I found that adding a dehumidifier to my Cal 40 kept our boat dry, light and mildew free. After a long day of sailing i can put wet gear including a soaked spinnaker down below and come back the next day and everything is DRY.
Step 1: Buy a Dehumidifier
You need to get a special unit that has an attachment for an external drain. This enables you to route the drain to the bildge so you can run the unit all the time. I bought a ComfortAire for around $130 at my local hardware store. You will also need a 5ft garden hose to attach to the unit. Each unit may vary so check and make sure to get the proper hose for your unit.
Step 2: Make sure that your bildge pump is working.
Check and make sure your bildge pump works well and the float switch works. You will be pumping water into the bildge and activating the float switch so it must work. You need to also make sure the shore power is connected and charging the batteries so running the pump will not drain your batteries.
Step 3: Attach hose to dehumindifier.
After you have the unit in the boat attach the hose so it will be ready to run and drain into the bildge.
On my unit there is a plastic piece that goes through the rear of the unit, you may have to punch a hole to make it fit--the hole is like a punch out. This will keep the unit from draining into the bucket in the bottom of the machine. I zippedtied mine so that it would not come out easy and turn the unit off.
Screw the garden hose to the external plastic piece.
Step 4: Placing unit in boat for use
Find a place in your boat where it will be stable from waves and harbor surge. Make sure you also have access to the bildge easily for the hose.
Insert the hose in to bildge, and turn on the dehumidifier--you are done and your boat will never be so FRESH.