Hanging anode question

[mrangry]

I have a 45ft grp yacht with a yanmar inboard diesel , a reduction gearbox driving a shaft terminating in a featherstream prop. The only anode the boat has is a single small prop anode which I have noticed is gone, presumable due to me not coating the two securing bolts with something like duralac. I plan to fit another anode when she is hauled out, but meantime I will replace the prop anode whilst still afloat.

I have read about people also using a hanging anode which would be grounded to the dc system but have a question regarding this. Am I right in thinking electrical flow will follow the path of least resistance, therefore will choose the propshaft over the conductor wire connected to the hanging anode, as the propshaft offers less resistance? Would this then render the hanging anode useless?

 

[Stemar]

I have an anode on each hull, connected to the engine, and there's a wire connecting the two ends of the flexible coupling. That should protect the shaft and prop - I hope! I've heard it said that an anode only works if it can "see" what it's supposed to be protecting.

It's always possible (probable?) that your prop anode left ion by ion, rather than in one go. ISTM that the first things it will protect are the mounting bolts, as they're in the most intimate contact. I know that on my old boat, the mounting bolts were either in good nick or gone with the rest of the anode if I'd left it too long.

 

[vyv_cox]

The hanging anode is ideally connected to the prop shaft when the boat is at anchor or berthed. This overcomes any possible resistance in the coupling or gearbox.

As you have a prop shaft a conventional shaft anode is the best protection. The anode for my Bruntons prop is held on with plastic bolts to avoid the problem you have had.

If you attach a shaft anode, ensure it stays in place using the 'two hammers' method. Tighten the screws as hard as possible, keeping the gap between the two halves fairly equal. Now with a hammer in each hand strike the two halves together. Tighten the screws. Keep doing this until they will tighten no more. The anode will not fall off.

 

[VicS]

I have a 45ft grp yacht with a yanmar inboard diesel , a reduction gearbox driving a shaft terminating in a featherstream prop. The only anode the boat has is a single small prop anode which I have noticed is gone, presumable due to me not coating the two securing bolts with something like duralac. I plan to fit another anode when she is hauled out, but meantime I will replace the prop anode whilst still afloat.

I have read about people also using a hanging anode which would be grounded to the dc system but have a question regarding this. Am I right in thinking electrical flow will follow the path of least resistance, therefore will choose the propshaft over the conductor wire connected to the hanging anode, as the propshaft offers less resistance? Would this then render the hanging anode useless?

A relatively small prop anode , vs the relatively large surface area of the prop, will not last long. In the owners hand book Darglow say,
"About the anode
The propeller is protected from electrolysis by the zinc anode. This is designed to be used in addition to other anodes on the boat (hull, shaft anodes etc). If it is the only anode on the boat it is likely to be consumed quickly."
( the fixing screws should be Loctited)

Owners manual and fitting instructions here:- https://www.darglow.co.uk/wp-conten...herStream-SHAFTDRIVE-fitting-instructions.pdf

As Vyv says the best bet is probably a shaft anode in addition to the prop anode.

If you opt for a hull anode there must be a good low resistance electrical path between it and the prop, which will mean bridging the flexible shaft connection or fitting an Electro-eliminator to make direct contact with the shaft. ( Note R&D couplings can be fitted with a special internal earthing conductor)

If using a hanging anode it must , as Stemar says, be able to see the prop and be reasonably close to it. Electrical contact would be best made by clipping to the shaft inboard of the stern gland ... BUT DON' T FORGET IT ! Other wise to the engine block or gearbox and bridging the flexible coupling. The circuit will then be anode .... hanging wire .... engine block and gearbox .... shaft and coupling... prop

Worth looking at this video if fitting a hull anode:-

( Note the low resistance aimed for between anode and prop)

.

 

[RupertW]

I tried the hanging anode approach when my prop anode had done its job and deteriorated to half its size but we were leaving the boat for an unknown number of months. Various lockdowns meant 11 months away. There was still some prop anode left but the hanging one had a barnacle but was otherwise pristine - therefore had done nothing.

The wire was about 5m long and just 4mm2 so maybe a thicker wire would have helped and maybe once the prop anode had completely gone then the least noble metal would have been the hanging one so it would have started to work. But I ended up thinking I’d rather snorkel down and attach one directly (but not completely securely) to the prop shaft if I had to do it again. The downside of that would be remembering many months later not to put the motor in gear before removing it and ideally replacing it properly.

 

[mrangry]

The hanging anode is ideally connected to the prop shaft when the boat is at anchor or berthed. This overcomes any possible resistance in the coupling or gearbox.

As you have a prop shaft a conventional shaft anode is the best protection. The anode for my Bruntons prop is held on with plastic bolts to avoid the problem you have had.

If you attach a prop anode, ensure it stays in place using the 'two hammers' method. Tighten the screws as hard as possible, keeping the gap between the two halves fairly equal. Now with a hammer in each hand strike the two halves together. Tighten the screws. Keep doing this until they will tighten no more. The anode will not fall off.

