I have to give a big shout out to Nicro Ventilation for their Day/Night Plus solar vents.
I installed 4 of the 4" units at least a year ago. One is over the V-berth, one over the aft seat of the settee, one in the head to exhaust compost gases, and one in the lazerette. They come with two fans so you can set them up either as exhaust or intake ventilation, as you choose. And, although they come with a removable grill, I don't think they fit all that well. Besides that, they attract dirt and dust, impeding airflow. The grills are definitely not needed for safety as reaching the on/off switch requires you to stick your finger up into the revolving fan blades. The switch is above the blades. But, since the motor doesn't have any real torque, this is not a safety issue. I've chosen to remove the grill altogether.
Even in Newport, we get enough sun, to run the fans all day and charge the built-in battery enough to have them run all night as well. OK, in the interest of full disclosure, I guess that's not entirely true. In the dead of winter the fan ran all day and part of the night although it was turning pretty darn slow by bedtime. But they run all night now. Of course we're only a month away from the longest day of the year, but still...
During most of the last 6 months the vent over the v-berth was covered by the upside-down dinghy, rendering the fan useless. The vent was still open but there was nothing other than convection and outside breezes to cause any airflow through it. Occasionally I would wake up to a cold drip of water caused by my breath condensing on the porthole directly above my pillow. As soon as I removed the dinghy, the fan started turning and I haven't been dripped on since.
These units are rated to move 1000 cu. ft. per hour.
Our composting toilet requires continuous airflow in order to dry out the compost and allow efficient aeration of the bacteria. A in-line fan is provided that runs off our house battery. But, since a vent was still required, I figured why not add a little extra air flow without any more drain on the house batteries. So I installed a Nicro vent on a dorade:
I didn't use dorades on the other vents because by pushing up on the tube that surrounds the fan, you can close the vent off. So if we get seas aboard, we can keep the water from entering the cabin by closing the vents. But, since the one in the head would be connected directly to an air discharge hose, it would be tough to open and close the vent. So I built a dorade instead. For those not familiar with dorades, they are a box with a partial baffle in the middle dividing the front and rear of the box. If water enters the vent, it'll fall straight down and is kept from entering the cabin by the baffle. It then drains out the half circle holes you can see directly below the Nicro. Air simply travels over the top of the baffle and then down through the hole in the cabin top.