Why ten torpedoes changed the attack problem

Royal Navy statistical reasoning estimated roughly one hit from a five-torpedo spread and three from ten. Interlaced or slightly staggered groups made it harder for a defending ship to find a safe passage between the tracks. Combing a spread means steering between those tracks to avoid the torpedoes, and even a five-weapon attack could sometimes be combed completely. Three hits were expected to cause severe damage or sink the ship. The same concern with survivability informed the preference for more twin gun turrets, which reduced the proportion of armament lost when one mounting was disabled.