Beat the Heat: Retrofit an A/C System

“I will not sleep on this boat. I’m going home–NOW!” said my overheated sailing mate and spouse, Monica.

“I will not sleep on this boat. I’m going home–NOW!” said my overheated sailing mate and spouse, Monica.

For years, our 1986 Hunter 40, Gratitude, had been an ideal weekend and summer vacation cruising platform for exploring New England. We’d enjoyed beautiful summer nights on the hook and day-long broad reaches to and from our home port of Westbrook, Connecticut. During that time, Gratitude had been treated to a number of upgrades that made two-handed cruising safe and quite pleasant. Doing much of the work myself not only reduced costs, but provided me with a great outlet for the stresses of my office job.

Gratitude at anchor; ever since the A/C installation, the name seems even more appropriate

Gratitude at anchor; ever since the A/C installation, the name seems even more appropriate

One year, as the leaves were turning, a job change forced a move to Washington D.C., where Gratitude looked great in her new home just south of Annapolis, Maryland. The challenges of the shallower depths of the Chesapeake Bay were offset by the extended cruising season. A Thanksgiving Day sail that first year followed by dinner aboard was a treat we never would have experienced “up north.”

Springtime commissioning also came early, with Gratitude back in the water on April Fool’s Day. “No joke,” I told my snowbound sailing friends in New England. Three months of sailing followed, as we began exploring the largest estuary in the world. Life was good.

The companionway before installation

The companionway before installation

Then came the Fourth of July and our first real heat wave. The six-inch fans that provided so much comfort in New England now only served to push around a lot of overheated air. As my crew threatened mutiny I realized Gratitude desperatelyneededa new piece of equipment—air conditioning.

I started my research at that year’s Annapolis Boat Show. What an event for a DIY project! Manufacturers and their local representatives offered lots of advice and ideas on how best to tackle this project. My tote bag was stuffed with product brochures, detailed spec sheets and business cards.

Armed with this detailed information and my scribbled notes, I spent the next few months exploring options and finalizing choices. As I began to visualize the final installation, I convinced myself that this could indeed be a successful DIY project.

 The companionway after installation

The companionway after installation

How much air conditioning?

The first step was to determine how much air conditioning I needed. Air conditioning (as well as heating) systems are rated in British Thermal Units, or BTUs, per hour, with each BTU equaling the amount of energy required to increase or lower the temperature of one pound of water by approximately 1ºF. Larger numbers indicate more powerful systems.

Back-of-the-envelope estimates from manufacturer’s representatives I’d met included the phrases “it depends,” “space available” and “your preferences”—which was reasonable given the many variables involved. When pressed for a guesstimate, most suggested I would be well served with a 12,000 BTU/hr or 16,000 BTU/hr unit in the saloon, and another 5,000 BTU/hr unit in the aft cabin.

The reps all stressed the need to both calculate the amount of living space that needed to be chilled, and to differentiate those spaces by the amount of direct exposure to sunlight they receive. Measuring the interior of an object with few square areas takes some imagination, but I applied a bit of helpful advice from the show—measuring the large “boxes” comprising most of the volume in each area—to create a close approximation. (See Figure 2) As part of the process, I used a layout diagram from the owner’s manual to create a small model, which quickly showed how best to use the natural divisions in the boat. Gratitude has three cabins—a small forward stateroom, a saloon and an aft stateroom—and I measured the maximum length and width for each. (Passageways, small heads and closets need not be included.) Don’t be concerned if some of the measured area falls outside the hull.

Layout of a typical marine A/C system

Layout of a typical marine A/C system

At the Annapolis show I’d picked up a copy of the The Cruisair Guide to Installation, which suggested using the average width. However, on the advice of some of the experts I’d spoken to, I used the larger numbers to create a fudge factor to allow for less than ideal efficiency. The guide also contained a simple chart with a “load factor” based on deck and climate. For the Chesapeake, I chose “temperate climate” and “below deck” to get a load factor of 60. Multiplying the load factor (60) times the square feet (279) suggested that I needed a unit with a capacity of 16,740 BTUs/hr, which was very close to the guestimates I had received at the show.

Where do I place these units?

My Hunter was built with a reinforced hull liner to maximize the belowdecks spaces. As a result there is little space under the sole and behind settees and cabinets for routing the large air ducts and water hoses needed for AC units. Installers experienced with Hunters suggested I use two units—one in the saloon with a duct to the forward cabin and another in the aft cabin.

The saloon has two large settees with ample stowage. However, a new holding tank and plumbing nearly filled the space beneath one, and the other housed the Adler Barbour refrigeration compressor. Repositioning some of that gear would allow for the installation of a 16,000 BTU/hr A/C unit. There was also a path from below the starboard settee through a cabinet in the forward head to the forward cabin. I considered installing a slightly larger unit in this location for the entire boat. But the search for an acceptable path to the aft cabin ended at the galley stove.

To calculate what size air conditioning unit you need, divide the boat up into “boxes”

To calculate what size air conditioning unit you need, divide the boat up into “boxes”

Meanwhile, in the aft cabin there was a small space beneath a closet that was just large enough to hold the smallest unit. However, working in that tight space, running the plumbing and ductwork was going to be a real pain. I had also used that space to store spare parts and tools, so those would have to be relocated. In addition, a 5,000 BTU/hr unit was overkill for the aft cabin by almost 35 percent.

