Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 30-05-2016, 19:21   #1
Registered User
 
Privilege's Avatar

Join Date: Oct 2009
Location: Bermuda
Boat: Privilege 435
Posts: 362
Images: 12
Is having a steel rudder inviting galvanic corrosion?

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.
__________________

__________________
Privilege is offline   Reply With Quote
Old 30-05-2016, 19:50   #2
Registered User

Join Date: Jun 2013
Location: canada
Posts: 1,699
Re: Is having a steel rudder inviting galvanic corrosion?

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.
__________________

__________________
smac999 is offline   Reply With Quote
Old 31-05-2016, 04:17   #3
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,373
Images: 7
Re: Is having a steel rudder inviting galvanic corrosion?

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.
__________________
RaymondR is offline   Reply With Quote
Old 31-05-2016, 06:13   #4
Registered User
 
Privilege's Avatar

Join Date: Oct 2009
Location: Bermuda
Boat: Privilege 435
Posts: 362
Images: 12
Re: Is having a steel rudder inviting galvanic corrosion?

Quote:
Originally Posted by RaymondR View Post
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?
__________________
Privilege is offline   Reply With Quote
Old 31-05-2016, 06:48   #5
Registered User
 
leftbrainstuff's Avatar

Join Date: May 2011
Location: San Francisco and Australia
Boat: Liberty 458
Posts: 1,978
Re: Is having a steel rudder inviting galvanic corrosion?

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
__________________
leftbrainstuff is offline   Reply With Quote
Old 31-05-2016, 09:02   #6
Registered User

Join Date: Sep 2013
Location: San Francisco Bay
Boat: Chung Hwa Boat Builders, Magellan 36
Posts: 264
Re: Is having a steel rudder inviting galvanic corrosion?

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
__________________
foufou is offline   Reply With Quote
Old 31-05-2016, 09:12   #7
Registered User
 
Cadence's Avatar

Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 6,058
Re: Is having a steel rudder inviting galvanic corrosion?

Check to insure your boat isn't what is providing the ground for the shore power. Just a SWAG.
__________________
Cadence is offline   Reply With Quote
Old 31-05-2016, 10:11   #8
Registered User

Join Date: Aug 2015
Posts: 112
Re: Is having a steel rudder inviting galvanic corrosion?

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?
__________________
out42 is offline   Reply With Quote
Old 31-05-2016, 10:25   #9
Registered User
 
Privilege's Avatar

Join Date: Oct 2009
Location: Bermuda
Boat: Privilege 435
Posts: 362
Images: 12
Re: Is having a steel rudder inviting galvanic corrosion?

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?
__________________
Privilege is offline   Reply With Quote
Old 31-05-2016, 13:01   #10
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,373
Images: 7
Re: Is having a steel rudder inviting galvanic corrosion?

Quote:
Originally Posted by Privilege View Post
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
__________________
RaymondR is offline   Reply With Quote
Old 31-05-2016, 13:57   #11
Registered User

Join Date: Sep 2014
Posts: 397
Re: Is having a steel rudder inviting galvanic corrosion?

SMAC is correct, you need a galvanic isolator the rudder has nothing to do with it
__________________
bsurvey is offline   Reply With Quote
Old 31-05-2016, 14:22   #12
Registered User
 
Cadence's Avatar

Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 6,058
Re: Is having a steel rudder inviting galvanic corrosion?

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.
__________________
Cadence is offline   Reply With Quote
Old 03-06-2016, 02:54   #13
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,373
Images: 7
Re: Is having a steel rudder inviting galvanic corrosion?

Quote:
Originally Posted by bsurvey View Post
SMAC is correct, you need a galvanic isolator the rudder has nothing to do with it
Does AC cause electrolytic corrosion?
__________________
RaymondR is offline   Reply With Quote
Old 03-06-2016, 07:57   #14
Registered User
 
Privilege's Avatar

Join Date: Oct 2009
Location: Bermuda
Boat: Privilege 435
Posts: 362
Images: 12
Re: Is having a steel rudder inviting galvanic corrosion?

Thanks everyone. I'll fit the isolator and report back in a couple of months with an update on the state of my anodes.
__________________
Privilege is offline   Reply With Quote
Old 03-06-2016, 22:25   #15
Registered User

Join Date: Jun 2013
Location: canada
Posts: 1,699
Re: Is having a steel rudder inviting galvanic corrosion?

Quote:
Originally Posted by RaymondR View Post
Does AC cause electrolytic corrosion?
galvanic isolators and iso transformers block DC current. which flows in the AC ground line.
__________________

__________________
smac999 is offline   Reply With Quote
Reply

Tags
corrosion, rudder, steel

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Galvanic corrosion - Bonding rudder bearing? svfinnishline Construction, Maintenance & Refit 1 17-02-2013 02:27
Shore Power and Galvanic Corrosion scalleia Electrical: Batteries, Generators & Solar 7 21-07-2010 20:05
The Galvanic Series and Corrosion GordMay Construction, Maintenance & Refit 8 22-05-2010 11:26



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:52.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.