. I happen to moor alongside a dry dock and I make a point of taking a look at the boats that come in and before they have been cleaned up. I have therefore come to the conclusion that if anyone wants a real benefit from anodes they must be positioned every two feet or so along the hull. As for the bottom, who knows.

[Gibbo]

That's the usual approach, though there is another argument along the lines that the anodes have a limited 'range', so the further forward you can get them within the physical protection of the counter, the greater the area of the hull that will be protected.

 

Indeed, but a steel hull with no prop or shoreline won't really have a problem anyway. Adding a prop creates a problem so anodes are added to help. If no shoreline is used then fitting them as close to the prop as possible is best.

 

If a shoreline is used then adding an isolation transformer or GI is better than hoping anodes will cure the problem.

 

Any more threads about ITs or GIs should be banned.

 

Gibbo

 

From pure observation (no science involved) the influence of a sacrificial anode extents no more than a 600mm radius around it. I happen to moor alongside a dry dock and I make a point of taking a look at the boats that come in and before they have been cleaned up. I have therefore come to the conclusion that if anyone wants a real benefit from anodes they must be positioned every two feet or so along the hull. As for the bottom, who knows.

 

It may appear that way. It's simply that it has more effect closer to it so you'll see less corrosion (often none) close to it and conclude that it did nothing 5 feet away.

 

If you remove it altogether the corrosion over the entire hull *will* become worse.

 

If you add too many you get another problem that causes corrosion. Particularly on aluminium. And on wood it can wreck it in a matter of months with too many anodes.

 

Gibbo

[Keeping Up]

And on wood it can wreck it in a matter of months with too many anodes.

On a metal boat you use a metal anode (a different metal), so presumably on a wooden boat you use wooden anodes - but which wood? And can you get PVC anodes for GRP boats?

[carlt]

On a metal boat you use a metal anode (a different metal), so presumably on a wooden boat you use wooden anodes - but which wood? And can you get PVC anodes for GRP boats?

The classic boat fraternity drags up the balsa anode april fool occasionally but the only problem with too many (ie any, apart from on the rudder) anodes is another hidey hole for rot to live behind.

 

All my spikes are galvanically isolated from the oak, though. The tannic acid doesn't like mild steel.

[Radiomariner]

From pure observation (no science involved) the influence of a sacrificial anode extents no more than a 600mm radius around it. I happen to moor alongside a dry dock and I make a point of taking a look at the boats that come in and before they have been cleaned up. I have therefore come to the conclusion that if anyone wants a real benefit from anodes they must be positioned every two feet or so along the hull. As for the bottom, who knows.

 

This is my observation also, and is also the opinion of my tutor on my diploma course.

Having anodes spaced 1.2m along the hull is the answer, but not only is it not cost effective, but so many protrusions along the hull could cause more serious problems. Big boats (on lumpy salty water) use Impressed Current Cathodic Protection where the natural galvanic current is actually sensed and a nuterelising current generated. Far far to expensive (and big) for canal boats, besides I am not sure that they work in fresh water.

 

Anodic protection of course is secondary protection. The first defence is a good surface coating. The more paint the better!

Some will argue that no paint is better than unsound paint, even under the waterline where the products of oxidation get trapped and spread under the paint layer. This is not galvanic corrosion and is a different subject. For this reason, the "soundness" and adhesion of that first coat is important.

Welds and the steel close to them are particularly vulnerable. Not only is the weld possibly a 'weak' area, but with the weld being of different material from the steel plate, a localised galvanic action action can take place between them. When doing the 'blacking' I paint over the welds first so they have two coats and if I have some blacking (I use comastic) left, I paint the welded areas again.

 

I guess a with a 10mm bottom plate protection against corrosion is not so important, but remember the welds. However if you have a 6mm plate like mine a good coating on the bottom could be worthwhile

[Jason Wilson and Family]

Do you have any other metals than steel, like aluminum, on your boat. The little bit I remember about this seemed to be that having two unlike metals next to each other caused corosion in between them depending on the types. But it still doesn't sound like what you're experiencing. I think in those cases it was one metal or the other, and your anodes weren't either.

[WJM]

Bog standard Liverpool steel boat with a yellow propellor!

[David Schweizer]

The Person who built my boat recconned that sacrificial anodes in fresh water were of little consequence unless there were a lot of chemicals in the water. When I bought it it, the boat was 15 years old and there was virtually no corrosion at all on the hull. I did have anodes fitted when I moved the boat down to the K &A because of all the nitrates that wash down from the cornfields in Wiltshire, but ten years on, both the anodes and hull they are showing very little sign of corrosion.

 

As far as blacking the base plate is concerned, I always understood that for rusting to take place you need water, light and oxygen. As thare is no light under the boat, and precious little oxygen, there is surely very little risk of significant rusting. I do however black the underside of the counter which will get some light and more than its fair share of air from the stirring action of the prop.

[Radiomariner]

I always understood that for rusting to take place you need water, light and oxygen. As thare is no light under the boat, and precious little oxygen, there is surely very little risk of significant rusting. I do however black the underside of the counter which will get some light and more than its fair share of air from the stirring action of the prop.

 

Just water and oxygen David, Light does not come into it. Look at all that rust in that dark damp with condensation bilge!

