With good bottom paint, how long... - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-18-2006
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With good bottom paint, how long...

I won't be sailing my boat over the winter, and am wondering if I can postpone hiring a diver to clean the bottom of the boat til Spring, or will that lead to so many barnacles/critters attaching themselves so firmly to the hull by then that I risk hull damage? The boat is moored in Nanaimo, BC, Canada. The bottom was last painted in Spring/05, cleaned thoroughly in Spring/06, sailed through the past summer, but is developing a slight green algae slime now by Fall. Other than sailing or diving the boat, any other advice on keeping the hull underwater clean over the winter?
Thanks for any advice or suggestions.
Frank.
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Old 09-18-2006
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Depends on the paint, but I now believe you can go 4 years in northern climates with 2 coats of hard bottom paint between haul-outs. As far a cleaning goes I would hire a diver, but you could probably slide if and only if you hang a zinc over the side. The big risk to the bottom is the prop and shaft if the zincs go, and they can fast if something goes wrong, with your boat or a boat near by.
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Old 09-18-2006
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Yes, it depends on the type of paint to some degree, and the idea of an extra properly connected "over the side" zinc is a good one.
You will likely have some barnacles/mussels in hard-to-paint areas like the very bottom of the keel but they shouldn't get too carried away.
One factor that might make things a bit worse for you is the general lack of fresh water in Nanaimo harbour areas. Many marinas are partly fed by creeks which helps keep the salt water growth down.
Consider a "half lift" - where you got lifted before lunch and have an hour in the slings for a quick clean. You can change zincs then too and it's probably cheaper than a diver's two hours. Plus you get the peace of mind before you head to the prairie winter!
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Old 09-18-2006
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How your bottom fairs over the winter is almost entirely dependant not on the type of paint used, but rather on the condition of the paint that is on the bottom now. If your paint is appox. a year and a half old (as you seem to indicate) then it is probably closer to the end of it's useful lifespan than it is to the beginning. Keep in mind also, that more frequent, gentle cleanings are better for the paint and will help it last longer than less frequent, more abrasive cleanings. Since it is obviously of concern, I suggest you have the bottom cleaned one last time before you park the boat for the winter. And almost certainly having a diver do it will be much cheaper than a haulout. I don't know what kind of boat we are talking about, but it would either have to be a very large one or have a bottom in terrible condition to warrant two hours of a diver's time to clean and zinc her.
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Thanks for your replies and your suggestions so far. The boat is an Ericson 30+ (30 foot length, 10.5 foot beam and almost 6 foot fin keel). I will discuss the options you mentioned with the boat-watch guy I use to keep an eye on her. He also dives and does basic maintenance stuff, and will have an opinion. With your additional information, I'll be in a better position to make the right decision, depending on cost.
thanks again.
Frank.
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Quote:
Originally Posted by FrankLanger
The boat is an Ericson 30+.
For a competent hull cleaner, cleaning and zinc replacement should take no more than 30-45 minutes, tops.
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Old 09-19-2006
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I'm a new boat owner. Tell me about zinks. How are they used?
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Old 09-19-2006
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Cellnav....

It's known as "cathodic protection". You connect a controlled piece of a more reactive metal onto the structure of the boat, IMMERSED IN THE WATER, making sure you have good contact with the electrical bonded system of the boat.

Classically, it's a fist sized piece of zinc bolted to the hull, and a wire led from inside the hull to all of the through-hulls. You probably have seen all those wires running from through hull to through hull inside the boat. That's the bonded system, and it connects all those expensive bronzes. Some owners drop a wee zinc "fish" over the side, and lead the other end of the wire back inside to the bonded system, or sometimes to the engine block, or whatever they want to protect.

Zinc is more reactive than the through-hull bronzes, and more reactive than iron if that's what it's wired to. Immersed in water, the effect is a very weak battery, where the zinc slowly reacts away. As it reacts, the zinc is always a few millivolts more positive than the rest of the system, and so the through hulls are a wee bit negative with respect to the zinc. This has the highly desirable effect of radically slowing the corrosion of the bronzes.

At intervals, the zinc is renewed. You know by looking at it.... it looks "crumbly". Since the zinc is sacrificed in this way, it is sometimes known as a "sacrificial anode".

In fresh water, zinc does not work so well, and some owners use magnesium. You see it on outboards sometimes. Magnesium is more reactive. Corrosion is much less a problem in fresh water though. Fresh water is good stuff, but it does freeze, as I learned expensively last year.
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Old 09-19-2006
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Cell Nav...Rockters explanation is a bit technical so here's the short version. Salt water and galvanic corrosion will eat your prop shaft up unless you bolt a donut shaped zinc to it which gets eaten away instead of the shaft. Zincs need to be replaced when they are pretty well eaten...sometimes in a few months...other times maybe once a year...you gotta watch em.
Same thing goes on INSIDE moost diesel engines in the cooling chamber (heat exchanger). Screw in pencil zincs are used there. Expensive if you forget about 'em. Ask me how I know!
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Old 09-19-2006
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Quote:
Originally Posted by CellNav
I'm a new boat owner. Tell me about zinks. How are they used?
Rockter's post provides good info, but let me expand upon that, if I may.

All metals immersed in water are subject to various types of corrosion. Metals in saltwater especially. So to protect underwater metals on our boats we attach sacrificial zinc anodes to them. Typically on a sailboat you would be concerned about your prop, shaft and strut. The issues we are mostly concerned about are galvanic corrosion (this occurs when two dissimilar metals are in contact with each other in an electrolyte, such as seawater) and electrolytic corrosion (this occurs when the metal part is attacked by an external source of electrical current and is often mistakenly called "electrolysis," which is a different process altogether.) Zinc anodes will protect metals from both types of corrosion.

Zinc is relatively low on the periodic table and is therefore less "noble" than the parts it is used to protect. When electrically bonded to a particular part, say a bronze or monel prop shaft, the zinc will deplete first, giving up electrons until it is completely gone, at which point the item it was attached to is no longer protected and will begin suffering damage. So zincs are supposed to waste away and therefore need to be replaced when they are approximately 50% depleted.

In a marine environment and everything being as it should, your zincs should last 6-9 months. Dramatically more or less and it's time to look for a reason. That being said, there are many factors that affect zinc longevity. Stray current in your bilge, an electrical problem on a neighboring boat or a problem with the shore power system can all affect your zincs. Even buried metal objects in your slip can cause rapid zinc depletion. Keep in mind also that when you are plugged into a shorepower system, your boat is in electrical contact with every other boat on that circuit. So a boat many slips away could be affecting the effectiveness of your zincs.

Hope this helps,

Matt
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