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Copper Coolship at a brewery in Prague

Coolship (the Anglicized version of the Dutch/Flemish koelschip) is a broad, open-top, flat vessel in which wort cools. The large surface area in relation to volume allows for relatively efficient cooling. It is also an integral step in the process of making Belgian lambic and other spontaneously fermented beers. Historically, coolships were constructed of wood, but later were lined with iron or copper for better thermal conductivity. Most modern constructed coolships are made from stainless steel, although copper coolships are still used in many Belgian lambic breweries as well as a few breweries outside of Belgium. Pre-dating the specific application of coolships by Belgian lambic brewers for inoculation during cooling, some German and English brewers also still use copper coolships (kühlschiff in German and "cooler" in Great Britain) as chillers to partially cool wort to 55-75°C (131-167°F) before either chilling the wort in a closed system or transferring the warm wort to a fermenter to cool and finally pitch yeast, a process which was once common throughout Europe [1][2][3][4]. Saison breweries used coolships during the 1800's in a similar fashion to German brewers to cool the wort before pitching yeast [5].

Contemporary usage also includes using a coolship as an open fermentor for the production of non-sour beer. For example, Anchor Brewing Co fermented beer openly, but because the wort is pre-chilled before entering the vessel, Anchor Brewing no longer refers to these as "coolships" but as "open fermenters" [6]. Contemporary experimental usage of coolships includes allowing wort to cool and become inoculated with microbes from the environment, as well as adding cultured yeast at some point in the process, and optionally using the coolship as an open fermenter to conduct the primary fermentation in rather than conducting the primary fermenation in barrels like in Belgian lambic brewing. This is not considered to be full "spontaneous fermentation" since yeast and/or lactic acid bacteria are added to produce a more controlled fermentation, but the resulting beer might benefit from or be otherwise altered by the wild microbes or the effects of the lack of head pressure/exposure to oxygen caused by cooling (and optionally fermenting) the wort in a coolship [7]. A less common practice that has been gaining some traction in the US is using a portable coolship. These coolships are designed to be transported, often in the back of pickup trucks. The coolship should have a baffle that prevents the dangerously hot wort from spilling out of the coolship. The coolship can then be transported anywhere to inoculate the wort. See this All About Beer Magazine article.

This article will focus on the use of a coolship to initiate spontaneous fermentation and will not significantly address the use of coolships to partially cool wort as in the German/English/saison methods of chilling wort or to conduct open fermentation.

See also the MTF Coolship Cooling Calculator. See Spontaneous Fermentation for information on dissolved oxygen in wort that is cooled overnight in a coolship.

Homebrew Coolships

Miniature Coolship by Devin Bell
Boil kettle coolship by Gail Ann Williams. Cheese cloth was used to keep out debris, and a chair was carefully placed to keep out wild raccoons.

Homebrewers may use coolships in their home brewhouse as a way to cool and inoculate beers to be spontaneously fermented. The purpose of a coolship for homebrewers is identical to commercial brewers. (For more information on the process of brewing with a coolship, see Spontaneous Fermentation.) However, the vessel selected as a coolship generally will be determined by the available resources of the homebrewer and the effect of the coolship will be driven by the surface area to volume ratio of the wort within the coolship.

Common Homebrew Coolships

Homebrew coolships range from repurposed vessels to custom designed equipment. There are costs and benefits associated with each design that should be considered.

Some homebrewers prefer to purchase or repurpose from the home suitable shallow, food grade containers used in cooking or catering. These include stainless steel or aluminum baking pans, large glass baking pans, food grade plastic trays and storage containers. These vessels are often cheap and may already be available in the home. It can be tough working with large vessels with no valve to drain the coolship into a fermentor. One must make sure the selected vessel is designed for use with hot liquids so it does not crack or melt. Cooling boiling wort in plastic containers that are not rated for boiling temperatures may leech chemicals from the plastic depending on what they contain, even if they are food grade.

Others design coolships using specialized equipment fabricated for that purpose or created out of stainless steel or copper parts. An easy route to design a coolship may employ restaurant supplies such as steel storage racks and stainless steel tubs. These designs allow for convenient features such as a ball valve for draining the cooled wort or screening to keep out break material from the boil kettle. The coolship can be built to an optimum surface area to volume ratio. These coolships are often the most expensive route but the most customized and durable.

A third option is to use the boil kettle as a coolship. In this method the kettle is simply left outside after the boil. Here there is no need to purchase or design a separate vessel and it may already provide a valve to drain the cooled wort and handles for easy movement. However, by using the boil kettle the brewer has no choice in the surface area to volume ratio of the cooling wort and there is no opportunity to remove the break material from the wort prior to cooling.

Regardless of the method selected, the surface area to volume ratio should be carefully considered due to its affects on cooling and microorganism populations in the spontaneous fermentation.

Surface Area to Volume Ratio

Abbreviated SA:V, the surface area to volume ratio is the proportion between the surface area of the wort (the dimensions of the wort exposed to the sides of the vessel), and the volume of the wort. The SA:V affects the cooling rate of the wort. In addition to this, the surface area of the top of the vessel which is exposed to air in proportion to the volume of wort potentially affects the inoculation rate of the vessel, although this aspect has not been studied that we are aware of.

