Transition temperatures and smectic B mesophase ranges are examined for a number of homologous series, and some general rules are proposed that relate the smectic B phase behaviour to molecular structure. A mechanical model is developed for the crystal and hexatic smectic B phases, based on the dynamics of rigid or elastic rotors, which suggests two possible types of behaviour. It is found that such predicted behaviour is observed for particular series, but in general the smectic B phase characteristics of homologous series may be between the limits established for the model.