Saildrive anode advice.

[mainsail1]

By chance, my boat has been lifted twice in four weeks which has never happened before.
I was horrified to see that the new anode that I fitted on the Brunton Auto Prop on the first lift is almost wasted away already.
Over the years the anode has disappointingly only taken about three months or maybe four to waste but less than 4 weeks is amazing.
Anyone have an idea why this could be?

 

[pvb]

Electrolytic corrosion is a strong possibility, although the prop should be electrically isolated by the rubber insert.

Did you fit a genuine Brunton anode, or a copy?

Presumably the saildrive has its own anode - has this eroded too?

 

[Daydream believer]

My Brunton anode lasts half a sailing season ( 3 months) & costs an arm & a leg. Did you source a genuine Brunton one, perhaps an alternative with less gold dust in it.?????
If you have an ambassador rope cutter your prop will not be isolated from the sail drive.
I use one sail drive anode per year & do have galvanic isolator etc. the saildrive is supposed to be isolated from the engine, but I am not convinced that it is because of the gear linkages etc that jump the link.
i only use shore power when on board visiting a marina which is 10 weeks per year. I cannot rule shore power stray currents out of the mix

 

[mainsail1]

Yes, I use genuine Brunton Anodes and, as you say, they don't last long but a month seems ridiculous.

I am not sure about the rubber insert, never seen it??

 

[pvb]

Yes, I use genuine Brunton Anodes and, as you say, they don't last long but a month seems ridiculous.

I am not sure about the rubber insert, never seen it??

Brunton Autoprops for saildrives have a rubber insert in the hub to reduce shock loads.

 

[mainsail1]

The sail drive anode has so far lasted over a year.

 

[Daydream believer]

Brunton Autoprops for saildrives have a rubber insert in the hub to reduce shock loads.

The idea is that in sudden shock the rubber shears. The prop will then slip on the shaft. It is then hoped that severe damage to the main drive might be reduced, although not guaranteed. There should still be enough grip for the boat to travel at reduced speed to safety.
I forget the recommended period, but a service cost me about £ 350-00 , 7-8 years ago, & I assume it included a new bush , (plus a beer for the MD's wife & half the staff).

 

[pvb]

I forget the recommended period, but a service cost me about £ 350-00 , 7-8 years ago, & I assume it included a new bush , (plus a beer for the MD's wife & half the staff).

I'm quite glad I stuck with a fixed prop!

 

[Tranona]

By chance, my boat has been lifted twice in four weeks which has never happened before.
I was horrified to see that the new anode that I fitted on the Brunton Auto Prop on the first lift is almost wasted away already.
Over the years the anode has disappointingly only taken about three months or maybe four to waste but less than 4 weeks is amazing.
Anyone have an idea why this could be?

Just a characteristic of the prop with large amounts of copper alloy (bronze) and stainless steel with differing potential mixed together in an electrolyte and a sacrificial anode that is too small in relation to the metal it is protecting. Irrelevant that it is a saildrive - shaft drives have the same issues. It is electrically isolated from the drive because of the rubber bush and the drive is isolated from the engine. The anti shock function is secondary - you would really have to hit something very hard to shear that - more likely to do other damage to the drive first. However failure of the bond between the hub and the rubber is not uncommon - it happens with standard aluminium fixed blade props as well. One consolation is that it is at least replaceable in a bronze hub unlike the aluminium ones which often split when pressing a new bush in. But then a complete alloy prop costs less than replacing a bush in a Bruntons!

Short anode life is common on feathering and 3 blade folding props because of the high proportion of stainless in the mechanism plus for some makes poor quality bronze alloys. Simple 2 blade folders like the Flexofold I have either do not need anodes at all or only small ones which last well over a season. Mine is now 11 years old and no sign or any wear or corrosion despite being in the water all year round. Now I have coated the blades with Velox keeps clean as well. The saildrive anode was changed last year after 5 years, but still had lost less than half its weight.

