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Privilege · 30-05-2016, 18:21

I have a fiberglass catamaran. I obtained a walk-on mooring but at low-low tide the port rudder touches bottom so I made up a shorter temporary rudder out of a steel plate welded to a galvanized pipe. I painted it with epoxy barrier coat. It seems to work fine on the boat and doesn't seem to effect handling or show signs of corrosion, however my saildrive zincs are lasting three months.

I have all underwater metals bonded together, including the rudder, and they are also bonded to a hanging zinc. The rudder also has a zinc but this and the 'fish' don't corrode as quickly as the drives zinc. I am plugged to shore power but no other boats around. Salt water.

Is the rudder likely to be the cause of the excessive corrosion? If it is, I can live with it for the convenience of the location but if there is likely to be another problem I need to address it. I appreciate that it's impossible to properly diagnose but any guidance would be appreciated.

smac999 · 30-05-2016, 18:50

what was the old one? fiberglass?

if you added a lot more metal that needs protecting then the zincs will probably last less. though if it's coated the metal shouldn't be "exposed" to water and it shouldn't make a difference from before.

do you have a galvanic isolator / iso transformer? you should if plugged in.

RaymondR · 31-05-2016, 03:17

Check the isolated voltage between the steel rudder and the rest of the bonding system with a multimeter if there is a significant difference then you probably have a corrosion potential. I would guess at "significant" being any more than about 0.1 volt but someone out there might have a better understanding.

Privilege · 31-05-2016, 05:13

Quote:

Originally Posted by RaymondR

Check the isolated voltage between the steel rudder and the rest of the bonding system with a multimeter....

I've been searching online how to conduct an isolated voltage test but can't seem to find the right way. Can you assist?

leftbrainstuff · 31-05-2016, 05:48

You need to check the rudders potential relative to the common bonding system.

You then size zincs so that they sacrifice consistently with other zincs. Surface area effects the galvanic current flow and therefore decay rate.

You measure the potential difference of each bonded item relative to a common ground or reference point. Probably the engine.

Have a marine electrician conduct a galvanic survey if you dont feel comfortable doing it yourself.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

foufou · 31-05-2016, 08:02

Your zinc problem is likely specific to your setup. All prior comments are spot on. As a teenager my family had a boat with a "barn door" steel rudder. Big zinc, no problems

Cadence · 31-05-2016, 08:12

Check to insure your boat isn't what is providing the ground for the shore power. Just a SWAG.

out42 · 31-05-2016, 09:11

Shore power may not be the cause of the difference but it could well be accelerating the process to the extent that it may not be making a non-issue into an issue. Assuming you have done the previous poster's wise suggestions about potential differences, can you unplug for a longish period then check?

Privilege · 31-05-2016, 09:25

Thanks for all your suggestions. Whilst waiting for delivery of a galvanic isolator (which might take some while to Bermuda) I'm going to disconnect the ground wire on the shore power inlet which apparently does the same thing although doing so could make your hair stand on end when it's not supposed to.

I'll also get an isolation voltage test done. On another thread, I've just read that aluminum anodes are the proper anodes for saildrives in saltwater. I've checked my SD50 manual and it states;

NOTICE:
Failure to use the correct anode material may result in inadequate protection and excessive corrosion of underwater drive system components. Use only zinc or aluminum anodes in brackish and saltwater applications. In freshwater applications, use aluminum or magnesium anodes...

I've always thought that zincs were the proper anode for saltwater. Thoughts?

RaymondR · 31-05-2016, 12:01

Quote:

Originally Posted by Privilege

I've been searching online how to conduct an isolated voltage test but can't seem to find the right way. Can you assist?

If the rudder is connected to the bonding system, disconnect the wire from the rudder and check for any current or voltage between the rudder and the wire from the bonding system. If it's not connected just check between the rudder and the nearest item which is bonded.

If there is no voltage difference there should not be any current anyway but it is the current which is doing the damage.

There is a fairly good treatment on corrosion in the Boatowner's Illustrated Handbook of Wiring, author Charlie Wing printed by McGraw Hill ISBN 0-87742-383-0

bsurvey · 31-05-2016, 12:57

SMAC is correct, you need a galvanic isolator the rudder has nothing to do with it

Cadence · 31-05-2016, 13:22

After reading a no. of recent posts. It would seem most people need a isolation transformer. I don't know if iso/trans is a marine product name. I would go to an electrical supply house not something saying marine. If spending the money buy a buck boost transformer and wire it such that the boost could be adjusted. 7KVA would probably serve most.