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-   -   Concrete keels - any facts? (http://www.sailnet.com/forums/sailboat-design-construction/63036-concrete-keels-any-facts.html)

kulokoo 03-19-2010 05:02 PM

Concrete keels - any facts?
 
Granted concrete is not the material I would use for ballast if I had the choice, but looking at old boats I see a Rawson 30 that would otherwise seem to fit my purposes (seaworthiness, cruising, liveaboard), taste and budget.

So what problems might one expect out of a concrete ballast keel that is 40 years old?

Things I've heard...

They cause corrosion
They soak up water
They disintegrate

Are these real problems or myths?

Also, are there fixes if problems come up, and if so how hard are the fixe$?

I am imagining if something goes wrong with a concrete keel it is harder to remedy than an external lead keel...


Thanks in advance for setting me on course!

sailingdog 03-19-2010 05:23 PM

If there are any leaks, there will be freeze/thaw cycle damage if the boat is someplace it gets below freezing for any period of time.

Concrete is a horrible ballast material for a couple of reasons, aside from the ones you've mentioned.

Concrete is not a very dense material. This means that it requires more space to provide the same amount of righting moment as lead or even iron. This means the boat has less interior volume and a lower righting moment, as the CG of the ballast is effectively higher. The boat will probably also have much more wetted surface area than it would if the boat had been made using lead as a ballast material. This means that a concrete ballasted boat will be slower and more tender than a similar design using lead or iron as a ballast material.

Second, it is difficult to properly repair once damage does occur. Lead is far easier to repair should you have problems.

Before committing yourself to this Rawson, I'd would recommend you read the Boat Inspection Trip Tips thread I started, as it will help you determine whether this boat is even worth going forward on.

Jeff_H 03-19-2010 05:59 PM

Sailingdog hit many of the functional issues with concrete ballast. I personally helped a fellow who owned a Buccaneer with concrete ballast that had gotten wet and frozen many times over the years. The concrete had been reduced to loose rocks that you could easily remove with your fingers surrounded by water saturated dust.

Of course the technique for constructing concrete ballast varies widely from simply dumping cement into the keel encapsulation envelope as was done on the Buccaneer, to casting a mix of boiler punchings and concrete in a separate mold and setting it like you would a cast lead keel as Bristold did on the Bristol 24 (AKA Corsair)

The other issue with this technique is never knowing what density the keel will end up being. The production manager at Bristol mentioned Corsairs where they accidentally left out the boiler punchings, and my family looked at a Cheoy Lee Frisco Flyer that turned out to have the a similar problem.

Jeff

kulokoo 03-22-2010 04:39 PM

Thanks, this makes me just about rule out a Rawson as I am more interested in sailing a boat than trying to re-engineer one with a problem.

Curious about the Buccaneer with the concrete ballast gone bad: were you and the owner able to fix it? Dig out the concrete and fill it with lead or something? Seems there is an interesting story there

sailingdog 03-22-2010 06:11 PM

On one boat I helped repair, we took the concrete out and filled the space with a mixture of epoxy and lead weights gotten from a tire shop. We then glassed over the top of the whole mess. It took up less space than the equivalent weight of concrete/iron and made for a deeper bilge.


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