Second hull anode on saildrive boat?

Jabs said:
I have a Brunton prop on a Volvo saildrive.
The prop is isolated from the saildrive shaft by a rubber bush.
It is isolated from the rope cutter by PTFE washers.

Tony
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We will get to the bottom of this one day. What you say agrees with what I thought originally, but if it is isolated then the hull anode is offering no protection to the prop, only to the SD casing to which it it is connected.
 
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Jabs said:
I have a Brunton prop on a Volvo saildrive.
The prop is isolated from the saildrive shaft by a rubber bush.
It is isolated from the rope cutter by PTFE washers.

I have fitted an extra anode to the gearbox, situated in full direct and close view of the prop and it is very effective.
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If the prop is isolated, i don't understand the need for the extra anode, or how it can be effective ? Or is this just effective at making the leg anode last longer ?

I am now trying a/f on the prop.

Bruntons concern is that is the prop edges get the A/F worn off, they could erode.

Tony
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Vyv seems to have had success antifouling his prop.
 
VicS said:
We will get to the bottom of this one day. What you say agrees with what I thought originally, but if it is isolated then the hull anode is offering no protection to the prop, only to the SD casing to which it it is connected.
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Which was the point of my original question at the start of the thread - an extra anode to provide extra protection to the (very valuable) saildrive leg.
 
Cantata said:
Which was the point of my original question at the start of the thread - an extra anode to provide extra protection to the (very valuable) saildrive leg.
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You really should not need it. The 130 drive has a massive anode - twice the bulk of the 120. As I said above, mine last minimum of 3 seasons to less than half remaining. Just monitor the wastage and if it is excessive - greater than once a year then investigate why - for example shorepower without a galvanic isolator. A hull anode won't reduce the wastage of the ring anode as the latter will erode completely first as it is closest to the housing so will only be effective when the ring is gone.

The only situation where you might need an additional anode is if you alternate between sea and fresh water for example if you moor in a canal and sail in the sea. Thena hanging magnesium anode is useful. Quandry who kept his saildrive boat in the Crinan had this arrangement.
 
Before I bought my flex o fold I was told that the Brunton's auto prop was not suitable for my SD20 sail drive because it needs a bushing to protect the sail drive. Looking at the hub it could well be insulated between the shaft and outer hub. There is a clear break caused by a plastic layer. Not sure if this is primarily a v or electric insulation.
I suspect the issue is that someone is fitting an inappropriate prop and the hull anode is a special mod
 
The bushing does two things. First provides a shock absorber, second isolation. Yanmar insist on both functions whereas I think Volvo are less insistent on the shock absorbent element.
 
It's not a 130 saildrive, it's a er MS25 I think, which has the same anode as the older 120. Anode wastage in one season (since I bought this boat) was 'hardly any', with the standard Volvo ali prop.
Boat is not left plugged in for long periods, at all, in fact hardly ever unless we are marina-hopping on holiday.
Galvanic isolator ready to install.
Just being manically careful, really.
 
As stated above, I have fitted an extra anode. It has reduced the rate of deterioration of the saildrive anode and the prop anode.

The Bruntons prop IS isolated from the shaft.

I have researched this topic with Volvo, Bruntons, MG Duff, and several electrical experts. There are as many opinions on this whole subject as there are questions.

Tony.
 
Jabs said:
As stated above, I have fitted an extra anode. It has reduced the rate of deterioration of the saildrive anode and the prop anode.

The Bruntons prop IS isolated from the shaft.

I have researched this topic with Volvo, Bruntons, MG Duff, and several electrical experts. There are as many opinions on this whole subject as there are questions.

Tony.
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It makes sense that the leg anode depletion will be slower with the hull anode fitted, but If the prop is isolated how can a hull anode have any effect on the depletion rate of the prop anode ?
 
To clarify:

I am talking about an EXTRA anode in clear view of the leg bonded to the gearbox.

It has made a huge difference to the life of both the prop and leg anodes.

Anodes appear to be a black art!

I do have a huge anode, further aft connected to the electrical ground.

I have a galvanic isolator.

Tony
 
Jabs said:
To clarify:

I am talking about an EXTRA anode in clear view of the leg bonded to the gearbox.

It has made a huge difference to the life of both the prop and leg anodes.

