collar anode choice (zinc or alloy and what size) with fitment of new Varifold prop?

[alexg]

I have just bought a new Brunton's 2-bladed Varifold prop for my 28ft Beneteau Oceanis which lives on the Lymington River in a marina (salt water, lots of current and plenty of galvanic action)

The original prop was a 3-bladed fixed affair which was great under power but seriously draggy under sail.

The shaft is a 25mm stainless steel jobby with about 40mm exposed between the exit from the GRP shaft tube and the prop.

In the original set up, I had a 15mm collar anode on the shaft and a nut anode on the end of the shaft aft of the prop. The hull has no anodes fitted. Between June and February, the nut anode was well wasted and the collar anode about 35% wasted.

With the new Varifold fitted, I can no longer use a nut anode at the end of the shaft, so my only anode will be a collar anode.

My questions are therefore:-

1.) Do I continue with a collar anode of the same width, or do I go up to circa 30mm (which will expose very little of the shaft either side)?
2.) Do I use a zinc or an alloy anode (I understand the alloy anodes tend to last longer)?

cheers

 

[Tranona]

Think you will find that your new prop has its own anode so you may not need any additional protection for it.

 

[VicS]

I have just bought a new Brunton's 2-bladed Varifold prop
My questions are therefore:-

1.) Do I continue with a collar anode of the same width, or do I go up to circa 30mm (which will expose very little of the shaft either side)?
2.) Do I use a zinc or an alloy anode (I understand the alloy anodes tend to last longer)?

cheers

I expect Tranona is right.... he usually is! But if you fit a shaft anode there is no need to expose any particular length of shaft AFAIK.

There are three types

Zinc for salt water only

Aluminium for salt water or brackish water

Magnesium for fresh water only

More info on the MGDuff website

Several factors to consider when comparing zinc and aluminium anodes.

Aluminium is slightly more reactive

Aluminum is trivalent, has an RAM of 27 and a density of 2700 kg m^-2

Zinc is divalent, has an RAM of 65 and a density of 7100 kg m^-2

The relative prices of the two vary

 

[alexg]

i do not believe that the two-bladed version of the varifold prop has an anode at all. the 3 and 4 blade variants do.

essentially, i need to know whether i should opt for the fattest collar anode i can fit on the small amount of exposed shaft, or stick with a new version of what i had before.

as for the material... i have read so much conflicting advice about zinc/alloy. alloy appears to last longer than zinc and is also well suited to salt water, but performs well in brackish water too. in view of this, surely alloy is a no-brainer, or am i missing something?

 

[Tranona]

OK if it does not have an anode then you probably don't need one. My FlexoFold 2 blade does not have one either. The anode is on the prop is usually there because of mixed metals in the construction of the prop, particularly the pins running in the bronze. However you may well find that Varifold have used compatible materials. Suggest you ask Bruntons whether you need an anode. If using a shaft anode in the gap between the cutless bearing housing and the front of the prop, mount it right against the prop and leave a minimum of 10mm of clear shaft to allow water to flow out of the bearing and cater for fore and aft movement of the shaft when going in and out of gear.

 

[VicS]

r am i missing something?

yes the meaning of the word alloy!

 

[vyv_cox]

I don't have a Varifold but I do have a two-bladed Autoprop. It might be a guess too far but it seems likely that their materials are similar. The Autoprop has a zinc anode whose life is very short, often less than a season. Adding a shaft anode makes a big difference, extending the Brunton anode life to about two seasons, with the same or a bit more from the shaft anode.

The important point about the anode material is not how long it lasts but how effective it is at protecting the propeller. Zinc is about 0.1 - 0.2 volts more anodic than aluminium in the galvanic series in seawater and will therefore offer better protection.

As VicS suggests, almost every metal we use is an alloy.

 

[VicS]

I don't have a Varifold but I do have a two-bladed Autoprop. It might be a guess too far but it seems likely that their materials are similar. The Autoprop has a zinc anode whose life is very short, often less than a season. Adding a shaft anode makes a big difference, extending the Brunton anode life to about two seasons, with the same or a bit more from the shaft anode.

The important point about the anode material is not how long it lasts but how effective it is at protecting the propeller. Zinc is about 0.1 - 0.2 volts more anodic than aluminium in the galvanic series in seawater and will therefore offer better protection.

As VicS suggests, almost every metal we use is an alloy.

Ah but you are comparing zinc with common aluminium alloys. Aluminium anodes are usually to a US military spec containing a small % of zinc and crucially a very small % of indium. The result is that aluminium anodes are more anodic than zinc by about 0.04 volt making them very slightly more effective, at least no less effective.