Anyway the subject was galvanic corrosion different from rust corrosion.

 

On the subject of rust; If the water was pure distilled, life would be rosy for submerged metal. In addition to chemical impurities, the water is teeming with microorganisms, aerobic bacteria, fungi. algea, diatoms, yeasts and others. As far as we are concerned, these form into four main groups:-

Slime formers that form a slimy covering over surfaces reducing oxygen movement and trapping particles of debris

Sulphur reducing bacteria that produce hydrogen sulphide from dissolved sulphates

Sulphur oxidising bacteria that produce sulphur and highly corrosive sulphuric acid

Iron oxidising bacteria that oxidise soluble ferrous iron to insoluble ferric hydroxide.

All these products get trapped next to the hull by marine growth and areas where rust corrosion has already started.

Need I go further?

 

You are right to make sure that the counter is well blacked, as this area is prone to heavy oxygenated water churned up by the propellor. It is also close to the water surface and forms part of the water line (Air/water interface) generally the areas most affected by rust.

 

Light! well it does have a bit of an effect. Certain marine growths really thrive on sunlight, thus trapping more debris, oxides and sulphides, but as its only the area in contact with the steel we are most concerned about a point will be reached where the amount or lenght of growth no longer matters and it is effectively forming a protective barrier. This is a highly debatable point. It will however cause drag, and slow you down or increase your fuel bill.

[John Orentas]

Look at all that rust in that dark damp with condensation bilge!

 

 

Here we go again ! Boats are dank leaky, wet and rusty.

 

You must have some bloody rough boats you lot.

[blackrose]

Here we go again ! Boats are dank leaky, wet and rusty.

 

You must have some bloody rough boats you lot.

 

 

 

 

Just water and oxygen David, Light does not come into it. Look at all that rust in that dark damp with condensation bilge!

 

Yes, light affects photosynthesis - promoting certain bacteria, algae and plants, so it should have nothing to do with corrosion unless the sulphur reducing bacteria etc you mention happen to be photosynthetic organisms? (I guess they are not.)

 

Anyway, I've never understood the oxygen/water theory (that there is no oxygen 2 ft below the surface so you don't need to worry about the baseplate). After all each water molecule is made up of 2 atoms of hydrogen and 1 atom of oxygen. And if there is no oxygen down there then what do the fish breathe?

Edited July 4, 2007 by blackrose

[Gibbo]

I've never understood the oxygen/water theory (that there is no oxygen 2 ft below the surface). After all each water molecule is made up of 2 atoms od hydrogen and 1 atom of oxygen. And if there is no oxygen then what do the fish breathe?

 

There is less *dissolved* oxygen. The oxygen the water is made from doesn't cause corrosion.

 

Fish don't need much oxygen. Far less than it takes to rust steel.

 

How long do you think Titanic would have lasted in an inch of water? What's the difference?

 

Gibbo

[John Orentas]

Yes, light is for photosynthesis, nothing to do with corrosion. However I've never understood the oxygen/water theory (that there is no oxygen 2 ft below the surface so you don't need to worry about the baseplate). After all each water molecule is made up of 2 atoms of hydrogen and 1 atom of oxygen. And if there is no oxygen down there then what do the fish breathe?

 

 

 

If I owned a dry dock and/or earned my living from bitumastic painting of narrowboat hulls, I think I would be looking enthusiastically for good logical reasons not to paint the bottom-plate.

[WJM]

After the war ('39-'45) the Irish Government sank the national fleet of steel commercial barges on the Royal Canal in Dublin. The explanation was that by being underwater they would be preserved. Unfortunately they didnt sink completely, the bits above the water corroded into nothing but the wet bits were still intact in the '80s

[Radiomariner]

After the war ('39-'45) the Irish Government sank the national fleet of steel commercial barges on the Royal Canal in Dublin. The explanation was that by being underwater they would be preserved. Unfortunately they didnt sink completely, the bits above the water corroded into nothing but the wet bits were still intact in the '80s

Oh dear, I am afraid I might have painted a rather gloomy picture, metal, especially iron based products do rust underwater but nowhere nearly as much as in an area where there is free oxygen and moisture.

A better alternative for the above Irish barges would have been to remove them completely from the water and store them somewhere dry (Cucooned or 'mothballed' like the US of A Liberty fleet). Ask yourself this however; which is the more economical answer for a vast number of barges that you are not likely to need but are keeping for a while, just in case.?

 

you must have some bloody rough boats you lot.

 

Yes John, I am very much a "do as I preach, dont do as I do guy" I get a lot of experience on how not to do things that way

The condition of my boat is not the best in the world, (yet a long way from the worst!)

I am afraid that while away at sea my boat suffered long periods of neglect, from which it has not yet fully recovered.

Tell you what John. Why don't your outfit take my boat in and do it up completley to advertise what you can do? For free of course!

 

Rgds

[David Schweizer]

....After all each water molecule is made up of 2 atoms of hydrogen and 1 atom of oxygen. And if there is no oxygen down there then what do the fish breathe?

I guess that you ducked out of the chemistry lessons at school Blackrose. Oxgen is a gas element and water is a liquid compound, their properties are totally different, after all would you put sodium and chlorine on your fish & chips, and enjoy them?