I was starting to not like where this project was headed. Originally, I had hoped to install a single unit in the saloon, but was now faced with doubling everything—two units, two through hulls, two water pumps, two filters. Worse yet, when I calculated the electrical demand for the two recommended units, I ended up past the limit of my single 30-amp shore service. The thought of adding another shore power connection was a tipping point and forced me to take a step back.

What do I do now?

First, I revisited the assumption of 6 feet of headroom, which had been used to calculate the load factor in the Cruisair Guide. In fact, on Gratitude, only the saloon carries that much headroom throughout. The forward cabin, on the other hand, has standing headroom, but also contains a 100-gallon water tank that raises the entire V-berth and dramatically reduces the total cabinspace volume. Similarly, the majority of the aft cabin is squeezed under the aft cockpit.

To recalculate Gratitude’s BTU/hr needs, I used the measured cubic feet (1,366) divided by the guide’s assumed cubic feet (279 x 6 = 1,674), which yielded a factor of 81 percent. Multiplying the original load of 16,740 by 0.81 suggested a capacity of approximately 13,500 BTU/hr. This difference was to prove extremely valuable.

I also reconsidered “how cold,” or more importantly, “how much colder than outside” we wanted Gratitude to be, and ultimately concluded that a 15ºF difference in air temperature with reduced humidity would make the cabins pleasant on most days.

Combined with my new reduced area estimate, this suggested that a single 12,000 BTU/hr unit would be more than adequate on most days in the Chesapeake. In addition, this unit’s electrical load would not require an additional 30-amp service. Now I just needed to find a place to put the thing!

Back to the boat

Much of the work thus far had been done in the comfort of my study, with the winter winds howling outside and Gratitude safely on the hard. Now I decided that a fresh look at installation options was in order.

The A/C unit finds a home atop the engine box

The A/C unit finds a home atop the engine box

Again, I wanted to find a solution involving just one system—as this would significantly reduce both cost and complexity—so I focused on the two large settees in the main cabin. I had originally dismissed the port settee because it was almost filled by a new holding tank and hoses, but a more careful inspection revealed that some of those hoses could be rerouted, opening up sufficient space for the A/C unit. There was also an easy path from this spot to the forward cabin, although the path to the aft cabin would have to go through the nav station and head. 

It was while searching for a better path aft that I found myself studying the engine compartment, located between the galley and aft head. On the Hunter 40 the top of this compartment forms a platform that splits the companionway steps into two parts. Behind the three steps comprising the upper segment, there is also space for three small shelves in front of the aft cabin bulkhead.

 I began to wonder if I could mount the unit behind these steps, as this location would provide an easy path for air ducts into both the aft cabin and the saloon. As an added benefit, I’d have much easier access for plumbing and electrical runs.

Investigating further, I made the surprising discovery that the entire unit could be removed, as the mirror on the bulkhead in the aft cabin covered a set of mounting screws. In less than 20 minutes I was able to disconnect and remove it to get a good look at what lay behind.

In fact, the space behind the cabinet proved to be surprisingly empty, containing only the wet exhaust hose and siphon break assembly attached to the bulkhead. The three shelves built into the unit were mounted from the rear and could also be easily removed— potentially leaving a perfect hole to mount the return air and supply grilles. 


I began thinking to myself that this just might work.

The next step was to put the cabinet into the trunk of the car and head home to work out the final installation plan and compile a detailed equipment list. Removing the shelves was easy, as the shelves and trim pieces had been pre-assembled and then simply secured in place with six wood screws. With the shelves removed I could compare the available space with the measurements on the detailed specification sheets I had collected at the boat show. As luck would have it, the box could easily accommodate the width and height of the Cruisair 12,000 BTU Stowaway Unit I had in mind. However, the depth would require some carpentry, as the side and top needed to be extended by approximately 3/4 of an inch. That work became the final item on the rough work plan.

The complete parts list included:

Cruisair 12,000 BTU Stowaway Unit

SMXII controller

three teak grilles

Groco Strainer, 2 through hulls and hose

120v water pump (500 gph)

20 amp/120v A/C breaker

miscellaneous clamps, wire, condensation drain hose


Now it was time to source parts and get on with the installation, so I placed an order for all the parts I needed. I decided to source the major parts through a local manufacturer’s representative rather than through the marine supply stores. This was another piece of good advice I picked up at the boat show: should I run into problems and need assistance, I’d have access to the right resources.

 The actual installation went remarkably well

The actual installation went remarkably well

I broke the job into these major steps:

physical installation of A/C unit and control box

fresh water supply and discharge through hulls

fresh water hoses, filter and water pump

condensation drain hoses

electrical and control wiring

cabinet modifications

A/C grille/ductwork installation

The actual installation went remarkably well, with only a couple of extra trips back to the chandlery. It took two long days to install the A/C equipment and another two days to finish the cabinet work. When it came time to re-launch, I performed a few quick functional tests to confirm that everything was operating properly, but the real test came a few months later when we had our first really hot day. The outside temperature climbed to a scorching 95ºF, but inside it was a cool 75ºF…and we were one happy and very comfortable crew!


>Bob and Monica Bell sail Gratitude, their 1986 Hunter 40, on the Chesapeake Bay


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