Coolship

Copper Coolship at a brewery in Prague

Coolship (Anglicized version of the Dutch/Flemish koelschip) is a type of fermentation vessel used in the production of beer. Traditionally, a coolship is a broad, open-top, flat vessel in which wort cools. The high surface to mass ratio allows for more efficient cooling. Contemporary usage includes any open fermentor used in the production of beer, even when using modern mechanical cooling techniques. Traditionally, coolships were constructed of wood, but later were lined with iron or copper for better thermal conductivity. See also the MTF Coolship Cooling Calculator.

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.

(This section in progress.)

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 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.

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 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 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

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, and the surface area to volume ratio . 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 . 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.

Some brewers claim that controlling the speed of cooling is important to assembling a desired blend of microorganisms in the wort . 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 . 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.)

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 = 100 sq. ft.

Surface Area to Volume ratio = 100/150 = 0.67 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 of the top surface of the wort = 6.25 sq. ft.

Surface Area to Volume ratio = 6.25/1.25 = 5 sq. ft. per cubic foot

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 match the 0.67 ratio of the 36 BBL coolship with our 10 gallon example, the dimensions would have to be approximately 0.915' x 0.915' x 1.49' with a surface area of 0.8375'. 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.

See Also

Additional Articles on MTF Wiki

• MTF Coolship Cooling Calculator
• Spontaneous Fermentation
• Lambic

External Resources

• King's Coolship, BrewingTV Episode 42.
• Lambic and Wild Ale blog; Cool Ship posts.
• The Sour Hour, Episode 11 with Rob and Jason from Allagash, Jean Van Roy from Cantillon, and Vinnie from Russian River.
• Koelschip, Lambic.info.
• Amos Browne (Browne and Bitter blog) shows that not all coolship projects succeed.
• Amos Browne's second attempt (Browne and Bitter blog) with lessons learned from the first attempt.
• Slow Learning: Sour Beer Experiments, by Gail Ann Williams.
• Black Project's Coolship explanation by James Howat.