HARTFORD, Conn. — A new, Spanish-designed submarine has a weighty problem: The vessel is more than 70 tons too heavy, and officials fear if it goes out to sea, it will not be able to surface. And a former Spanish official says the problem can be traced to a miscalculation — someone apparently put a decimal point in the wrong place. “It was a fatal mistake,” said Rafael Bardaji, who until recently was director of the Office of Strategic Assessment at Spain’s Defense Ministry. The Isaac Peral, the first in a new class of diesel-electric submarines, was nearly completed when...