One of the things that always bothers me is to open the cabin hatch and be greeted by hot humid air and the smell of...well you know...dampness.  The solution is to add ventilation.
For this project I turned to Jamestown Distributors and found a UFO vent by Vetus.  I chose the model with a translucent center.  It lets a little light below deck.  Vetus also makes an Electric Ventilator Fan, which I opted to build myself out of a reclaimed computer fan, some scraps of oak, and some left over screen from my Screen Door Project.  The Electric Ventilator Fan only costs about $80 but if you are like me and enjoy a challenge then this project can be fun.
First the UFO Vent
Contrary to the criptic instructions, this vent is very easy to install.
I installed mine in the foredeck hatch.
1.  Cut a 2-7/8 hole in the hatch lid.
2. Place the black gasket over the hole.
3.  Place the O-Ring in the top groove of the translucent insert.
4.  Place the white splash guard over the gasket with the fingers pointing up.
 5.  Drill holes and screw down the splash guard with six #6 stainless steel pan head screws.
6.  Place the translucent insert over the fingers of the white splash guard.
7.  Place the stainless steel splash guard over the translucient insert making sure that the translucient insert seats properly.
8.  Screw down the stainless steel splash guard with four #6 stainless steel pan head screws.
Next comes the Electric Ventilator Fan
I opted to build my own but this looks like a very nice unit.
Home Built Ventilator Fan
 I build my Fan Assembly around a computer fan I had in my spare parts box.  To allow better ventilation I place 4 rectangular holes around the sides and stapled screen on the inside edge.
The UFO fan comes with a bug screen for the large round hole.
Originally I thought I would need the extra section of wood that the fan is mounted on.  I thought the hatch would be too thin to mount the UFO and the fan.  However it turned out not to be necessary.  As you can see in the last photo everything mounted just fine without it.