Roger & Roger-
I've had a similar model with similar results. 8.3 amps x 12 volts = 100 watts. Nice, and in easy conditions, it will keep a single window marginally clear in situations where the moisture in the boat air is not too challenging and the outside temperature is not really terribly cold. But it's not a cure-all for the defogging problem. Has just as much heat to offer as a 100 watt light bulb, since most of the 100 watts in an incandescent bulb actually is radiated as heat, not light.
Realistically, though, the problem of defogging can be anything from a minor annoyance easily blown away with a small fan to conditions that could defeat even your car's defrosters even after the engine is fully warmed up. Moisture comes from breathing, perspiration, unvented heaters/cooking stoves, wet clothes, foul weather gear, and dogs. I've found that I can deal with most of the above, sans as case of really wet clothes, wet foul weather gear from trying to troll in the rain, and of course, my 175 lb Great Dane, Shadow, when she's a wet, stinking mess!
The steps involved in defogging will vary somewhat based on the situation, but usually include the following:
Turn on ventilating fans on the windows- I have two mounted ones that can oscillate, and a similar third one mounted to a Stanley metal giant clothes pin style spring clamp with a cigarette lighter plug that can be moved around the boat as needed. These are the most efficient defogging units of the various devices used.
Provide ventilation to external air to get rid of the excess humidity. A vent, cracked window, etc.
Turn on your Wallas, Force 10, Dickinson, Toyomi, Webasto, Espar, or whatever to raise the temperature in the cabin and thus lower the relative humidity. This does not apply to unvented heaters, e.g., propane heaters w/o external venting, as they will simply add to the water vapor problem.
The Origo alcohol stove (original equiment) on the C-Dory is a very bad water vapor producer and a problem itself when used in cold weather.
Use another fan like the ones in the front windows to circulate air throughout the cabin, thus spreading the water vapor around evenly, distributing the heaters warmth, and assisting in getting fresh air into and out of the cabin. An exhaust fan would be useful here.
Turn on your small defogging windshield heater directed at the helmsman's starboard front window.
I've not done this, but in extreme climates, one could install a tube from the heater output area over to the front windows powered by a couple of computer CPU fans, one at the intake and another at the output, placed at the bottom of and directed upward on the window directly in front of the helmsman. Probably could be done with clamps like the one mentioned above and the same type lighter plug so it could be removed when not being used, as space near the cabin top is at a premium in our boats. The Toyomi, Webasto, and Espar units are really forced air diesel furnaces and have their output directed in tubes, so this would be a natural for them.
The real task here is to figure out how to get as much hot dry air as possible to the windows as well as the cabin with what we've got to work with. With an inboard engine, you could use hot water from the cooling system to get the job done with a cabin heating unit and ducting.
If you've got really wet clothes, gear, or whatever, change it out and put it in a plastic garbage bag to be dried out later, rather than let it fog up the windshield for the next hour or so.
If you have to go out in the rain to tend the rods, put up the bimini to minimize the pick up of rain on your clothes.
Stop trolling, anchor, and still fish if it means you don't have to go out in the rain as often.
Go back to your covered berth and fish under a roof. In my case, at least, there are more fish (bass especially) concentrated under the marina than in most places in the lake!! Since you're not driving right now, turn on the coking stove, heat up some water, and make a couple of Irish Coffees and contemplate how good life is with the C-Dory!!! AND Think about how to devise a better defogging system! Hmmmmmmmmmmmmmm???!!! Joe.