zinc as ballast instead of lead in aluminium keel

If I was constructing a boat for sale or commercial purposes I would be heavily involved with the naval architect and or engineer. However I am doing this for pleasure so paying someone else to do the fun parts would ahh.... remove the fun. Also let me give you a non nautical example that might illustrate my philosophy. I needed to add a veranda/patio to my house, I could buy a kit that was designed by an engineer to meet the usual one in a hundred year wind speed and that minimised the materials (to the advantage of the kit company I suppose). This would be essential if my patio was going to fly/float but of course it just sits there. So what I did instead was to pay the same price as the engineered kit and use stronger (heavier too) materials. In the last storm, while others were collecting their tin roofs from their neighbours, I was sitting under mine nice and dry. Note that weight was not an issue, a boat or plane might be different.

 

Yes I've seen the same problem with steel punchings in concrete as used in small trailerable fixed keel boats, usually the water gets into the bilge and down into the concrete. I'm not too worried about large quantities of zinc corroding as it would be well sealed. I'd probably make castings that were slightly smaller than the keel and then bed them in elastomer, weld a plate over the top and then fill the rest of the cavity with more elastomer. The advantage of zinc over lead (apart from the toxicity of handling it) would be that any penetration of the seal would result in a small quantity of zinc corroding compared to having lead which would corrode a small quantity of aluminium - of which there is only 6mm thickness.

I work in electronics, so measurement is not a problem and I have done some work on cathodic protection for cars. I'm not sure that a zinc anode on the outside of a keel would have any protective effect on the lead - aluminium junction inside as there is no return path for the currents involved. However once the aluminium is pierced there would be a path and the anode may start protecting, but who wants a pinhole in their hull.

 

Is this a new build? If it is you need to watch the stability issue of a higher ballast CoG raising the boats CG.

You can use Zinc sure, but you would have to cast the blocks external to the keel the melting point at around 30% more than lead is too high to cast directly into alloy plate and the extra heat makes casting a lot harder and more dangerous than casting lead. That's an OHS issue too.

Some Alloy plate lead filled keels have been cast with zinc sheets between the Alloy and the lead, it softens, forms to the plate and protects the hull in the interface but it still needs capping. The problems occur in wet bilges that are not well sealed and salt water getting in the interface .

I've seen holes from unprotected/unsealed lead ballast and salt water.

Agreed lead is toxic but with some basic precautions and common sense it's safe to work.

You don't need antimony for internal ballast nor for low stressed external ballast. If your lead scrap comes from vehicle batteries it's in there anyway.

Providing you cap the cavity and pressure test it it will be fine whatever metal you choose, even depleted uranium

 

This is my first post, and look forward to borrowing some of your wisdom on my projects. I have done some zinc casting. I found that it was too light for making lead-free tackle so went with bismuth (sometimes mixed with tin). Bismuth was about 6x more expensive than zinc, and melted much easier. The zinc took so much more heat, and both zinc and plain bismuth were quite brittle. Lead is "easy" to work with in comparison.

 

Welcome to the Forum Earl.

I think bismuth is definitely out as a ballast material

 

I just checked the prices of bismuth. Almost $20/pound; I would still go with lead.
Have you checked the numbers on depleted uranium? It's got the weight/density, but a few environmental/safety concerns.

 

Since this is just going to be stacked internal have you considered scrap steel (cheapest of all) and then galvanizing it? I have heard that zinc plates on easily (according to plating pros...) and is self healing if it gets dinged or scratched. I understand that it doesn't eliminate your worry about possible galvanic corrosion but don't anticipate anything achieving that. Isn't it possible to monitor for galvanic voltage?

 

Thanks, that all makes sense. I'm going to cast some aluminium so I presume zinc will be ok as it has a lower melting point. I will apply all the usual precautions. I'll cast outside and then place it in the keel embedded in elastomer. Everything will be sealed just as if it was lead. I checked the price and it is actually comparable to lead.

Too many hassles with lead (apart from the corrosion protection) especially with children around we are talking parts per million and the fumes will just settle all over the place.

 

What is your plan: Making a giant battery?

 

whatever happened to the K.I.S.S. principle????

 

i'd rather poor concrete in my hull than go all the trouble with zinc...

 

http://www.amazon.co.uk/The-Golden-Keel-Desmond-Bagley/dp/1842320114

 

Not a great solution with alloy. The surface is activated by both acid and alkali. Concrete and plaster and even lime wash passivate steel surfaces but activate aluminium alloy.