Every boat will benefit from better ventilation, it’s one of the things few boat builders consider. Opening port holes are great but must be closed if it’s raining or if you leave the boat. Here’s a simple solution which allows the ports to be left open in all weathers whilst still admitting light.

The first thing I did was make a cardboard template to test the validity of  the idea then I used 3mm thick lexan. Lexan (polycarbonate) is excellent to work with and has good anti scratch and UV properties. A jigsaw and a sharp blade is all you need to cut the Lexan. To clean up the edges, a small block  plane is perfect.

Two upright supports were made with three slots at an angle of 45 degrees.  The slots in the uprights and the horizontal pieces are only cut halfway. The pieces slot together and it is pretty solid even without gluing. The slats work perfectly at deflecting the heaviest rain but water will find its way in at the top of the port hole so to stop this a small strip of self adhesive neoprene is stuck to the top edge of the assembly.
 
Now we have a nice flow of air in all weathers and we can still look out of the porthole. The cost was nothing as I had the lexan and neoprene kicking  about. I made two, one for the galley and one for the heads and they took  about 3 hours to make. A perfect Sunday afternoon project!