While many have considered Air conditioning of some sort. I don't think I want the hassles - I'm not at a slip. I was just brainstorming this possibility...
Or a marine air conditioning unit, one designed to run from a 12V source?
look at cruisair dot com. Find the section NEW! Cruisair Cuddy dc Kits. This new low-profile 3500 BTU self-contained unit is made for boats too small for an onboard genset. It is powered by a user-provided dedicated power module. Read about it in the PDF file linked
I'd also recommend 1 or 2 Nicro vents which work well. I have one on the forward hatch and when it's operating it cools the boat 5-10 degrees.
Is there truly no leakage from rain, spray etc with the nitro installation? I am not talking about getting it set and sealed properly, I am talking about leakage from the fan housing itself? Seems like if it is open to pass air then water can get in.
Ive buried the nose on my with a Nicro 3 inch on the front hatch and never gotten more than a couple drops in at worst. They are designed to be pretty resistant to letting water in from up top.
My problem with the Nicro was, the battery lost all power to retain juice after the 2nd season. I bought a brand new rechargeable battery from Home Depo and it still doesn't hold enough juice to keep it whirring through the night. What the...? I remember distinctly it used to spin at first.
I have 3 of the $ 80 fans from West Marine and another plug in portable. They do well as long as the temps drop below 75 at night for me.
Inorder to get more air moving through the cabin when it is not raining -
Stern anchor with a yoke on both stern cleats. Move the bimini all the way forward and leaned back on the campainway and open the forward hatch. Set the boom so that it will stop the end of the bimini thus holding it open. 4x to 5x the air flow
If you expect to much wind during the night or big waves, rig a bow line to the bow cleat and a quick release.
The only place I can see using that type of vent is on the main sliding hatch, simply because it requires such a large hole to be cut for mounting and the front hatch isn't flat. (I've mounted three of the thermostatically controlled MaxxAirs on my travel trailer). Another reason is they are designed to not allow normal rain fall in, but are not sealed well enough to prevent water ingress from a boarding sea, so a top of the cabin mount is best.
The reason I think it's not a good idea is it will significantly reduce head room in the only spot inside where you can stand upright if you are over 5'11" tall, and, it or any other vent in that location will limit how far you can slide the hatch forward unless you make a mounting so that it is flush on the inside instead of the outside. A raised mount on the outside will reduce the view straight ahead for the helmsperson.
last place goes to a little blue and white oscillating fan that makes a ton of noise and developed shaft corrosion sufficient to stall it out after a couple of seasons in salt water.
Granted the walmart fan has not seen salt water yet, and is only one year old but it blows a lot of air, is almost silent and appears to be low power consumption.