Anodes appear to be a black art!

I do have a huge anode, further aft connected to the electrical ground.

I have a galvanic isolator.

Tony
Click to expand...

The purpose of which is ........ ?
 
Jabs said:
To clarify:

I am talking about an EXTRA anode in clear view of the leg bonded to the gearbox.

It has made a huge difference to the life of both the prop and leg anodes.

Anodes appear to be a black art!

I do have a huge anode, further aft connected to the electrical ground.

I have a galvanic isolator.

Tony
Click to expand...

The hull anode can have no influence on the prop anodes as there is no electrical connection to them. As you say you prop is isolated from the housing, so how can an anode bonded to the housing have any impact? The purpose of the prop anodes is nothing to do with the drive housing, but to protect the mixed metals in the construction of the prop. That is why essentially one material props such as the standard aluminium prop and some folding props such as my Flexofold 2 blade do not have any anodes at all.
 
All I can tell you is that it works.

I had tried everything else. It does appear to defy logic but if you atart to think of the seawater as being both the electrolyte and also a conductor, you can get somewhere.

You can argue that the electrical connection between the prop and the hull anode is through (high resistance) saltwater.

So why do I have more total anode degradation at my home berth which is thirty feet from piling to which there is no electrical connection?

Note to Vic S. Don't understand the question??

If it's about the galvanic isolator, that is to ensure there are no earths floating!

This whole subject is an art, not a precise science!

Tony
 
Hi Vyv,

The prop has a boss which bolts onto the shaft. The shaft and retaining system butt up against this boss.

The boss has a resilient rubber bush which drives the prop hub. This electrically insulates the hub, hence blades, from the boss and shaft.

Best option is to look at the Boat Show. They are keen to talk about the products.
I suspect the boss and hub have some sort of drive spline system to carry the substantial torque loads involved. I imaging the, should the rubber be eroded, there would be metal to metal between the hub and boss.

As soon as I'm back, I'll look for drawings.

It is an amazing piece of kit from an engineering point of view.

Tony
 
Jabs said:
All I can tell you is that it works.

I had tried everything else. It does appear to defy logic but if you atart to think of the seawater as being both the electrolyte and also a conductor, you can get somewhere.

You can argue that the electrical connection between the prop and the hull anode is through (high resistance) saltwater.

Tony
Click to expand...

Thinking back to my Uni chemistry days, I don't think it can work like that. Imagine a beaker of salt water with two strips of dis-similar metals in it. One will be the natural anode and the other the natural cathode because of their relative positions in the table.

If you connect the two together with a cable electrons start to flow from through the saltwater from one plate to the other and back along the cable to the first plate and so a circuit is formed and anode depletion begins.

If the only electrical connection between the two metals is the saltwater, then surely electrons would have to flow in both directions across the water to form a circuit and surely that can't be possible? :confused:

Richard
 
Jabs said:
All I can tell you is that it works.

I had tried everything else. It does appear to defy logic but if you atart to think of the seawater as being both the electrolyte and also a conductor, you can get somewhere.

You can argue that the electrical connection between the prop and the hull anode is through (high resistance) saltwater.

So why do I have more total anode degradation at my home berth which is thirty feet from piling to which there is no electrical connection?

Note to Vic S. Don't understand the question??

If it's about the galvanic isolator, that is to ensure there are no earths floating!

This whole subject is an art, not a precise science!

Tony
Click to expand...

You misunderstand how electrolysis works. The sea water is the electrolyte, not the connection. For a hull anode to protect the prop it must be connected electrically to the enode which as it has lower potential will erode in preference to the material of the prop. As there is no connection between the prop and the anode (or indeed anything else as it is electrically isolated) the only protection is the dedicated prop anode.

The question from VicS is why do you have an anode connected to the electrical ground? It can serve no purpose.
 
Jabs said:
..............................
..........

I do have a huge anode, further aft connected to the electrical ground.

Tony
Click to expand...

VicS said:
The purpose of which is ........ ?
Click to expand...

Jabs said:
Note to Vic S. Don't understand the question??

Tony
Click to expand...

What is the purpose of this "huge anode"? You say it is connected to the electrical ground but what underwater appendages are bonded to it for protection by it ?
 
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