Over the last 30 years we’ve been involved in many aluminum and fiberglass yacht construction projects. Our experience has been that properly built, aluminum holds a substatial edge overall in maintenance issues. “But what about corrosion, and the horror stories that are rumored?” you might be thinking.
To anwser that we offer the photos above and below. Those are sacrificial zincs, formerly attached to the hull of Wind Horse as sacrificial anodes, so that any corrosion comes off their mass rather than the hull.
And they are the original zincs, from when Wind Horse was launched, 6.5 years, and 50,000+ miles ago. The two end zincs(top photo) are from the middle of the hull, the center two are from near the rudders. Of the original material we estimate overall less than a quarter of the sacrificial zincs have been consumed. We have been carrying six spares all these years, so we are going to change them as we want to reduce “stuff” in the basement. But the reality is these could go another six to ten years.
There are a number of factors involved with such a small amount of corrosion and anodic activity:
- Proper wiring, including the use of an isolation transformer.
- Correct paint system, with careful attention to preparation from the bare aluminum, through primers, to anti-fouling.
- The right aluminum alloys.
- Placing the appropriate size of sacrificial zincs where the area they protect is balanced with the other zincs.
To keep an eye on this process we fit a simple indicator system that you can check periodically. This tells you the zincs are working correctly.
Posted by Steve Dashew (November 14, 2011)
November 14th, 2011 at 9:59 am
Wondering if the amount of time spent at anchor vs. in marina contributes to this result? I also have bee wanting to ask if you have a view on Seakeeper and related gyro technology. I suppose it will mainly be utilized to hide design flaws as perceived by the ocean-crossing type group. As a boater whose wife seriously suffers from sea-sickness (and has since her teens when we first started dating many years ago) the impact it COULD have on my personal near-shore cruising can not be underestimated…now to try and understand it better….thanks again for the excellent resource(s).
November 14th, 2011 at 11:31 am
Hi Ray:
If you have an isolation transformer any shoreside problems are separated with the isolated ground that then occurs. Wind Horse was tied to a dock in Ft. Lauderdale for six months last year.
November 14th, 2011 at 10:12 am
Hi Steve,The diminutive wear on the zincs is impressive, how do you connect them to the hull? On steelhulls we usually weld on the steelbar incorporated in the zinc. I once tried to weld on stainless bolts to fasten the zincs but that was a poor idea as the stanless steel corroded away. Can you describe your monitoring system?
reg.
Nils
November 14th, 2011 at 11:28 am
Hi Nils:
The zincs are cast with a steel bar inserted that has a pair of holes. The hull has threaded inserts that match up to the holes in the steel. These inserts stand off the hull and provide a small gap between the backside of the zinc and the hull. The stainless studs are double nutted, coated with Lanacoat, and t hen threaded through the flat bar into the aluminum inserts.
November 16th, 2011 at 5:55 pm
Hi Steve,
A friend and I were just yesterday wrestling with the question of what type of bolts to use to attach zinc anodes to our aluminum hulls. Do you get the electrical connectivity between the hull and anode via the stud, or via contact between the standoff and the anode? Is your threaded insert (Helicoil) one of the coated ones (Primer Free II) or just plain stainless? Does the Lanocote not insulate electrically? I know it must be necessary to keep the threads from galling. One resource I have (MGDuff in Europe) says to use galvanized hot dipped bolts to attach anodes to aluminum, presumably because of the more similar low “nobility ” of the steel, zinc and aluminum versus stainless, which is much higher on the scale (lower voltage potential). Sorry for so many questions, but this is a subject to which there seem to be many contradictory answers…Obviously you have done it right!
Thank you,
Brian
November 16th, 2011 at 8:30 pm
Hello Brian:
Bolts are stainless steel – 316 variety. Contact is between the steel plate in the zinc and the aluminum boss which is welded to the hull. Seems to work although I understand the iron/zinc notion too. One of the keys is the Lanocoat barrier between the stud and boss.
November 16th, 2011 at 7:15 pm
You mentioning controling the zinc this is verry interesting. Could you provide some details of your indiator system to control the zinc.
S/V Tangara
November 16th, 2011 at 8:35 pm
Hi Jean-Claude:
The monitoring (watching) of the zincs is done using a silver chloride half reference cell and measuring the voltage between it and the hull. This typically runs between .87 and .93 of a volt. Lower means the zincs are not large enough or dirty, higher indicates too much zinc.
March 12th, 2012 at 2:05 pm
I was looking at the new zincs on Wind Horse up at Jarrett Bay and was wondering whether you have considered aluminium Navalloy “zincs”. They have a 0.35V potential vs. 0.30V potential of zincs relative to aluminium outboards (where I use them). Web site is http://www.performancemetals.com
All the best
Don
March 12th, 2012 at 8:21 pm
Hi Don:
We’ve been using the same approach for many years. It is one of those things that if it works, we’d rather not mess with the formula.