Galvanic corrosion
Discussion
While beaching the boat on saturday to scrubb off the accumulated crud underneath, we noticed that there is significant corrosion to the propellor. It is a folding prop, on a sail drive. The propellor itself has two small zincs which had both completely disappeared, and there is a larger zinc on the saildrive leg. There is also another zinc on the hull which is bonded through to the engine etc.
It seems that the two on the prop should not have gone that fast. The boat has been in the water since the beginning of May. Anyone have any ideas what can cause corrosion this quickly? Luckily was still had the old fixed prop which we refitted.
It seems that the two on the prop should not have gone that fast. The boat has been in the water since the beginning of May. Anyone have any ideas what can cause corrosion this quickly? Luckily was still had the old fixed prop which we refitted.
Is the bonding correctly connected?
Do you connect to shore supply when the boat is left?
Are you moored in an area with lots of stray currents?
I lost a prop after the old man went 18 months without a haul out. 30 minutes of revving the arse of the engine I went through all the possible reasons for no propulsion, the final straw was seeing the prop shaft spinning and nothing.
I had to get a diver to confirm!
Have you got a copy of Nigel Calder (Sp?) Electrical and Mechanical Book for Boats? Very useful.
What do you sail?
Do you connect to shore supply when the boat is left?
Are you moored in an area with lots of stray currents?
I lost a prop after the old man went 18 months without a haul out. 30 minutes of revving the arse of the engine I went through all the possible reasons for no propulsion, the final straw was seeing the prop shaft spinning and nothing.
I had to get a diver to confirm!
Have you got a copy of Nigel Calder (Sp?) Electrical and Mechanical Book for Boats? Very useful.
What do you sail?
ALawson said:
Is the bonding correctly connected?
Do you connect to shore supply when the boat is left?
Are you moored in an area with lots of stray currents?
What do you sail?
The boat is a Leisure 27 - a bilge keeler with a Volvo engine and saildrive. The boat is on a swinging mooring and not often connected to shore power. As it isn't in a marina, I doubt there are many stray currents aboout.Do you connect to shore supply when the boat is left?
Are you moored in an area with lots of stray currents?
What do you sail?
The anode on the hull is bonded through to the engine block, which is electrically connected to the saildrive which has its own anode. Both of these have minor pitting.
The only thing that occured to me was the prop itself is made of dissimilar metals - the hub is an aluminium alloy of some sort and the blades are steel, and that the connection between the prop and saildrive is somehow suspect. I don't know how it is possible to improve this as the prop is just mounted on a splined shaft with a castellated locking nut.
It was lucky we decided to beach it, as the corosion was bad enough to be able to see the outside of the bushes the pins that hold the blades on fit through. Losing a blade was not far away.
I take it your saildrive looks like this.
Whats the connection between the prop and the saildrive, I have has a ss split pin put through by prop shaft just incase the keyway were to corrode due to anode falling off!
If you main anode if ok and the ones on the saildrive are ok then I wouldn't worry "that much" especially as you can easily dry out and inspect, also are you saying that you have two additional anodes on the prop over the ones (No 6) on the saildrive.
Looking at the attached galvanic series I would expect the zinc anodes to protect everything, however I am not sure wether this would stop bimetalic corrosion!
Whats the connection between the prop and the saildrive, I have has a ss split pin put through by prop shaft just incase the keyway were to corrode due to anode falling off!
If you main anode if ok and the ones on the saildrive are ok then I wouldn't worry "that much" especially as you can easily dry out and inspect, also are you saying that you have two additional anodes on the prop over the ones (No 6) on the saildrive.
Looking at the attached galvanic series I would expect the zinc anodes to protect everything, however I am not sure wether this would stop bimetalic corrosion!
ALawson said:
I take it your saildrive looks like this.
Whats the connection between the prop and the saildrive, I have has a ss split pin put through by prop shaft just incase the keyway were to corrode due to anode falling off!
If you main anode if ok and the ones on the saildrive are ok then I wouldn't worry "that much" especially as you can easily dry out and inspect, also are you saying that you have two additional anodes on the prop over the ones (No 6) on the saildrive.
The sail drive does look like that, but the prop looks like this (sorry about the size):Whats the connection between the prop and the saildrive, I have has a ss split pin put through by prop shaft just incase the keyway were to corrode due to anode falling off!
If you main anode if ok and the ones on the saildrive are ok then I wouldn't worry "that much" especially as you can easily dry out and inspect, also are you saying that you have two additional anodes on the prop over the ones (No 6) on the saildrive.
The anodes on the prop are labelled 7/8 and there is one each side. These were new this season and have completely disappeared. The method of fixing is a bit different to that one, as instead of having parts 2,3 and 4 there is a single large stainless steel nut that has a split pin through it. I would have thought that the saildrive anode would also protect the prop, if the prop anodes disappeared but that hasn't happened and the hub has been damaged.
I don't think it is cavitation damage. There was a lot of white metal salt crusted to the hub and the damaged areas have gone kind of spongey - very soft and crumbly.
The prop is the Volvo one designed for that saildrive, so shouldn't be any problems there.
I will do some more research, as to replace the hub will probably cost a load of money and it will be stupid for it to just happen again.
The prop is the Volvo one designed for that saildrive, so shouldn't be any problems there.
I will do some more research, as to replace the hub will probably cost a load of money and it will be stupid for it to just happen again.
Maybe the is a current leak on your boat i.e bilge pump connection live and in water. Had a case recently when the boat was over bonded and the copper/brass skin fitting was also bonded as was the stern drives but in close proximity once the bonding to the copper/brass skin fitting was removed the indicator on the meter dropped to normal .Note the boat had always chewed through anodes according to the owner. Seems to be o.k now .
Edited by Rum Runner on Tuesday 14th August 22:29
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