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 ====Cooling Rate====The cooling rate of the exposed wort is influenced by a number of factors including the ambient temperature, the [https://en.wikipedia.org/wiki/Thermal_conductivity thermal conductivity] of the coolship material, and the surface area to volume ratio <ref>[http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/heatingandcooling/heatingrev6.shtml ''Energy transfer by heating''. BBC website, Bitesize section. Retrieved 7/24/2015.]</ref>. The most important factor is the ambient temperature, but the easiest variable to control is the surface area to volume ratio. The greater the surface area of a given liquid the faster it will cool <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>. 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]. The surface area to volume ratio also affects the inoculation rate. The more surface area to volume ratio, the more microorganisms that will be collected in the coolship for the given volume of wort <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>.

Scaling a commercial-sized coolship down to homebrewing volumes will produce a coolship that does not match the surface area to volume ratio of the larger coolship. For this reason, the surface area to volume ratio should be the driving factor in determining how to design the shape and depth of a homebrewing coolship. In his 2015 National Homebrewer's Conference presentation, Wild and Spontaneous Fermentation at Home, James Howat provides a comparison of the surface area to volume ratios between a 36 BBL coolship and a 10 gallon coolship scaled down linearly from the 36 BBL coolship <ref name="Howat"></ref>. To keep the comparison simple, it compares only on the surface area of wort exposed to the air because this is where the most heat escapes from the wort. (A comparison including the total surface area of wort would show an even larger difference between the examples.)

::Surface area (2wl + 2lh + 2wh) = 260 sq. ft.::Surface Area to Volume ratio = 260/150 = '''1.73''' sq. ft. per cubic foot :Example of a 10 gallon coolship:::Dimensions of the 10 gallon coolship: 2.5' x 2.5' x 0.20'::Wort volume: 9.35 gallons = 1.25 cubic feet.::Surface area (2wl + 2lh + 2wh) = 14.5 sq. ft.::Surface Area to Volume ratio = 14.5/1.25 = '''11.6''' sq. ft. per cubic foot</code> This example indicates a substantially lower ratio for the 36 BBL coolship. At the lower ratio, the wort will cool considerably slower. As a result difference in cooling rate between the two coolships, the inoculation rate and ratio between microorganisms will differ. Designing a coolship for homebrew volumes may result in a coolship that does not appear similar to its larger companions. To get as close to the 1.73 ratio of the 36 BBL coolship with our 10 gallon example, the dimensions would have to be as close to a cube as possible with dimensions approximately 1.1' x 1.1' x 1.03'. This still only gives a SA:V of 4.9 sq. ft. per cubic foot. A homebrewer desiring a more shallow vessel may insulate the coolship to slow the cooling rate although this may affect the inoculate rate. A typical homebrew-volume boil kettle is closer to the larger coolship (estimated 1-2 sq. ft. per cubic foot) and insulating the kettle when using it as a coolship may slow the cooling rate closer to that of a commercial-volume coolship. ====Inoculation Surface Area====The ''top'' surface area to volume ratio affects the inoculation rate. Some brewers believe that this inoculation rate should be somewhere near what commercial coolships are. An example to find the SA:V ratio of just the top surface using the same two coolship dimensions follows: <code>:Example of a 36 bbl coolship:::Dimensions of the 36 coolship: 10' x 10' x 1.5'.::Wort volume = 1122 gallons = 150 cubic feet. ::Surface area of the top surface of the wort (w x l) = 100 sq. ft.

::Surface area of the top surface of the wort (w x l) = 6.25 sq. ft.

</code>

This The example indicates a substantially lower ratio for above shows that the 36 BBL shallow 10 gallon coolship. At the lower ratio, the wort will cool considerably slower. As also has a result difference in cooling rate between the two coolships, the inoculation rate and much larger ''top'' surface area to volume ratio between microorganisms will differ.