We have 5,000 pounds of lead ballast in our boat, which could be replaced with a lot of batteries. Cost, density, and limited life have been the main deterrents. Cost is obvious. Density is a bit more subtle: the effectiveness of ballast is directly related to its density as compared to that of water. The job of ballast is to hold a boat upright in the water. To do that it needs to be denser than the water around it. While batteries are heavy, even the heaviest are not nearly as dense as lead, and newer ones are lighter still, which is good for cars but bad for ballast. Note that this is mostly an issue with mono hulls, as catamarans rely on form stability as opposed to ballast stability, and light weight is an advantage in this case. As for limited life, our ballast is in a fin shape bolted to the bottom of the boat. If this were replaced with batteries, the batteries would need to be fin shaped or fit in a fin, and they would have to be replaceable, since battery life is limited.

Tom

I think if you designed the batteries to fit the keel fin of a single-hull sailing yacht, there would be an opportunity for the single-hulled application of this technology. Especially if you use bolted-on plates to cover the access panels and design the compartment to be watertight. Of course, then there's the issue of ensuring that the people involved in the installation and removal of the batteries were properly trained, and so forth; this is definitely a step in the right direction though.

Just my $.02