drag d without decreasing net mouth expanding force l thereby to largely improve the rate l d further the volume of water flow on the outer side surface of otter board becomes larger by that passing through said through hole 13 from inner side to outer side of otter board and the outer water flow constitutes an accelerated laminar flow so that the pressure of water flow outside of otter board body is decreased in comparison with an otter board having no through hole 13 and otter flap therefore net mouth expanding force l and consequently rate l d are further increased in an experiment for the surface area of otter board body 11 as 100 and for the area of flaps 14 as 13 the angle u03b8 of said flaps was changed from 0 u00b0 to 20 u00b0 and the corresponding values of net mouth expanding force l and drag d of the otter board were determined for the values of l and d as 100 when u03b8 is 0 u00b0 the percentages of l and d were 107 and 80 respectively so that l was increased by 7 and the d was decreased by 20 resulting in the value of l d increased by the function of 1 34 further when the angle u03b8 is larger than the angle of attack u03b1 said flaps become rather a resistant plate with smaller l and larger d this second embodiment has a biplane construction comprising 2 wing bodies 22 22 and a bottom sled 23 said wing bodies 22 22 are connected in parallel relation with each other by means of connecting rods 24 24 fixed at the upper and.