I have one in my C36. Not withstanding that analysis, it works perfectly up to about 85 degrees. Still acceptable to 90. I can run it on a Honda 2000 if needed. I guess it comes down to how many on board etc and how cold you want it. It isn't as simple as creating a vacuum and displacing cold air. And I do it where it retains the water. It'd be even better if I used a drain, as it is optional. And this with no sealing of duct. I think it's a 10000 Home Depot deal. Cheap and can wedge into the aft berth entrance. On a 30 it'd freeze me out.Think about this: There is a big, flexible duct that is exhausting very hot air to the outdoors. Where does all that air come from? The indoors, or in this case, your boat's cabin. Now, if air is being sucked out of the boat, that means that either your boat will become a vacuum and the a/c unit will not be able to work, or air will be pulled from the outdoors into your cabin to replace the air that the a/c unit is pushing out through the duct. That means that the unit pushes the air that it has cooled out of your boat and pulls hot, humid air into your boat. Now lets think about all that heat - where does it come from? The a/c unit uses lots of electricity to cool hot, humid air. The heat that is pulled out of the air goes out the duct, but also all the electrical energy is converted to heat and that supposedly goes out the duct, too. Trouble is, as Bob4203 explained, some of the heat radiates and is convected out of the duct, back into your boat. Since an air conditioner actually makes more heat than it removes, the unit is always fighting itself. Then you need to deal with all the condensate that you get by dehumidifying a constant flow of humid air passing through the boat. All in all, those portable units are terribly inefficient and are only moderately useful if you are in the path of the cool air jet and you don't mind being in a cold draft in order to feel cool. Window units reject heat and condensate directly outdoors and do not pull conditioned air from the indoors. They are a better choice.
I'm not a mechanical architectural engineer. Mine works perfectly for $300. I can remove it easily in the winter. Freezes me out at night and helps in the middle of the day. On a C36 and not the 30. But again... I'm not an architectural electrical engineer.I had one given to me; I do know how they work. Gave it back within three days. I'm an architectural mechanical engineer. My specialty is energy efficiency. Except for dealing with condensate (yes, some units sling the condensate onto the condenser coil - so do some window units - but they can be overwhelmed then drip), tell me one thing about my post that was inaccurate. By the way, how many cubic feet of space does your boat's cabin contain? My 22,000 Btuh home unit cools 13,000 cubic feet in Alabama heat and humidity. But you need 11,000 Btuh to cool your boat? See what I mean? You might feel cool, but you are wasting a lot of energy. And you need to jerry-rig an inconvenient configuration to make it work. I do agree that you can try one though, then take it back to the store if you don't like it. That's what my friend did with the one she gave me.