I do agree that we may see masts which are diamond wire supported for their entire length. Adding a new set to the top is a likely way to experiment.

Photos of my mast crack are attached. As you can see, I was a lucky guy when I caught the problem just before emminant terminal failure.

You are so right about the disimilar metals and the rivets. But I must also contribute the failure to the straight horrizontal line shape of the top of the hound and the fact that the mast above this point is in almost constant motion during most sailing. Each tack reverses the bend and each bump or puff causes some pumping.

GARY




See third and forth pictures in subsequent posts.

GARY

Gary,

Upon looking at the failure, I am convinced that the heavy corrosion of the mast tube in the vacinity of the masthead played a key role in weakening the tube. It does so in three ways. The first way is obvious: corrosion erodes some of the metal away leaving it thinner and therefore weaker. The second way is that the corrosion always eats a very jagged, irregular path through the metal. This acts the same as a deep scratch or hole causing stress concentration. The third way is that it causes chemical/molecular changes to the metal making it weaker and brittle.

It might be a good idea on these painted masts to remove the masthead and fabricate a barrier gasket out of something and re-install. This would stop the galvanic action for good. A piece of mica is a good candidate.

I am not a fan of painted aluminum parts on boats. Anodizing holds up much better and stops galvanic corrosion much more effectively than the typical conversion coating used to prepare the mast for paint or powdercoat.

You really must not tolerate corrosion on the aluminum parts of your boat. If you start to see any corrosion, it must be dealt with by dressing out and applying conversion coating and re-painting. I'm a bit worried about the mast and beams on my old P19 and it's maybe about 1/100 as advanced as the corrosion I see in the pics.

Jimbo

Jimbo, can you expand further on the remedial action to take? What do you mean by conversion coating? Name(s)of suitable product(s)? Method to be used?

Thanks

You have to start by getting to the origin of the corrosion. If it's poking out from under the masthead, for instance, you really need to remove the masthead fitting. The next step is to dress the corrosion. This removes the oxide and gets you back to clean metal. The red Scotch-brite Surface Conditioning Discs on a 3M Roloc mandrel are a favorite for this step. These are best used with a small right angle air grinder, but a drill will work OK too, just a little slower. One of those little nylon brushes (the orange one works best) are also excellent. You can find those at home centers, lately. The drill is the perfect driver for a nylon brush.

Whatever you do, NEVER use any kind of wire brush! These always leave microscopic paticles of dissimilar metal behind and cause terrible galvanic corrosion later. Don't do it!

After grinding and smoothing the area down to clean metal, you should etch or acid clean the metal with one of the etchants sold for that purpose like Alumiprep 33 from Henkel, 225S metal cleaner from DuPont or DX533 from PPG. Follow all label or data sheet instructions on dilution. Definitely do not let these products dry on the surface! If the product dries out before you rinse, you have to start over with the etch. Next you will apply a chromate conversion coating like Alodine 1201 from Henkel, 226S from DuPont and DX503 fom PPG. When done properly, these produce a visible change in the color and texture of the metal and leave a gold/straw color. Again, these products, like the cleaners must never be allowed to dry on the surface, or you have to start over with the conversion.

It's all really easy after you get the hang of it. The best way to use these of course is by dipping in a tank of the product, but that's not always practical like for a mast. All these manufacturers have application guides to help you. Awl-Grip's application guide has instructions for conversion coatings as well. They recommend the Henkel products.

Chromate conversion coatings are definitely environmentally hazardous products, ground water pollution being their major danger. For that reason, be sparing/conservative of the product and catch all used product you can. Never dump it on the ground or pour/flush it into any sewer or storm drain! To dispose of it, simply set it aside in an open container and let it evaporate completely leaving a brown solid cake or granules. This can be disposed of cheaply as dry waste, perhaps even for free at your local landfill's hazardous household waste division.

The acid etch cleaners can definitely burn your hands so wear gloves when you are using them. The burns don't hurt right away so it can fool you into thinking it's OK to have your hands in it. The chromate conversion is poisonous (though not terribly so at least in liquid form) so again wear gloves and promptly rinse away any that gets on you. Just use good industrial hygene practices and you'll be fine.

Jimbo

Relatively new boat? Its been around at least 10 years

I'm thinking more like 15 years. Both the Mystere 6.0 and the Nacra 6.0 came out well before spinnakers were being used on beach cats.

Excellent info Jimbo, many thanks. I intend to 'have a go' at the main beam on my boat this winter.

I'll start making enquiries into the availability in the UK of the products you mention.

Thanks again.

Jimbo,

That is not a stress crack but is a crack that developed through galvanic corrosion. This corrosion is caused by minute electrical currents setup by two disimilar metals that rob the 'anode' material of stucture at the molecular level...the aluminum in this case. It surprises me that I haven't seen any manufacturer put magnesium anodes on our aluminum parts. My monohull has anodes on it's mast as do most power boat motors and such.

