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skip2 12-19-2003 02:31 AM

Corrosion in the marina
 
I have a 43'' sailboat, and am tied up in a 300-slip marina with all fiberglass (mostly sailboats) near me; there are several vacant slips in the area. Over my protests, a few days ago a 56''steel trawler (live-aboard)was allowed to come into the slip next to me;and we share a power pole (A and B side). I''m very concerned about possible corrosion effects due to this large steel boat so close to me-but can not find anything specific in writing to support my concerns. The Trawler is a new boat (made in China), in very good nick, and equipped with an islolation transformer. However, the marina (not in the US) wiring is questionable, with many feeder lengths hanging in the water, etc. Can anyone provide any insight/what to watch for or check, or supporting arguments as to why a steel boat should not be berthed next to fiberglass boats--- or do I really not have a problem? Many thanks,

Silmaril 12-19-2003 08:55 AM

Corrosion in the marina
 
There are many good books on the subject of yachts and electical issues including electrolysis. Nigel Calder''s books come to mind.

In general, I think the main issue is stray currents, which when coupled with an un-isolated ac source on your boat spells trouble. For safety reasons, all of your grounds on your boat need to be common, whether ac or dc, and that is where the trouble begins. In a perfect world, there would never be any current "leaking" into the water and we wouldn''t have to worry. But seeing as we don''t have the "perfect" world, you have to protect yourself.

Use a galvanic isolator whenever you are plugged in. It breaks the leak to ground for your vessel, and isolates you from stray current in the water. For a battery to work, you have to complete a circuit, and your boat, the water (esp. salt water) and the shore power connection make a wonderful battery, so by using a galvanic isolator, you break that circuit.

Prior to my installing the Guest 30amp unit on my boat, I was going through shaft zinks at about two per month! I left myself plugged in all the time to keep everything topped off during the season. I had two bullets on the shaft and inspected them often. Now, with the isolator installed, my two zinks last an entire season, from May to November!

If your boat is completely bonded, and you are worrying about extra protection, you can hang a zink fish attached to your stern rail over the side while at dock.

I also carry a plug in circuit tester, and use it whenever I travel. I plug it in before I plug my boat in. As long as the lights come up green, I am good to go!

The Guest galvanic isolator also has a ground fault detection display built right in, and will break your circuit if it detects a fault.

The most important thing is to get your vessel out of the stray current circuit, in addition to giving any stray current something to feed on other than your valuable underwater metals. That is why zincs are called "sacrificials", the least noble metal goes first (zinc), then the next(bronze) then the next(steel). I guess if everything below the waterline was made of platinum (one of the most noble metals), we wouldn''t have to worry! If you always have a zinc out, it will go first. Be aware that if you have bronze through-hulls, they MUST be electrically bonded to your boat to protect them also.

petercra 12-19-2003 05:41 PM

Corrosion in the marina
 
Whether or not to bond through hulls to the rest of the boat''s underwater metals is one of the great debates. Nigel Calders, Boatowner''s Mechanical and Electrical Manual, 1990 ed, p. 107 discusses this and calls bonding the conventional wisdom.

Recognize, however, that a through hull in a fiberglass hull with rubber hoses is electrically isolated and unlikely to corrode significantly in the absence of strong stray currents in the water. There''s simply no circuit for the electrons to flow through to balance the copper and other ions which leave the through hull and dissolve in the seawater. Of course, the lack of bonding means they probably aren''t connected to a zinc to protect them either. But, connections can go bad and zincs can erode to nothing.

I feel safer with all underwater metals isolated.

GordMay 12-19-2003 10:36 PM

Corrosion in the marina
 
Being as these underwater metals reside in water (by definition), and often transport water:
How do you effectively isolate them?

I think that (so called) isolated metals are merely (more or less) poorly connected.
Regards,
Gord

paulk 12-21-2003 02:27 AM

Corrosion in the marina
 
Sounds like Silmaril has the best advice: watch your zincs to see if anything happens to them. If you can put one on a cable over the side, you''ll be able to pull it up to check it without going swimming.


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