A spherical mini submarine of diameter 3 m is designed to be operated by two persons. The submarine is used for deep sea exploration where the water temperature may be as low as 6 degrees C. It was found that under such conditions the inside air temperature is uncomfortably cold. Specify the capacity of an electric heater that will maintain an inside temperature of 21 degrees C. Power source limitation requires that the heater does not use more than 500 W. Furthermore, weight consideration limits the increase in weight to a maximum of 34 kg. The inside heat transfer coefficient is estimated to be 12 W/m^2-C. However there is considerable uncertainty regarding the outside heat transfer coefficient. A very rough estimate suggest that it could be as high as 100 W/m^2-.