I really cannot understand why a boat that is supposedly a ''quality long distance crusier'' would ever use concrete and iron ballast. While there is a difference in cost between a cast lead ballast keel and a concrete and iron one, the difference (probably less than $3 to $5K) makes absolutely no sense over the life of the boat and generally makes me wonder what other corners the builder has been cut.
To answer your question, low density ballast is a major compromise. It means that much more ballast is needed to accomplish the same stability and that ballast comes at the price of increased displacement, increased drag, and often decreased carrying capacity. This greater weight and drag is not something to be ignored, it means that the sail plan has to be larger and the engine and fuel
capacity needs to be larger. It means that you need to carry more sail in heavier going which is bad news since iron and concrete ballasted boats usually don''t have the stability to carry that extra sail. Adding weight and drag begins a design cycle that cumulatively just keeps making a boat heavier and heavier or else reduces the safety factors on the strength of the boat.
Iron and concrete ballast also brings up the whole encapsulated keel debate. This is another one of those ‘no one universally right answers’ item. In other words an argument can be made for either type of keel. (For the record, I personally strongly prefer a bolt on keel rather than an encapsulated keel.) Here’s the way I see it.
Bolt-on keels tend to offer more performance since the ballast must be cast and without the keel stub skin thickness tend to be lower relative to the center of bouyancy. They also have significantly less wetted surface and frontal area making them theoretically faster on all points of sail. They are simple to repair
and generally can be repaired satisfactorily no matter how bad the mistake.
On the down side they are more expensive to build; requiring precision casting, bolt hole drilling and a lot more hand fairing. They are higher maintenance requiring fairing every 10 years or so and new keel bolts at some point in the boat’s life.
Encapsulated keels are less expensive to build. There’s less labor and less precision required. Boat builders will often use less expensive forms of ballasting with encapsulated keels, such as iron or lead scrap cast in concrete, resin or other binder which further reducing costs. If they are not damaged in a grounding, encapsulated keels are less expensive to maintain.
On the down side, as mentioned above they are less efficient from a sailing standpoint. Their real downside is the difficulty in doing a proper repair
. Typically, in a hard grounding a number of things happen on an encapsulated keel. Typically the skin of the keel encapsulation gets ruptured and separates from the ballast. This allows water into the small cavities between the keel and the ballast and once wet it can mean the ‘beginning of the end’ for the boat as this permanently wet fiberglass blisters itself from the interior and the wet areas spread around the ballast. This is especially a problem on a boat that is hauled out for cold winters where freeze/ thaw cycles can really pry the skin loose from the ballast. The problem gets worse when the ballast contains ferrous materials. Here the ballast begins to rust and can reduce the ballast into a loose mass of concrete and rusting iron. Having watched a boat being rebuilt in which this had happened, I can assure that this is not a cheap process or one that I would want to go through.
Beyond that, in a grounding the ballast is often forced upward as well. In an encapsulated keel the membrane of the hull is at the outside of the keel and the membrane above the ballast is often quite thin. In a bad grounding the ballast keel is often is pushed through this membrane causing a serious and difficult to repair
damage and leak.
We grounded a boat with an encapsulated keel that we never could permanently fix for as long as we owned the boat. The problem would get worse with every year, spreading from a small dimple on the leading edge of the keel to an area that was much of the bottom and sides of the keel.
Lastly, it is very hard to lay-up the glass in the keel cavity. As a result the glass work in this vulnerable area of the boat is often inferior to the glass work else where on the boat.
To some extent this arguement is one of ultimate durability vs low maintenance. A bolt on lead keel will last forever but will require higher maintenance to do so. Sooner or later, an encapsulated keel will spell the end of the useful life of a boat, but will require less maintenance until the time that the ballast keel has delaminated from the keel cavity.