Air returns in a house and a small boat are different creatures. Both serve as a means of supplying air to recirculate thru the evaporator, but approached in different ways.
In a house, returns are placed in different rooms to assist in exchange of air. Merely pumping air into a closed room (think closed door) doesn't do much good. Air must be exchanged. Houseboats and some larger boats have rooms and doors, ducted air returns are necessary for proper exchange.
Sometimes as an alternative, doors will be cut shorter at the bottom to provide a return air path.
A airtight "return tube" although it won't hurt is seldom necessary. When using a sealed tube make sure nothing can obstruct it. An errant sheet of paper, a towel tossed accidentally over the grill, any minor obstruction close to a "sealed" return can be sucked and held into place very easily totally obstructing the return air. Just like a clogged filter would. Result in the summer is a frozen coil. Most marine A/C units are located under a seat or storage compartment They are dropped into place, plumbed, then a tube takes the air to a location to be blown into the cabin. The two must be separated a little to prevent the air supply and return from making a circle of air, rather than circulating around in the cabin.
Just make sure there is a filter (mine are attached to the coil) and my front A/C has a washable filter on the coil and in a rectangle vent hole in the bulkhead under the front berth.
An example that comes to mind is a "mom & pop" restaurant that smells like bacon in the morning. They may have an exhaust fan, but don't want to use it because it sucks the heat out of building. All has to be done is provide a make up air tube vented to the outside near the grill. Air comes in the tube goes to the fan and exits taking the odor with it. Makes a circle rather from the doors/ windows all over the building. You create a path of least resistance. I have even seen exhaust fans in a block building suck air down the chimney down the gas water heater exhaust pipe and blow out the pilot light..
That's my morning snowed in lesson for Sat. BoateEd looked empty this morning so I thought I'd impart something for all who cared to read.
