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The third option is to use your boil kettle. At the 2015 National Homebrewer's Conference in San Diego, James Howat's presentation, ''Wild and Spontaneous Fermentation at Home'', brought up the issue of [https://en.wikipedia.org/wiki/Surface-area-to-volume_ratio surface area to volume ratio] <ref name="Howat">[http://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars/ ''Wild and Spontaneous Fermentation at Home''. Presentation by James Howat at 2015 NHC.]</ref>. The ''surface area to volume ratio'' of a hot liquid, directly affects the cooling rate of that liquid (it affects the cooling rate of all objects, not just liquids) <ref>[http://www.fmf.uni-lj.si/~planinsic/articles/Cheese%20cubes_EJP.pdf The surface-to-volume ratio in thermal physics: from cheese cube physics to animal metabolism. Gorazd Planinsic and Michael Vollmer. European Journal of Physics. 29 (2008) 369–384.]</ref>. In other words, the greater the surface area of a given volume of liquid, the faster it cools. For example, imagine 100 liters of hot liquid is in a very wide and flat container. It will cool much faster than if it was in a perfectly square container, and even faster still than if it was in a spherical container. See [http://wordpress.mrreid.org/2011/10/20/spherical-ice-cubes-and-surface-area-to-volume-ratio/ this article for another explanation of how surface area to volume ratio affects cooling].
James Howat's example of how to find the surface area to volume ratio of a coolship (note that the below is not a true surface area to volume ratio equation, but a simplified version that only measures the top part of the coolship, which makes sense since most of the heat will escape from the uncovered top of the coolship) <ref name="Howat"></ref>:
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