Cracks always happen when the metal is under cyclic stress, but in this case the metal was weakened and perhaps embrittled by the galvannic corrosion. So which was it, stress or corrosion? The point is that you just can't tolerate corrosion on your boat! These boats don't have a lot of safety margins engineered into them. If they did, they would be thicker, stronger, heavier and slower, and we know we don't want that! So you just have to stay on top of stuff like this.

The anode is a gret idea, BTW. I think zinc is the standard metal used for this purpose and you can buy zinc anodes at any marine store. One for the mast and one for each beam. Sounds like a plan, especially for these boats with painted masts and beams. Adds a few ounces weight at most.

Jimbo

I predominantly sail my boat in lakes but hit the ocean 4 or 5 times a year. I've been planning to add a few anodes around the boat. If you do decide to attach anodes to your major aluminum components, be sure that it is electrically bonded to the aluminum. Remove any paint, anodizing, or powder coating where the anode meets the aluminum. They must be capable of conducting current to the anode for it to have any affect.

Yup--Corrosion at horizontal joint of dissimilar metals covered with crappy looking old powder coat on non-anodized mast! No surprise to me.

In 30 years I've NEVER seen a anodized (Hobie or Nacra) mast break like this. Their holes wallow out where stainless steel pop-rivets are wet with salt water, tho.
Also many extrusions are thicker on the side walls.

Why does the anode get all the attention?? Galvanic corrosion requires several things to occur. An anode, a cathode, an electrolyte and a conductive path.

In addition to maintaining a coating on the anodic mast, it would also be beneficial to paint the cathodic SS hound. More beneficial, in fact, because another variable in how quickly the anode corrodes is the surface area ratio between the cathode and the anode. A scratch in the protective coating on the mast near a large uncoated cathode will produce more concentrated corrosion than if the mast were unprotected.

The conductive path. If the hound was bedded with a dielectric sealant and the rivets installed coated with the same, it would minimize (in addition to what you've achieved with the paint) the conduction portion of the circuit. The RTV would also minimize the electrolyte getting into the joint. I don't know the dielectric properties of the RTV sealants readily available at auto and hardware stores.

The electrolyte. In this situation with the mast-hound joint, it's a very small circuit. Since the mast is out of the water the majority of the time, the circuit will be that small crack in the joint were dew or rain can collect and work with the salt that has collected there. A dielectric RTV filet could prevent that from happening. I think that this mechanism would also negate the usefulness of a zinc sacrificial anode because the sacrificial anode has to be part of the circuit. In this case, attaching a zinc to the mast won't prevent the galvanic corrosion since it doesn't share the same electrolyte path for the ions that the mast-hound circuit does. However, the zinc anode would protect the mast while the boat is turtled.

I had suggested a piece of mica for the insulating gasket. Most electrical/electronics supply houses still carry mica sheets. It's an inorganic mineral with very high dielectric properties. Some other things you might put there like plastics can creep or flow out of the way over time negating their usefulness and possibly causing the fitting to become loose.
A thin piece of laminated glass/epoxy (G10 grade) like .020" thick should work, too. That's a stock item in the MSC catalog.

Jimbo

The Mystere Viper that was on our beach for a while had two sets. 34' mast. That was a mast that would bend in several directions at once. I think it has a carbon mast now.

On mine I stripped the power coat off, treated the bare alum. with a corrosion block and etch. I then clear coated it with Imron and after three years I have a couple small spots that started to bubble up under the clear. They were easy to clean up and spot in.

Steve,
Because a sacrificial anode is the easiest way to prevent damage to the important parts.
The anode is the part of the battery which loses material during electrical activity.
By attaching a piece of a "baser" metal, it corrodes instead of the important parts.

That mast looks like classic galvanic corrosion to me.
The base metal almost always corrodes from the inside and then the cracks become visible from the outside.
I have seen crossbars break at the location of pop rivets.

Sam, my question was rhetorical. My point was that corrosion can be addressed in a number of ways and tailored to attack the mechanism that one is faced with. Specifically, that sacrificial anodes won't work in the case of a dry-stored boat and that isolating and minimizing the exposed surface area of the cathode might be better options.

jimbo, one problem with the mica or other insulating sheets is that it doesn't prevent conduction and possible corrosion at the rivets, nor does it seal the mast at the same time. At several hundred or thousand volts per mil of dielectric strength, some of the insulating compunds don't need a goopy, shifting mess in order to work. Again, not that we can find them at the local auto parts store. Maybe an electrical supply house?

It should still work since you are drastically reducing the contact area between the anode and cathode. The mica could be combined with a layer of sealant to make the joint permanently dry.

Jimbo