Cooling Rate

The cooling rate of the exposed wort is influenced by a number of factors including the ambient temperature, the thermal conductivity of the coolship material (for example, Jester King Brewery noted a faster, more preferable cooling rate for a 30 bbl coolship made from copper versus stainless steel because copper is a better thermal conductor [8]), and the surface area to volume ratio [9]. 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 [10]. 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 this article for another explanation of how surface area to volume ratio affects cooling.The more surface area to volume ratio, the more microorganisms that will be collected in the coolship for the given volume of wort [11].

Some brewers claim that controlling the speed of cooling is important to assembling a desired blend of microorganisms in the wort [11]. Microbes survive and multiply at different temperatures and cooling too long or too fast may produce a beer that lacks desirable character or possesses an excess of undesirable character. A larger surface area of wort will allow for greater inoculation of microbes although if the wort cools too quickly the majority of inoculation will occur at cooler temperatures and affect the ratio and growth of various microbes in the wort. (For more information on the effects of the cooling rate see Spontaneous Fermentation.)

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 [11].

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 (2wl + 2lh + 2wh) = 260 sq. ft.
Surface Area to Volume ratio = 260/150 = 1.73 sq. ft. per cubic foot (56.8 cm2 per L)
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 (380.6 cm2 per L)

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 may 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 may want their coolship inoculation rate to 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:

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 to Volume ratio = 100/150 = 0.67 sq. ft. per cubic foot (22.0 cm2 per L)

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 of the top surface of the wort (w x l) = 6.25 sq. ft.
Surface Area to Volume ratio = 6.25/1.25 = 5 sq. ft. per cubic foot (164 cm2 per L)

The example above shows that the shallow 10 gallon coolship also has a much larger top surface area to volume ratio.

Coolship SA:V Calculator

Mark B. Fry's coolship SA:V calculator will calculate the SA:V for cylindrical kettles (updated by Joe Idoni to include a second tab for rectangular coolships). In order to use the files they must be downloaded. Click "File", "Download as", and select "Microsoft Excel (.xlsx)":

Testing Cooling Rates

These resources have tested the cooling rates of various coolship types.

Commercial Coolships

This table shows the dimensions of various commercial coolships (sources noted below the table). The volume of the coolships has been calculated from the dimensions, and therefore it is slightly larger than the practical filling volume. Surface area:volume is calculated for the top surface exposed to air for both the maximum fill and the approximate batch sizes as well as the total surface area (including the walls and bottom of the coolship). Some breweries use multiple coolships at once. When multiple identical coolships are used, numbers in parentheses following brewery names indicate the number of coolships used and the volume given is the total combined volume of these coolships. Metric is used so that the calculations are easier. For brewers who prefer ft2:ft3, 10 cm2:L is equal to 0.305 ft2:ft3.

Brewery Coolship Material Volume (total if multiple) (HL) Surface Area:Volume (top, min) (cm2:L) Actual Surface Area:Volume (top, estimated) (cm2:L) Surface Area:Volume (Total, min) (cm2:L) Brew Size (HL, approx) Length (cm) Width (cm) Height (cm)
3 Fonteinen (4) Stainless Steel 62.3 35.7 55.6 90.8 40 400 139 28
Cantillon* Copper 75 33.3 38.5 74.7 500 500 30
De Troch (2) Stainless Steel 57.3 38.3 68.7 88.7 32 349.3 314.5 26.1
Lindemans (new) Stainless Steel 234.3 19.9 23.3 46.4 200 1113.3 419.3 50.2
Timmermans Copper 236.1 26.8 57.5 58.6 110 810.1 781.3 37.3
Omer Vander Ghinste Copper 198 18.2 25.7 43.0 140 600 600 55
Van Honsebrouck Stainless Steel 352.5 17.2 26.4 39.6 230 778.9 778.9 58.1
Liefmans (2)** Copper 542 28.2 31.9 62.2 480 1749 440 35.4
Black Project (full)*** (Closed) Copper 13.6 21.9 24.8 68.4 12 243.8 121.9 45.7
Black Project (half) (Closed) Copper 6.8 21.9 24.8 76.6 6 121.9 121.9 45.7
Jester King Copper 84.9 21.9 50.2 53.6 37 609.6 304.8 45.7
Mantra (Closed) Stainless Steel 18.9 32.8 35.2 83.1 17.6 381 162.6 30.5
Monkish Stainless Steel 22.6 16.4 23.2 54.2 16 243.8 121.9 91.4
Orpheus Stainless Steel 28.3 NA ~26 76.7 (actual) 10 91 (radius) NA 43.2 (fill height)
The Referend (full)**** Stainless Steel 55.4 21.9 34.5 56.1 35.2 508.0 238.8 45.7
The Referend (small) Stainless Steel 19.4 21.9 36.3 63.4 11.7 177.8 238.8 45.7
The Referend (large) Stainless Steel 36.0 21.9 33.6 58.2 23.5 330.2 238.8 45.7
Sour Cellars Stainless Steel 7.4 17.5 18.0 70.7 7.2 135.9 95.3 57.2
Strange Roots Experimental Ales Stainless Steel  ?  ?  ? ?  ?  ? ?  ?
Trinity Stainless Steel 27.2 10.9 17.5 46.5 17 243.8 121.9 91.4
The Veil Stainless Steel 31.7 16.4 29.5 50.4 17.6 243.9 213.3 60.9
Yazoo***** Stainless Steel 25.7 14.1 21.3 49.2 23.5 198 183 71
Little Wolf****** Stainless Steel 16.1 22.2 27 79.6 12 650 55 45
Shady Oak Barrel House******* Stainless Steel 5.98 30.3 34.2 94.6 5.3 183 99 33
Atom Brewing Company******** Copper 5.678 25.4 28.26 84.1 5.1 119.4 120.7 39.4

