The phenomenon referred to as waterhammer can be used to create the pressure required for reverse osmosis of seawater, this has been described in a previous paper [1]. A subsequent paper then investigated economic feasibility and concluded that it could achieve significant cost savings as well as eliminating the need to burn fossil fuel energy for seawater desalination [2]. It is necessary to investigate the theoretical feasibility of using hydro-ram for this purpose. A major question is whether the hydro-ram (typically used for pumping relatively small quantities of river water to a higher level) can pump the large quantities of seawater required for desalination. Literature is reviewed in respect of the various designs and especially large designs and the theories of its operation are explained. Formulas for determining the output of a particular size of ram are identified. Seawater corrosion is also considered. It is concluded that it is both economically and technically feasible for this simple piece of apparatus to be used for the desalination of seawater.