Long wires can also be used for transmitting (non-directional) a wide range of frequencies by use of antenna tuners which can "adjust" the antenna electrically to various wavelengths. Directional (yagi type) antenna elements are usually 0.5 +/- 5% wave length long and spaced 0.15 to 0.25 wave length. A 30 megahertz wave length is 10 meters so the elements would be about 5 meters. A 10 megahertz wave length is 30 meters, a 4 megahertz wavelength is 75 meters. For maximum directivity the boom lengths would be multiple wavelenghs long to hold the multiple elements. Also you would need to get such an antenna at least a half-wave high to be effective. Therefore directional antennas at the normal 3 to about 10 megahertz frequencies (At night) on any boat were not practical due to size. 57 years later satellite UHF signals could be used (non-existant in 1939-45).