Cooling on a multimillion dollar luxury yacht is no “problem, mon”. If you can afford a mega yacht you can afford the money to fill the mega diesel tanks and run the mega generator at anchor day and night or pay the $4.00 to $6.00 per foot at the marina dock with 100 plus AC power (extra charge, of course). When in the out islands these “posers” are a real PIA to the cruisers who are attempting to sleep when the sun goes down. Those times you are forced to be at a dock the 30 amp power cord may be used to run the two air conditioners. After all, marinas frequented by itinerant cruisers of all persuasions are not very quiet places. Most of the time, in a marina, you are not given a choice regarding the direction the boat is pointed so natural ventilation is usually a problem.
At anchor the boat swings with the wind (unless overcome by current/tide) so the placement and orientation of hatches is important. The key is to be able to open the boat to take advantage of the breeze while not allowing rain in. Millennium Dragon has two small port lights over the sink cabinet in each head. These hatches allow the slightly pressurized air flowing under the bridge deck to enter the boat in the forward hull compartments. These hatches are always open when at anchor but they must be closed when departing the anchorage. Waves over two feet may splash through the hatch onto the sink cabinet. This is not usually a problem other than the clean up as you want to keep salt water out of the boat and when doing so you become upset with yourself for forgetting the *#%!@ hatch...again. There is a large hatch over the center of the head compartments that faces forward. As the heads are used for showers, fresh rain water is usually not a problem if this hatch is left open but I still like to close all deck hatches when it rains or when we leave the boat. There are port light down the top sides of each hull but these should be closed when it rains. The hatches in the staterooms are usually kept closed when we are not in the compartment as they are over the foot end of the queen size berths. Any light sprinkle results in wet bedding. The port lights at the aft end of each stateroom (in the aft crossbeam above the head of each berth) may remain open in all but the heaviest down pours as, only then will any splashing enter the boat. I build “eye brow” steps over these port lights to keep the rain out and provide safer access to the aft decks. They work very well. When the small ports over the sink cabinets in the heads and these ports are open a nice breeze flows through each hull. The main salon also has two large hatches in the aft (salon/cockpit) bulkhead. These hatches are under the hard bimini and may also be kept open. The companionway door also has a large screen that is fixed to the door frame with Velcro so, with the door open, a great deal of air is sucked out of the boat while at anchor. There are four overhead hatches in the salon that provide excellent ventilation when there is a breeze on a clear day.