prevent lateral flow thereof it should be noted that rails 26 28 need not extend very far forwardly of stern 22 in watercraft intended for high speed travel a constriction member such as constriction member 38 of the first embodiment together with flap 40 may be provided at the respective trailing ends 31 a 31 b of air scoops 30 a 30 b to prevent reverse airflow as in the first embodiment a dinghy 10 a equipped with the novel air chamber is depicted in fig1 u2013 13 rails 26 28 are added to the hull of the dinghy to frame the longitudinally extending sides of air chamber 32 the construction of air chamber 32 is disclosed in co pending u s patent application filed dec 23 2003 bearing ser no 10 707 590 and having the same title as the present disclosure which disclosure is incorporated herein by reference fig1 depicts air scoop 30 at the bow of dinghy 10 a and fig1 depicts how air passageway 36 extends from said air scoop to the leading end of air chamber 32 fig1 further depicts constriction member 38 at the leading end of air chamber 32 and transom 24 that forms a barrier at the trailing end of said air chamber as is clear from fig1 elongate rails 26 28 may be added to virtually any watercraft dinghy 10.