Simple and Inexpensive Air Conditioning

The heat is already upon us in northern Florida and summer has not yet arrived. Already, the air conditioner is running daily, and day and night most of the time. On our previous boat, we did not have the luxury of a built-in air-conditioning system, and early on, we decided not to install a central system. It would be another piece of equipment to maintain and would also require a generator installation for use when we were cruising. We opted for an inverter system to run the 110 appliances and tools, and to take a little different approach to the AC problem. Of course, on our current trawler Beach House, we have a built in heat/AC system and a generator. Living aboard in the Chesapeake in the summer, then many years in south Florida did require that we cool the interior of the boat if we wanted to be comfortable. While we were cruising through the Bahamas and Caribbean, we never felt the need for an air conditioner. But at the docks for periods of time, to replenish the cruising kitty, we decided on a less traditional setup.

A place to mount the unit had to be determined first. Actually, we tried installing one in the hatch on the cabin top. It cooled the area directly under the hatch, but the rest of the boat got pretty warm. There was also the issue of making it watertight during heavy rain storms. Next, we tried the companionway hatch. As long as we could use a secondary entrance, like the aft-cabin hatch, this could work. In a sailboat, the main companionway may need to be used and this will require stepping over the AC unit when coming or going. Most companionway entrances have groves cut to accommodate drop boards. In the case of some powerboats, one of the large windows can be carefully removed, and the mounting process close to the same. Since the space where the drop board can go is small, and the thickness of the space for this is ½”, we began with a ½”X2’X2’ piece of marine or exterior plywood. You can determine the size that will work for you. If using the space from an existing window, try and match the window thickness. Then the frames can be reused for a nice finished look. We also used a tube of silicone caulk, some sandpaper, a little paint and an aluminum strip like those used on the bottom of doors. Tools will consist of a power saw with a fine tooth blade, a straight edge, level, screwdriver or screw gun, metal snips, disposable gloves, pencil, caulking gun and paint brush. And, of course, you will need the air conditioner. We usually purchased them from the local building supply or discount department store. For our boat, we need a minimum of 6,000 BTUs, and 8,000 BTUs makes for very comfortable conditions. The size is determined by the volume of your cabin and the available mounting space. The physical dimensions of the unit should be kept at a minimum.

The first step is to cut your plywood to fit the entire space of your opening. Be sure the hatch will slide shut with it in place if there is an overhead hatch. Next, with the plywood in place, draw a level line as near the bottom as space will allow. This level line will be where the bottom of the AC will rest. Once these two steps are completed, lay the plywood down and set the AC on the level line with the face of the unit pointing up in the air. Trace around the unit with a pencil using the unit itself as a template. Once finished, you can cut out the opening into which the unit will slide. Take a moment to fit the AC into the opening to be sure it is snug. You should have very little space around the unit. Then, remove it so you can do a few more tweaks.

Approximately 2” above where the top of the unit would be, we drew a line straight across the entire piece of plywood. Use the straight edge or the level. Then, cut this line on an angle that is such that the rain will not run into the cut edge - very much like the angles that are cut into your drop boards. This piece can be lifted out like your hatchboards to make getting in and out easier. We then attached our door bottom aluminum trim, with the rubber strip attached, to the top edge on the outside to give us more protection from rain and washing the boat. Again, slide the AC unit into the opening to be sure it is a good fit. If it looks okay, remove it again, and go ahead and sand and paint the plywood to seal the wood from moisture and to dress it up a little. 

Now it is time to mount the unit. Remember that when it is running, it will draw humidity out of the air. That moisture collects in the bottom of the unit and has to drain somewhere. Mount the unit on an angle so that the water will drain to the back. Sounds simple, but there are a few things to adjust. If your water and holding tanks are a bit forward so when they are full the bow is down slightly, fill them. That way, the boat is trimmed down in the bow a bit. With the AC mounted on an angle to allow drainage, it will tilt back a little more as the tanks are drained, instead of tilting forward when the tanks are filled. Use the level to determine that you have sufficient angle. If the unit tilts forward it, will drain INSIDE the boat as moisture accumulates. Many units have small drains in the rear to which you can attach a plastic hose to let it drain overboard. The front of the unit should only be sitting a couple of inches inside the boat. Once you are confident that the unit is sitting where you want it, simply load your caulking gun with a tube of marine silicone caulk. Fill the space completely around the AC on both the inside and outside edges with the silicone. Use a moist finger to smooth it out, but leave plenty. Be careful until the silicone dries that you do not move the unit. After 24 hours, we have been able to lean on the box without it moving. You now have a very inexpensive air-conditioning system. The whole thing can be lifted out in one piece if necessary.

If there is any drawback to this system, it is that you might have to step over the whole thing when coming and going, depending on where it is mounted. But this has never been a big issue.  The top board can easily be removed to allow you to step onto the companionway ladder. And it can be run from a small gas-powered portable generator like the Honda 1000 or 2000 when the power fails. We used this system for 12 years before finally installing a built-in marine heat/AC unit. We can usually find a good AC for between $78.00 and $120.00. They will last for four to five years or longer, and then just buy a new one and you're back in business in a couple of hours. We have watched many of our friends nearly suffer cardiac arrest after receiving repair bills for their central air systems or realizing the cost to replace them. I know this approach is not for everyone, but for those of you on a tight budget or that have space constraints, it is another option. The same approach with a little modification can be used to mount your unit in an overhead hatch if the companionway won’t work. Stay cool this summer!


  1. Chuck, hope you and Susan are well and your recovery is going good. I was wondering if you are familiar with C Quarters in Carabelle. I may need a haulout and minimum survey for insurance upgrade. Does C Quarters have a dependable service of this type? will

  2. Will, We do know C-Quarters very well but they won't be able to help you with any of this. There is a good marina just across from C-Quarters called Dockside Marina. Good folks and they did a good job for us. They have a travel lift and may be able to recommend a surveyor. Or you can contact BoatUS for the surveyor, even if you aren't a member. We're doing fine and recovery is coming along, albeit slower than I would like. Thanks.

    1. Thank you very much, take care and we hope to bump into you guys one day. Will


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