Marine Air Conditioning (BTU) Sizing Calculator
Calculate the required cooling capacity in BTU/hr for marine air conditioning on your yacht or boat based on cabin dimensions, insulation quality, climate zone, window area, occupants, and equipment heat loads.
Air Conditioning Sizing Results
How do I calculate BTU for marine air conditioning?
Marine AC sizing starts with the cabin volume and then applies adjustment factors for insulation, climate, sun exposure, hull material, window area, occupants, and equipment. The base load is derived from the cubic volume of the space multiplied by a BTU-per-cubic-foot factor, then adjusted upward or downward for the specific conditions.
$$BTU = (Volume\;Load + Window\;Load + Occupant\;Load + Equipment\;Load) \times (1 + Safety\;\%)$$
Why is marine AC sizing different from residential?
Boats and yachts present unique challenges compared to buildings. The hull is surrounded by water and air with no ground insulation, windows are often large relative to the cabin size, engine rooms generate significant adjacent heat, and the vessel moves between different climate zones. Marine AC systems also use seawater-cooled condensers, which are more efficient in warm water but require different sizing considerations.
What is the typical BTU per square foot for a yacht?
A common starting point for marine AC sizing is 14–25 BTU per cubic foot of cabin volume, depending on insulation and conditions. For well-insulated mid-level cabins in subtropical climates, 18–20 BTU/ft³ is typical. Sun-exposed flybridge areas or poorly insulated spaces in tropical climates may need 25–30 BTU/ft³ or more.
How does window area affect the cooling load?
Glass transmits solar radiation directly into the cabin, creating significant heat gain. Single-pane glass in direct sunlight can add 150–250 BTU/hr per square foot of window area. Tinted glass, double glazing, and Low-E coatings can reduce this substantially but cannot eliminate it entirely. Large saloon windows and panoramic glazing are one of the primary drivers of oversized AC requirements on modern yachts.
How much heat does each person generate?
A person at rest generates approximately 400 BTU/hr of body heat. With light activity (moving around the cabin, cooking), this can increase to 500–600 BTU/hr. In a small cabin with several occupants, body heat can be a meaningful portion of the total cooling load.
What is the safety factor for marine AC?
A safety factor of 10–20% is standard practice for marine air conditioning. This accounts for duct losses, equipment ageing, extreme weather conditions, and uncertainties in the heat load calculation. Under-sizing an AC system on a yacht is far more problematic than slight oversizing, as the system may not be able to maintain comfortable temperatures during peak conditions.
What is a ton of cooling?
One ton of refrigeration equals 12,000 BTU/hr. Marine AC units are commonly described in both BTU/hr and tons. A typical yacht cabin might require 6,000–16,000 BTU/hr (0.5–1.3 tons), while a large superyacht saloon could need 36,000–60,000 BTU/hr (3–5 tons) or more.
$$Tons = \frac{BTU/hr} {12{,}000}$$
$$kW = BTU/hr \times 0.000293071$$
Does hull material affect cooling requirements?
Yes. Metal hulls (aluminium and steel) are excellent heat conductors and transfer solar and ambient heat into the cabin more readily than fibreglass or wood. An uninsulated aluminium hull in tropical sun can significantly increase the cooling load compared to a well-insulated GRP hull. Proper insulation is especially important for metal-hulled yachts.