Alwest (and some Alcan) boats have a “passive” cabin bilge vent on the port side of the superstructure, just aft of the window in the head. This passage comes up between the shower and the superstructure and appears to be designed to use the temperature differential between the cool bilge and warmer cabin wall to create a “chimney effect” to draw air out of the bilge.
This can be made to work much better if it is fan driven.
On Catherine Ann, we use the lower forward cabin as our main sleeping area ( transformed into a queen size cross wise berth) and continuously purging the main bilge draws stale odors from this area as well.
The means to do this is a surplus 5” square 12V computer cooling fan. (an investment of about $10.00). Actually, the hood that covers it, cost more than the fan?. The fan draws a fraction of an amp and runs continuously whenever we are on the boat. It is installed from the outside, into a square opening provided during the re-sheathing of the cabin exterior (see previous tech article).
There is no internal ducting required since this fan is gradually exhausting air from all interconnected hull and bilge spaces.