Single pole and 2 pole conflict

The same as outside. Unless you can keep it completely dry and free of salt spray, there will be electrolisis.

 

Ive never seen one. anodes need to be submerged in an electrolite to work. If you bonded a big anode in a small pool of bilge water, the metal in the area of the anode mass would be attacked, not the anode ? . I always find anodes to be a bit of black science. Is there a good Boatdesign thread on anode s

 

There is no black science or art to it. The anode is the sacrificial metal. I think you need to study electrolisis more to understand what it does. Every metal has an electrical potential. Metal molecules migrate from the lower to the higher metals in the scale. I know of an aluminum sailboat that sunk because of a bunch of copper pennies dumped in the bilge ( by a vindictive ex-wife). An anode would've prevented that.

 

It is a Black Science !!!!!!!!!!!!!!!!!!! I'm preparing to be hauled. I have almost 2000 euros worth of anodes on the bottom. I find it difficult to have a profession appraisal concerning when to renew these anodes. Expensive stuff. MgDuff, the local anode mafia ,naturally say...OUT with the OLD in with the new. Kimme 2000 euros.
When is an anode expired. When is its alloy depleted ?

 

According to boatzincs.com web site they should be replaced when they have corroded to 50% of their original weight.

 

Mgduff states...."To ensure correct level of protection anodes should be replaced when 80% consumed or if it is anticipated that they will be more than 80% consumed at the next scheduled docking. "

But anode mass seems to be only part of the story. Different size and Shape anodes behave differently. . "Evidently" and I only repeat what Im told, anodes behave something like a light bulb and radiate their protective field with a Wide BEAM. the edges of the anode and its depth protrusion are important or this beam become narrow. I will first notice that the edges of an anode erodes.
Many of the anodes on the boat are " Flush Anodes" and the edge surface is important. Its unclear when these Flush fit anodes become excessively recessed into the hull to provide the wide beam of protection. At 50 or 80 percent mass reduction they would be "invisible ". The flush pie plate anode is between the stern thrusrter and propellor in the pic.

 

Either you didn't try hard enough or there in no one with marine knowledge on your area. It only takes a calibrated plate and a voltmeter to figure out the level of protection you need. It will measure galvanic potential and stray currents. It is NOT black science. All you need is basic understanding of electricity.

 

Well if you ask your local supplier............even sooner, I guess?

As everything in our business, it depends.

First you have to ask, how long did they need to loose 50% of their mass. Because when that takes years, they are not doing their job well. then you have to ask, how long would the rest of material last before it falls off in crumbs. And do I have the time to wait for the next haulout? Many people haul every autumn and are fine with a anode of 50% for the entire summer.

Regards
Richard

 

Well Gonzo...you would have a very good buisness because the only advise from the Pros..MgDuff... is to buy new ones. Im stubborn and keep the anodes year after year, but the majority of yachts throw out TONS..that correct the scrap recycle box at the shipyard weighs tons...each year.

 

The service life off an anode and its weight at haulout is a poor indicator to its effectivness. If the anode is not correctly bonded to the metal hull, it will last a very long time as your hull becomes the sacrificial anode. .

 

Not exactly. That is what the proper measurments are for. And I already do good business. If you need advise in Spain I can recommend a couple of knowledgeable people.

 

Yes, and if they wait too long before the next haul-out 100% won't be enough!

 

Only if there is an electrical potential. However, it should be noted that electrode potentials serve only as a guide to the possibility of galvanic corrosion. The actual magnitude of the electrical potential is determined not only by the difference in electrode potentials between the particular dissimilar metals but also by the total electrical resistance, or polarisation, of the galvanic circuit. Polarisation itself is influenced by the nature of the metal/liquid interface and more particularly by the oxides formed on metal surfaces.

No it wouldn’t. Painting the bilges would have prevented it.

You cannot determine exactly where corrosion and thus a “cell” will occur inside a 'dry' hull. But you do need an aqueous solution for it to initiate. Your penny would be classic crevice corrosion.

The nature of sea air is very aggressive and is aggravated by moisture and sea spray consisting of very fine droplets of sea water. It is more aggressive than sea water. An aqueous solution which contains chlorides and thus locally increases the pH, will easily initiate an attack under the penny between it and bare aluminium and causing basic crevice corrosion.

However, you would need to have ESP to know exactly where to place an anode and exactly what size of anode to ensure that all 3 are in contact.

So, I’ll ask again, how many boats have you designed which have anodes inside their bare hulls?

Painting the bilges with white epoxy primer cures all. Not everyone does it, we do. We do not experience any problems, and never heard of fitting anodes inside the hull!

We also use MgDuff.