*An extra vessel is available to hold additional wort in case the wort volume exceeds the coolship capacity

**These coolships were used to cool wort to an intermediate temperature (~50-60 C) before finishing cooling with a Baudelot chiller, rather than for inoculation. They are no longer in use.

***The Black Project coolship is a single vessel with a divider allowing the coolship to be half filled with the depth and exposed surface area remaining the same as if it were completely filled.

****The coolship at The Referend Bier Blendery is a single vessel divided into two unequal compartments. This allows the coolship to be filled with three different brew sizes while maintaining the same fill depth and exposed surface area.

*****The coolship at Yazoo is custom designed to be used year round and double as an open fermenter. Fabrication by Badger Fabrication in Wisconsin.

******The Little Wolf coolship is not designed to be a coolship, but rather a vessel designed for soaking all of the hoses used in the brewery, thus the less traditional dimensions [12].

*******Shady Oak Barrel House coolship has a top that can be clamped down with a pressure release valve for transferring hot wort within the coolship to another location [13].

********Atom Brewing Company's coolship slopes to 15.5 inches; calculations do not include this slope.

This table is based on data collected by Belgian Beer Geek on the characteristics of coolships in various breweries and from MTF member contributions[14].

Overhangs and Other Innovations

Some breweries have opted to craft overhangs that simulate a wooden ceiling above their coolship. The idea is to simulate some lambic breweries that have relatively low ceilings made from wood. During the cooling of the wort in the coolship, vapor rises to the ceiling, condenses, and then drips back down into the coolship, potentially carrying microbes that are living on the wood of the ceiling (see also Sources of Microbes in Spontaneous Fermentation).

Other examples of innovative customizations to the coolship design include Yazoo's Embrace The Funk Coolship wort immersion chiller for experimenting with cooling at warmer ambient temperatures, and Black Project's coolship that can be split into half batches.


Any metal tank manufacturer can generally build a coolship, so local resources are likely to be available [15]. However, below are manufacturers that specialize in their construction for commercial breweries:

Some commercial brewers have reported positive results from using industrial maple syrup boil pans. See this MTF thread for more information.

Miscellaneous Information

Open Fermentation

Some commercial brewers have used their coolship to double as a fermenter for open fermentation (not to be confused with Spontaneous Fermentation). For example, see this MTF thread. For more information on open fermentation, see Michael Dawson on BrewingTV and this Beer & Brewing Magazine article.

See Also

Additional Articles on MTF Wiki

External Resources


  1. Lauren Lerch. "Coolship Field Trip". Crafty Beer Girls blog. Retrieved 03/18/2018.
  2. "Malz – aus Gerste und Weizen ". Zum Uerige website. Retrieved 03/18/2018.
  3. Various members of MTF. Milk The Funk thread on the use of coolships in historical German brewing. 07/28/2019.
  4. Martyn Cornell. Zythophile website. 10/18/2019. retrieved 10/18/2019.
  5. Georges Lacambre. Dec, 1851. Translated by Lucas Blasty on Milk The Funk Facebook group.
  6. "Anchor Terminology: Coolship". Anchor Steam website. 04/12/2012. Retrieved 09/10/2018.
  7. Brandon Jones of Yazoo Brewing Co. Milk The Funk Facebook group thread on using a coolship and pitching Bootleg Biology yeast. 10/18/2019.
  8. "Jester King Brewery"; Interview with Averie Swanson from Jester King Brewery. Brewing Network Session Podcast. 01/30/2017. ~1:22:00 minutes in.
  9. Energy transfer by heating. BBC website, Bitesize section. Retrieved 7/24/2015.
  10. 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.
  11. 11.0 11.1 11.2 Wild and Spontaneous Fermentation at Home. Presentation by James Howat at 2015 NHC.
  12. Stefan Wiswedel. milk The Funk Facebook group thread on Little Wolf's "coolship". 10/15/2018.
  13. Dustin Carver. Milk The Funk Facebook group thread about Shady Oak Barrel House coolship lid, clamps, and PRV. 11/16/2018.
  14. MTF thread about coolship dimensions
  15. Conversation with Joe Wells on MTF. 06/11/2016.