BALLAST

Ballast gets a considerable amount of coverage is my book 'METAL BOATS'  See sample below

In a metal, sailboat or trawler yacht, the keel sides, bottom of keel and web floors all form part of the ballast. In some cases such as in ultra-shallow draft boats, the bottom plating may also be considered to contribute to the ballast. Back in the early seventies, we did not make this sufficiently clear and this caused some builders of our designs, to install more ballast that was intended. Fortunately in most, if not all cases, this did not materially harm the handling of the boat. However carrying too much ballast can detract from the performance.

No matter who designed your metal boat, make sure you are aware of the amount of ballast that has to be added in addition to that already created by the keel and supporting structure. Of course the above mainly applies to steel hulls; aluminum hulls require the full amount of ballast quoted by the designer. In any case, during the construction stage, it is wise to only install between 70 and 80 percent of the total ballast required. The remainder can be added for trim ballast as and when required.

PLEASE .... NO CEMENT or CONCRETE in YOUR ballast mix.
Why? It causes corrosion   See METAL BOATS BOOK