I think it has something to do with pressure distribution.For the Same volume which of these will hold the water at less pressure cylinder or sphere.And which has larger surface area or volume between the cylinder or spheretrue, aerodynamics would most likely be a factor, but i think becauues of its ability to withstand the pressure of waterSpheres and cylinders aare one of the stronger shapes, thats why butane cans are cyldrical, but have an inverted half sphere on the bottomAs for your questionI believe the cylinder will hold the water at less pressure simply because it would be bigger and the metal will expand (even a fraction) much easier than a spherewhich has the larger volume? assume a cylinder 4 meters tall with a diameter of 4 metersand a sphere is 4 meters in diameterAnd assume they are both filled 90% of their capacity(im orunding results to 2 decimal) Volume of cyliner pie x r2 x H 50.27m3 x .9 45.243 Volume of sphere 4/3¶r3 33.51 x .9 34.41 there you go, a cylinder will hold more waterOther related question