Way back when, us beach rats used to love these hollow molded bellyboards with a squirt gun plug on the tip of the nose. We would stand on them on the sand to squeeze air out, then insert the stopper, which made for a concave bottom plus more spoon in the deck.
As I remember, Ziffys had five or so 3/4" ribs down the bottom, so they would hold some traction on a wave face. The handles caused tremendous drag and created a fountain effect. We never did figure out the purpose of the raised aft pontoons.