I had hoped to fit a shaft anode but there isnt enough of the shaft exposed to fit one. Apparently the stainless bolts also react with the zinc causing it to fail prematurely around the two fixing bolts

 

[Farmer Piles]

I bought my boat last winter and she had been in fresh water with magnesium anodes. The prop shaft anode had not only corroded - obviously doing its job - but had become loose on the shaft so making very poor contact. The prop had been new and there was a little pinking on the tips pf the blades. Before I put her in the water, the sea, I put a stonking great shaft anode on. It has stainless securing bolts so some corrosion due to them, but in stayed put and there is no fresh damage on the prop.
I have twin bronze rudders and stainless trim tabs. They, too, have anodes on them; the rudder ones ate through but they lasted the season and were perhaps a bit small - plus I used stainless bolts. The trim tab ones are not too bad at all.
I'm going to have a good crawl around all her skin fittings now that she is ashore.

 

[vyv_cox]

I had hoped to fit a shaft anode but there isnt enough of the shaft exposed to fit one. Apparently the stainless bolts also react with the zinc causing it to fail prematurely around the two fixing bolts

Your best bet is to paint the prop. This reduces the area of the cathode, less work for the anode to do. Hammerite special metals primer and a prop antifoul on top. Velox is great for me but others may work better where you are.

Since I did this my anode life has doubled, two years on the prop and four on the shaft.

 

[VicS]

I bought my boat last winter and she had been in fresh water with magnesium anodes. The prop shaft anode had not only corroded - obviously doing its job - but had become loose on the shaft so making very poor contact. The prop had been new and there was a little pinking on the tips pf the blades. Before I put her in the water, the sea, I put a stonking great shaft anode on. It has stainless securing bolts so some corrosion due to them, but in stayed put and there is no fresh damage on the prop.

I have twin bronze rudders and stainless trim tabs. They, too, have anodes on them; the rudder ones ate through but they lasted the season and were perhaps a bit small - plus I used stainless bolts. The trim tab ones are not too bad at all.

I'm going to have a good crawl around all her skin fittings now that she is ashore.

Why do you have anodes on either of these ?
.

 

[Tranona]

I had hoped to fit a shaft anode but there isnt enough of the shaft exposed to fit one. Apparently the stainless bolts also react with the zinc causing it to fail prematurely around the two fixing bolts

Not sure where that comes from. The zinc is sacrificial instead of the zinc in the yellow metal of the prop, not the stainless. If you don't have an exposed shaft (usually with a P bracket), then you will need a hull anode bonded to the shaft, usually via the gearbox with a bridge across the flexible coupling if you have one. The most common use of a hanging anode is on a boat with a saildrive where it is relatively easy to connect it to the drive housing. In general though they are not very effective because of the distance away from the stern gear through the water and only take over after the closer anodes have gone.

 

[NormanS]

The hanging anode is ideally connected to the prop shaft when the boat is at anchor or berthed. This overcomes any possible resistance in the coupling or gearbox.

As you have a prop shaft a conventional shaft anode is the best protection. The anode for my Bruntons prop is held on with plastic bolts to avoid the problem you have had.

If you attach a prop anode, ensure it stays in place using the 'two hammers' method. Tighten the screws as hard as possible, keeping the gap between the two halves fairly equal. Now with a hammer in each hand strike the two halves together. Tighten the screws. Keep doing this until they will tighten no more. The anode will not fall off.

I suspect that you are actually referring to a shaft anode, when you talk about two hammers. Bruntons propellers are too expensive for that treatment. ?

 

[Farmer Piles]

Why do you have anodes on either of these ?
.

Very good question!!
Because they were on there when I bought her.
Now that you have asked a very logical question, I am asking myself the same thing. Also thinking that the reason the anodes eroded was because they were bolted to S/S.

 

[Farmer Piles]

A very dark place this murky world of anodes and electrolysis!

Second only to anchors and anchoring!!

 

[vyv_cox]

I suspect that you are actually referring to a shaft anode, when you talk about two hammers. Bruntons propellers are too expensive for that treatment. ?

Yes, brain fade there?. I have corrected it.

 

[vyv_cox]

A very dark place this murky world of anodes and electrolysis!

Second only to anchors and anchoring!!

Far more scientific than anchors. The principles are fairly simple, the practicalities a little more complex but can be justified.

 

[Farmer Piles]

Quite, just a vague attempt at a little humour. Especially given the acres of text on anchors and their use.

 

[Spirit (of Glenans)]

Poat deleted.

 

[Slowboat35]

I too find my Darglow prop anode crumbled to bits in 12mths (and the prop blades covered with a skin of rock-hard calcium deposits) yet the bullion-bar hull mounted anode connected with thick copper cable to the enhine just grows barnacles and weed.
Beats me!

 

[Spirit (of Glenans)]

I too find my Darglow prop anode crumbled to bits in 12mths (and the prop blades covered with a skin of rock-hard calcium deposits) yet the bullion-bar hull mounted anode connected with thick copper cable to the enhine just grows barnacles and weed.
Beats me!

The prop anode is doing its job, as designed. Hull anodes are nowadays considered unnecessary.
My boat a Dufour 30 Classic only has a cone anode on the fixed blade prop and doughnut anode behind it, on the saildrive.
I coat the prop in Special Metals Primer and prop A/F and the anodes hardly show any sign of corrosion and last for years.

 

[VicS]

I too find my Darglow prop anode crumbled to bits in 12mths (and the prop blades covered with a skin of rock-hard calcium deposits) yet the bullion-bar hull mounted anode connected with thick copper cable to the enhine just grows barnacles and weed.
Beats me!

The calcium carbonate deposit on the blades suggests over protection or something else going on.
Presumably shaft driven rather than saildrive ? If so I'd measure the volts between battery negative and the prop/prop shaft.

Have you measured the resistance between the hull anode and the prop as described in the video in #4 ?