galvanic isolator question

[Sadlerfin]

We have a Bav 30 2006.  When we purchased the boat she had been left unattended for at least a season but connected to shore power. When lifted for the surveyor to do his job it was found that the Anode was mainly intact and the prop had degraded to a point of almost falling off. We changed the Prop and fitted a new Anode.

The boat is now in a marina connected to shore power. At the last life out I was concerned that the Anode had only lost say 30% of its body so we changed it anyway. We keep her in all winter connected to shore power and lift her out for a yearly inspection each spring. She is coming out next week, hopefully we'll find a well eaten Anode, however, my concern is if the Anode is not degraded then is there likely to be any damage elsewhere eg Prop and other metals?

Our understanding is this could be an earthing problem so if the Anode is not working would a galvanic Isolator be a good idea and how best do we bond the anode to the shaft

All this is not my area of expertise so advice would be appreciated.

Thanks Derek

[tiger79]

A galvanic isolator is always a good idea if you're going to use shore power! I fitted one a few years ago. Also, I don't like to leave shore power connected when my boat's unattended, so always unplug it when I leave the boat.

[Symphony]

Is it the anode on the saildrive housing or an anode on a folding or feathering propeller? The one on the saildrive does nothing for the prop and in theory the standard aluminium propeller does not need an anode nor does it generally corrode. However it can corrode if the the isolating washer behind the cone nut is missing. Equally the saildrive anode does not protect a folding/feathering prop because it is also isolated from the drive housing, and many types have their own anodes because the prop itself is made of dissimilar metals.

30% anode wear in one season on a saildrive anode is quite common. although the extent of wear on the split anode you have is deceptive because the bulk of the weight is hidden. I have the same anode on my boat and have just replaced it for the first time since new in 2015 and it was about 30% worn by weight, although it looked well worn with most of the visible part eroded. The boat has been in the water all year round.

A galvanic isolator is a good idea - in our club it is compulsory if you use shore power, but from what you say I don't think you have a real problem. Check the wear this time and suspect you will find it low. 3-4 seasons' life is common.

[dawntreader]

I fitted a galvanic isolator soon after purchasing the boat in 2006. I have only ever replaced the anode once - 2011 - as the anode had weakened close to the securing screws. My propeller and saildrive are in pristine condition. This is despite being on marinas in many countries this side and the Caribbean side of the Atlantic. We have been living on the boat more than the house the past 4 years so it has been extensively subjected to galvanic currents. I now paint the anode close to the securing screws to reduce exposure in that area (tip from this forum). I have discussed the lack of wear with Volvo engineers as it did worry me that the anode may have not been working but they said the measures  I have taken are why there is no wear.

[Lyra]

If there is nothing on boat that should be operating while mooring, I would not stay connected to shore power.
For maintaining the batteries against self discharge a small solar panel will do the job - for a small panel a regulator is not required so the setup should be simple (this is what I did when my boat was ashore once for several months).
I now have a permanent large panel, so I never leave the boast connected to shore power. Apart from not having to worry about galvanic corrosion, it also saved me grief on one occasion when there was an electric fault in the marina that resulted in 380V - many boats had their chargers destroyed as well as additional equipment.

[Sadlerfin]

Thanks for the replies, I will make two points.

1, We do not have a folding Prop so just one anode on the saildrive housing.

2, For 6 months the boat IS connected to shore power in order to run the de-humidifier and in v cold weather a heater.

Reading the above I can't understand why the prop had degraded, the engineer told me he had never seen one like it before.

Thanks for the help, am still not sure what to do will have to see how the anode is when we take her out next week 

[tiger79]

Thanks for the replies, I will make two points.

1, We do not have a folding Prop so just one anode on the saildrive housing.

2, For 6 months the boat IS connected to shore power in order to run the de-humidifier and in v cold weather a heater.

Reading the above I can't understand why the prop had degraded, the engineer told me he had never seen one like it before.

Thanks for the help, am still not sure what to do will have to see how the anode is when we take her out next week

You need to check whether the prop is correctly electrically isolated.  And also confirm that the saildrive is electrically isolated from the engine.

You coud also review whether a dehumidifier and heater are really necessary, especially in the south of England.  I haven't used either in my boats over the last 25-30 years, left afloat all year round.  Ventilation works fine!

[Symphony]

Thanks for the replies, I will make two points.

1, We do not have a folding Prop so just one anode on the saildrive housing.

2, For 6 months the boat IS connected to shore power in order to run the de-humidifier and in v cold weather a heater.

Reading the above I can't understand why the prop had degraded, the engineer told me he had never seen one like it before.

Thanks for the help, am still not sure what to do will have to see how the anode is when we take her out next week

As I suggested earlier a common cause of corrosion of an aluminium saildrive prop is not fitting the Delrin isolating washer in front of the prop before the securing cone goes on. The prop must be electrically isolated from the drive shaft which is stainless (as are most of the internals). This isolation is achieved by a rubber bush in the prop boss isolating the splined drive from the prop itself and the washer which isolates the cone nut (which is screwed to the shaft).

Provided the original coating on the blades and hub is intact, and even better if an additional coating such as Trilux or Velox is applied, the prop should last virtually indefinitely. There are many saildrives 30 and 40 years old still with their original props. Another cause of corrosion is if the coating breaks down, particularly on the tips so that the bare aluminium gets exposed, but this is mainly localised and takes a long time to erode large bits of metal.

[Craig]

Small solar panel should keep batteries fully charged, no worries. No need for shore power if leaving the boat unattended, even in Britain.

If you have de-humidifier or heater, why not connect these directly to shore power rather than through the boat's systems.

I would follow the directions in this post as well, in order to isolate the prop as there will be times you will be on shore power.

Craig
"Shirley Valentine"
Gold Coast
Australia