We are big believers in fans to keep the air moving around in the boat. We have lived aboard for 14 years and find fans are a must in both summer and winter, summer to keep things cool and winter to stop mold
and mildew during our wet Pacific Northwest
We have tried most brands and used to have several of the Hella units, they work well but are unfortunately expensive (typical boat stuff if you want it to work!) About 5 years ago we found a local product that works great, their website is Chimeron Creations, 12 & 24 Volt Marine Fans
if you want to check them out, they will do mail order to anywhere.
We have 7 of their small fans inside the boat and 2 large ones out in the cockpit
. The fans in the boat run 24/7, they are very quiet, draw very little electricity, and can be adjusted to any speed by the rheostat. IMHO they are a lot better looking than the Hella units and they cost about half as much.
They last about 3 to 4 years running 24/7 before they wear out which is about the same or better than our experience with Hellas. However the great thing is that the fan unit is easily replaceable in about 10 minutes with a readily available 12 volt computer fan that you can get most anywhere at a cost of less than $20! The unit is made so that a piece of the frame comes off the back with 2 screws and the fan just slides out of the wood enclosure. Slide in the new fan, connect 2 wires and you're done.
Great product IMHO. FTR, I have no connection with this company.