 

[Daydream believer]

deleted

 

[mainsail1]

Many thanks for the answers. Today, I have sprayed the Autoprop blades with zinc as an experiment to see whether the new prop anode degrades as quickly now.

 

[Tranona]

If you have used one of those zinc sprays described as "cold galvanising" doubt it will substitute for zinc as an anode, partly because the zinc content is minimal and secondly because the bolted on anode will erode first because it is closest to the two dissimilar metals. It may have some effect because, if it stays on it will reduce the surface area of the blades exposed to the electrolyte . If you want to go down the route of reducing the area then suggest you coat at least the blades with Velox which is also an effective long term antifoul. Others have reported longer anode life after applying Velox. antifoulingpaint.net/applyingvelox.html Available from most chandlers in the UK.


This is my boat and the Velox has now been on for 2 years and last week was lifted after 9 months in the water, mostly stationary, and the inside of the blades were clean and a light slime on the exposed outside. Fair amount of work preparing and applying, but one of my better decisions.

 

[mainsail1]

Thanks Tranona. Sounds good advice.
Yes, my reason for putting "cold galvanising" on was to reduce the metal area exposed to electrolysis although I realise zinc will degrade for just that reason. I will try Velox when the zinc has degraded. To remove it now would be a bit of a pain.

 

[Daydream believer]

Yes, my reason for putting "cold galvanising" on was to reduce the metal area exposed to electrolysis although I realise zinc will degrade for just that reason. I will try Velox when the zinc has degraded. To remove it now would be a bit of a pain.

One has to be careful with reducing the area exposed to electrolysis.
I asked the MD of Brunton his thoughts on antifoulling my prop & he said that if I encountered difficult conditions & an area of unprotected prop was exposed then that part would accept all the corrosion. This MAY cause imbalance.
If there were no antifoul protection then the entire prop would accept that corrosion & its effect would be limited as to have no major effect on balance.
If one is experiencing high corrosion then one might make sure that there is no damage to the tips. The pitch of a Brunton is set by the bevel on the tip & if one were slightly out then that could cause serious vibration.

 

[mainsail1]

Interesting.
Well, I thought I was probably walking into a minefield but after 20 years of wasting £100's of pounds on prop anodes from Brunton I thought the time had come to experiment. Time will tell as always...............................Might be quite nice if Brunton tried to fix the problem.

 

[Daydream believer]

Well, I thought I was probably walking into a minefield but after 20 years of wasting £100's of pounds on prop anodes from Brunton I thought the time had come to experiment. Time will tell as always...............................Might be quite nice if Brunton tried to fix the problem.

Apart from supply cheaper & bigger anodes, what would you expect them to do to fix the problem of electrolysis in differing metal?. I think one would hardly like it made in aluminium. If you want plastic then a Kiwi prop would be the answer.
One can hardly blame Brunton for an issue that has plagued props for ages. I have seen some that have never needed an anode & some folders that have disintegrated round the gears in 3-4 years. There are so many factors, Shore power,, a stray current , all sorts of things beyond the control of the prop builder

 

[yotter]

+1 for Kiwi prop, plastic and stainless steel is the way to go

 

[penfold]

Were you berthed in a marina? It could be a stray current from another boat.

 

[duncan99210]

This is an interesting thread. I have a Bruntons Autoprop on a Saildrive and my prop anode usually lasts about 2-3 x 4 month seasons. Weh I first got the boat, the anode didn’t survive a season before falling off: I questioned the Bruntons staff at boat show and was told to use nail varnish round the fixing holes. I do this and haven’t lost an anode since. The wastage of the anode is slow and never reduces it so that the inset bit of the anode is exposed:it annoys me that I throw away about half the weight of the zinc as it erodes to the point where the fixing holes erode away.
Worth pointing out that we rarely go to a marina and connect to shore power, most of the season is spent at anchor.

 

[mainsail1]

I am very aware of stray currents and do not connect to shore power if I am away from the boat. For the month in question the boat was on a pontoon in Lymington.
I will try the trick of shielding the Anode fixing holes with some kind of paint. Interesting idea as they are an early point of failure as you say.