Marine air conditioning has gotten complicated with all the system types and sizing calculations flying around. As someone who has sweated through enough humid summer nights below decks before finally investing in proper AC, I learned everything there is to know about cooling a yacht. Today, I will share it all with you.
Why Marine AC Is Its Own Category
Your home’s central air won’t survive a week on a boat—I’ve seen people try. Marine air conditioning faces salt spray, constant vibration, extreme humidity, and power constraints that residential equipment simply cannot handle. Purpose-built marine units address these challenges with corrosion-resistant materials, compact designs, and efficient operation.
The marine environment also creates unique cooling challenges most people don’t anticipate. Boats absorb tremendous solar heat through fiberglass and canvas. Engines add heat even hours after shutdown. And unlike homes with consistent power, boats must generate their own electricity or rely on shorepower when available.
Understanding Your System Options
Self-contained units are the most common choice for boats under 50 feet, and for good reason. Everything—compressor, evaporator, and fan—fits in a single compact unit that mounts in a locker or under a berth. Raw water flows through the system for cooling, eliminating the need for bulky external condensers.
That’s what makes self-contained systems endearing to us motoryacht owners—the simplicity of a single unit doing everything it needs to do.
Split systems separate the condensing unit from the air handler, allowing more flexible installation. They’re quieter in the cabin because the compressor mounts remotely, but they require more complex installation with refrigerant lines running between components.
Chilled water systems dominate larger yachts in the 60-foot-plus range. A central chiller cools water that circulates to multiple air handlers throughout the boat. While expensive and complex, these systems offer precise zone control and simplified maintenance for larger vessels.
Getting the Sizing Right
Probably should have led with this section, honestly.
Marine AC is sized in BTUs (British Thermal Units). The rule of thumb: plan for 1,000 BTUs per foot of boat length, then add capacity for high solar load areas like pilothouses and salons with large windows that bake in afternoon sun.
A 35-foot cruiser typically needs 16,000-20,000 BTUs. A 45-foot motoryacht might require 24,000-36,000 BTUs distributed across multiple zones. Oversizing slightly is better than undersizing—an overworked system runs constantly, consuming more power and wearing out faster than it should.
The Power Reality
A 16,000 BTU system draws roughly 13 amps at 115V—perfectly fine on shorepower but a significant load for a generator. Running air conditioning at anchor requires either a properly sized genset or a substantial battery bank with serious inverter capacity.
Newer inverter-driven compressors offer improved efficiency that’s worth the premium. Variable speed motors adjust output to demand rather than cycling on and off constantly. They’re quieter, use less power at partial load, and start more gently—important on inverter systems where surge capacity creates limitations.
Installation Isn’t for Beginners
Marine AC installation isn’t a weekend project—trust me on this one. The system requires raw water plumbing (typically tapping into an existing thru-hull or adding a dedicated seacock), electrical connections, ducting if using directed airflow, and condensate drainage that actually works.
Professional installation typically runs $3,000-5,000 depending on system complexity and accessibility. DIY is possible for mechanically capable owners willing to learn, but mistakes with raw water plumbing below the waterline can quite literally sink boats.
What It Costs to Run
Beyond electricity, marine AC requires regular maintenance you can’t skip. Impeller pumps need annual replacement before they fail at the worst possible moment. Raw water strainers require frequent cleaning, especially in seagrass areas. Filters in air handlers should be cleaned monthly during heavy use.
Budget $200-400 annually for routine maintenance on a single-zone system. Repairs—particularly compressor replacement—can run several thousand dollars, making extended warranties worth serious consideration on expensive units.
Heating When You Need It
Most marine AC units include reverse-cycle heat pump capability, providing both cooling and heating from a single system. Performance drops in very cold water (below 50°F), but for three-season boating in most regions, heat pump operation provides adequate warmth on cool mornings.
For colder climates or extended shoulder-season cruising, consider supplementing with diesel-fired heating. Webasto and similar systems provide reliable heat independent of shore power or generator runtime.
Who Makes the Good Stuff
Marine Air, Dometic (formerly Cruisair), and Mabru dominate the market for reasons that become clear when you compare build quality. All produce reliable equipment when properly installed and maintained. Price differences often reflect features like digital controls, inverter compressors, and warranty terms rather than fundamental quality differences.
Budget $4,000-6,000 for a quality 16,000 BTU self-contained unit. Add $3,000-5,000 for professional installation. Multi-zone systems on larger boats can easily reach $15,000-25,000 fully installed—a significant investment that pays comfort dividends daily.
My Honest Assessment
For coastal cruising in summer, marine AC transforms the onboard experience in ways that are hard to appreciate until you’ve lived it. Sleep becomes genuinely comfortable. Entertaining below decks becomes possible even in August. And the boat remains usable during the hottest months rather than sitting idle at the dock.
For occasional weekend use in mild climates, the cost may be harder to justify financially. But for anyone planning extended cruising, living aboard, or simply enjoying summer evenings below decks with family and friends, marine air conditioning delivers comfort worth every dollar of its considerable investment. Mine paid for itself the first summer I had it.