 

Hi CDK, thanks for all your suggestions.
With regards to isolation I was planning to isolate electrical equipment from the hull not the engines.
Rather than bolt mount everything as usual direct to stand off brackets or frames in ER or behind the WH console, I'll insert isolation mounts as blocks to direct mechanical<>eclectric current paths.

Total scope of additional work requires less than 10hrs but results for Client in long term reduced maintenance would be significant plus our shipyard rep. doesn't get burned.

 

I still dont like the single pole...earth on hull engine instalation. . Bad practice on a metal or any boat.

As for installing and investigating the Anode field and the " Black art "

The following are notes and clips I have. Plenty of details to think about.:idea:


" I STRONGLY recommend that you buy a good quality digital volt-meter, such as a FLUKE (there are many good models). IN ADDITION buy a silver-silver chloride half cell for reference in measuring ANY and ALL places around the hull which might be susceptible to corrosion. One good model is: Stelth3 Model SRE-011-SPB "


"You drop the reference half-cell over the side (attached to an insulated wire) and measure with the DVM voltages between the reference cell wire and points around the hull, including the engine, shaft, rudder post, etc. These readings remove all doubt as to whether or not these points are "protected" by your zincs."

"If all of your underwater exposed metals are bonded in the vessel then your readings should ALL be the same and be at least 200 mV greater than the least noble metal in your system. For example, if you have a fiberglass boat with bronze thru-hull fittings, a bronze prop and a stainless-steel shaft then the least noble metal is the steel and with proper zincs you should read at least -800 mV. Don't forget to bond and read your bronze raw water strainer which is "in circuit" with the sea water zinc-bonding system. Don't forget to bond bronze depth sounder transducers. "

"With aluminim you don't want to see readings at or above 1050 mV because the aluminum may suffer alkali corrosion from too much zinc ionic current. In addition, your anti-fouling paint will begin to suffer as well."

"These measurements can show you just how much zinc to use in your bonding system. You can begin with no zinc and drop a zinc over the side (attached to a wire connected to your bonding system) of the size to use on your prop shaft, for example. In this case coat the surface which would normally contact the shaft with grease so that the test is representative of the actual surface area that the zinc will present to the water. Note the "protection" potential increase over that of the previous "free" reading. If you reach the protected value then that is the surface area of zinc that you need. Add more zincs if needed to reach a protected reading. Beyond that more zinc surface area will drive the readings towards the "overprotected" values. NOW you can imagine that if you want long lasting zinc you need increased THICKNESS of the existing zinc surface area and NOT more zincs added.

"Note that the reference half-cell should be placed approximately at a depth and location somewhere where you are going to have the zincs mounted (within, say 6 feet or so) to obtain a measurement unaffected by metals away from the boat."

"The recommendations generally range between 200 to 300 millivolts ~

Ideally (in a fully “Bonded” boat), all fittings should measure exactly the same voltage; but differentials of between 30 - 50mV are often deemed acceptable.

These recommendations are for Salt Water only. Fresh Water applications usually require Higher Voltages.

"When a newly installed Zinc Anode begins to erode, the surface becomes pitted, creating a larger surface Area. This generates a slightly Increased voltage potential, which may result in a temporary “Overprotection” (causing accelerated Anode depletion, amphoteric attack on Aluminum, & Wood deterioration) "

"As the Zinc continues to erode, it’s Mass is decreased, generating a Decreased voltage, which results in “Underprotection”, and rapid corrosion of All immersed Metals. Replace Sacrificial Anodes immediately. "


"When you first immerse either a zinc or a reference half-cell the readings will change and settle out with a varying reading with a stable average (assuming an adjacent boat or field near your boat is not affecting the reading over time). One phenomenon is called polarization potential which happens when any two dissimilar metals are placed into an electrolyte solution. It takes awhile for the positive free zinc ions to migrate through the salt water to cathodic metals which they are supposed to protect. As those ions contact and adhere to the cathodes a local capacitive effect takes place which affects the steady-state reading. In addition, during the "plating process" the half-cell potential at the zinc anode slightly drops due to the ionic flow which is a current not formed by electrons. The electrons flow through the bonding wires and back to the zinc. "

"Twenty or so hours is a good time for observing the steady-state protection values yet twenty minutes gives you a good idea. If you are chasing particular galvanic corrosion problems you definitely need to check at the 24 hour point to make sure of your protection potential of around 250mV. "

"One vexing galvanic problem often found is with stainless or aluminum fittings which ride just in and out of the water. They are often not bonded well or are connected to a battery negative distribution point in the electrical system by mistake (or idnorance). and are often difficult to protect properly."

"Visualize that the zinc ion migration will be affected by currents around the boat. In fact, most often you have essentially no protection when underway. Problems do not often occur due to this because even blue water sailers spend an aggrigate average time not moving over the course of a year. "