zinc as ballast instead of lead in aluminium keel

I'm thinking of using zinc instead of lead in my aluminium hull. Has this been done before?
The advantages would be zero corrosion (the ballast should corrode first) and low toxicity. The disadvantage is extra cost (but reasonable) and lower density (but still quite high).

 

You want to have just the right amount of cathodic protection, not too much, not too little, just the right amount. And it needs to be outside the boat. And it is should be spaced out, not all in one lump. You should hire an NA or engineer experienced in Al corrosion prevention to run the calculations and design a protection system for your boat. This isn't something you can just pick up in an evening.

 

seen lots of aluminum boats with internal lead ballast & no problems (not in 23 years, anyway) as long as it is done correctly.

 

Zinc would be a nightmare.

I prefer gold bars as ballast. If you cant get your hands on enough gold , go with lead.



A good ballast arrangement ,in the bilge , is to fabricate a long 90 degree angle iron mold . Cast the lead in this angle iron mold then cut the balast pigs into handy Toblerone chocolate bars style lead pigs. This triangle shape stacks and conforms well to the shape of the hull and they are easy to remove for hull skin maintenence. Insulate the lead from the hull for best results.

If you do go with gold use the same mold but remember to put a few handy padlocks on em after they are laid.

Nothing worse than getting out on the water, sheeting in then have the boat tip over because your crew took the gold bars down to the pub to exchange for beer.

 

I can not address the likely galvanic corrosion this might cause which always occurs with dissimilar metal contact, but if it is lower density of the ballast material, it means you will need more of it. this means a larger keel, which means more drag. The larger keel could also affect the balance of the boat under sail, so you should get some professional guidance, or you could end up with something less than satisfactory.

I am not sure this would be worth the cost and effort. It might be easier and cheaper to look for ways to isolate the lead from the aluminum contact rather than redesign the whole boat.

 

Well, most metals would be better than zinc. http://www.simetric.co.uk/si_metals.htm

 

And if you dont take care of the zinc, as it erodes from cathodic reaction, your stability goes to pot.

 

It might help if I clarify a few things.

(1) The ballast is internal so the type of ballast will not alter the drag.

(2) The keel runs the length of the boat and is quite wide and deep (and flat on the bottom). By my rough calculations it would have 6.7cm depth of lead or 10.64 cm of zinc. Thus raising the center of mass very slightly.

(3) Because the zinc is internal, any corrosion would be contained and not reduce the mass, though I presume zinc-oxide/chloride/whatever would have more mass.

(4) Lead is toxic

(5) Zinc is less toxic and can be cast just as easily. Consider galvanising baths are open to the air and workers lungs.

(6) There is a possibility that lead will corrode the aluminium (plenty of horror stories out there), zinc is protective of aluminium. This is despite best practice in sealing it which I will do anyway.

(7) The extra $1000 or so is cheap insurance. I won't hire an engineer for this. Plenty of cheap boats out there that I could buy and sail straight away if I had that kind of money. All risk is mine and part of the fun.

(8) The zinc is ballast and not there to primarily protect the aluminium, this is done by the usual anodes.

What I'd really like is someone to say they have seen it and it sucks (or maybe not). If it is safe enough to hang zinc off a hull then maybe it is safe enough to put inside.

(excuse me if this ends up in the wrong part of the thread, I haven't been here for a long while ... to busy welding)

Thanks for the input so far.

 

How about tungsten, all those tig electrode stubs have got to be good for something. Similar density to gold. 30 bucks a kilo. Not radioactive like plutonium - but that's only an alpha emmiter so the antifoul should hold it in - could be used as heating though.

Tungsten would be an aid to navigation because you could just drive through the reef without a scratch. Might need a big sail or rocket assist though.

 

I would worry about the electric charge generated by zinc if it came into contact with sea water.

Ive sailed several lead ballasted aluminium keels with no problems if you keep the lead dry..no seawater.

Lead shot in vegatable base oil is used. The oil displaces water and air voids.

Perhaps you could also use zinc shot in oil.

 

Lead is cheaper, but what about the antimony that is needed to make the lead strong enough for an external keel? My recollection is that it adds a significant amount to the cost. Zinc is much denser than iron which is used for keels in europe. Zinc is very mobile. It will protect steel bolts in contact but can cause alloy changes in brass and bronze.

 

"Zinc is much denser than iron "
Where do you get that info?
Iron 491lbs./cu.ft. or 7.87g./cu. cm.
Zinc 445lbs./cu.ft. or 7.13g./cu.cm.
http://www.coolmagnetman.com/magconda.htm

Antimony would usually not be necessary for internal ballast where the ability to hold keelbolts was unimportant, especially if scrap lead was used instead of virgin lead. Scrap lead usually has some antimony and perhaps tin, quantities varying with the type of scrap.

 

My mistake going by memory.
http://www.avlandesign.com/density_metal.htm
cast iron 7150 kg/mm
zinc 7134

Cast iron is 0.2% denser than zinc but varies with carbon content.
I used to design zinc precision castings and weight was always a concern.

 

Paying a naval architect to do the smart stuff is even cheaper insurance.

And no one seems to be betting on your zinc solution for some reason(s) ....

 

The zinc will expand massively when it corrodes. It could blow the keel apart. Rusting iron ballast can do the same if encapsulated. You'd need to set the densitiy as that of the corrosion product and pack the keel with shavings of zinc that could expand to take up the volume. You don't want the stuff on the inside corroding. You want the stuff on the outside corroding. I tried to see if there was a more electrically compatible lead alloy, but didn't find much on the anodic index of lead alloys with a five minute search. That's the direction I'd go - cast a compatible alloy. And be careful it's all of a kind. As long as the anodes are doing their job, the lead - zinc issue doesn't exist, but you need to test the system with reference anodes. Put that $1000 into galvanic test gear and you will be set for the life of your boat, and could make some jingle on the side with it as well, or at least trade it for beer.