Marine Air Conditioning: General Design Thoughts

The installation of air conditioning in my boat has been a thought process spanning more than a year. In preparation for the possibility of this project, I took a tour of my boat builder's factory. I was able to see how the factory installed the air conditioner, its location, and the routing of the various components.

To further determine how the factory unit is installed, I was able to obtain reference drawings showing the locations of the various components. While the air conditioner the factory installs is not the Mermaid unit, it still provided useful information as to the routing of the ductwork, location of the thru-hulls, and other components. Knowing where the various items will fit is of great benefit.

One issue is that although the Mermaid unit is generally the same physical size as the factory unit, the BTU rating is about 30% more. This means I have to use a minimum of two 4in ducts where, I believe, the factory unit uses one 4in and one 3in duct. This might present a challenge, since I may have to enlarge the cutouts to facilitate the larger duct. Mermaid also recommends using a larger return air grille than the factory unit as well. I would like to use insulated ductwork, but lack of space may prevent this.

Fortunately, even with the heat coil, the AC power requirement is still less than the 20A circuit breaker I will be using, so at least I will not have any power issues to contend with. As I will explain later, the connection to AC power in this particular boat is going to be the easiest part of the installation - it's already done! The boat manufacturer pre-wired an AC breaker in the AC power panel, and even added an "Air Conditioner" label. All I had to do is to order a special cable assembly from the manufacturer which mates to a special connector on the back of the AC power panel. This cable and connector was around $25, which makes the power connection a cinch. I simply need to wire in the AC power cable's pigtail and plug it into the breaker panel. Its worth mentioning that had this option not exist that I would have to wire the air unit to power myself. And the appropriate wiring, as like all wiring in a boat, according to US Coast Guard Regulation 33CFR183.425, is STRANDED wiring.



I plan on placing my air conditioner components as close as possible to the factory locations. After all, on a boat this size, there is not a lot of extra room, so using the existing pathways for the ductwork will hopefully make the job easier.

Since the boat did not come with factory air, the rear storage compartment was finished off with a rear and left sidewall. This results in not enough room to install the air conditioner. Therefore, I will have to modify the storage compartment by cutting out the rear wall in the rear storage bin. If factory air were installed, this cabinet would have been likewise modified.

The removal and re-installation of the storage compartment is pretty difficult, due to the close tolerances involved. For this reason, I took about 1/8in off the bottom edge of the cabinet, which meant that I had to completely remove the cabinet, take it home, and run it through my Jointer/Planer. This greatly eased the chore of installation and removal. As the project went on, I had to remove this cabinet several times - so this is a modification that was well worth the time.

Another problem to overcome is a satisfactory distribution plenum at the top of the blower. The issue I have is a limited height in the rear berth compartment for the plenum. None of the stock plenum configurations I could find were going to be suitable. I finally decided to make my own plenum using sheet aluminum. To do this, I purchased an inexpensive box-and-pan brake from Grizzly Industrial (Grizzly's Web Site), which should made the job pretty easy.


One thing I did discover from an air conditioning expert is that you should NOT make a square or rectangular plenum, as it inhibits airflow. This is apparently common in the boat building industry, but it would never be acceptable in the air conditioning industry. Therefore I will attempt to create a plenum with a "curve" in it to avoid this issue.

Here is a rough idea of the plenum I will attempt to make. Although it doesn't seem to be that much different than the square box shown above, it helps direct airflow. I do not have any direct experience on this matter, so with a leap of faith, I am taking what I am being told by an experienced technician as fact.

At this point I can sketch an idea of how to assemble the system. I expect that over the course of construction that a few mistakes will be made, and I will change out a few items. This is basically my "minds-eye" view of all of the components.



I need to ensure that there is a sufficient path for the condensate drain. The method I will use is to raise the air conditioner a bit, providing a positive flow path for the condensate. Within the restrictions of the cabinet, and the plenum I will use, I can raise the unit 2 inches. I plan on using a foam rubber pad for vibration isolation, sitting on top of a base I will make. The base will also be useful in locating the mounting screws away from the edge of the boat side.


The next consideration is to determine where to locate the thru hulls. I may have to deviate a bit on the exit thru hull - simply due to the exit water line location of the air conditioner is unknown at this point. As long as I keep the exit thru hull well above the waterline, there should not be an issue.

The entry thru hull is near the transom, and close to the centerline as planned. As luck would have it, the boat builder has already glassed in a stiffening pad in this area to facilitate the thru hull.

One last thought about the entry thru-hull. Make sure all of the components are made from bronze. The temptation might be to use a bronze thru-hull and ball valve, but to use a brass nipple, since they sometimes easier to find. This might present a problem, since galvanic current between the dissimilar brass and bronze, when immersed into water (especially salt water), might corrode or weaken the brass. Some boat builders run a ground wire to the thru hull connection, and this might not be a bad idea, since it protects the thru-hull by shunting the current to the system ground, and eventually to the sacrificial zinc anodes. And, since the seacock ball valve usually has dissimilar metals within, it might be a good idea to ground the thru hull regardless. If grounded, a brass nipple can be used, but I plan on grounding and using a brass nipple. The other option is to use a non-metallic thru-hull.

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