We use Eolian all winter long (at the dock anyway), therefore we heat her all winter long. On houses, storm windows are used to provide an extra layer of insulation against the winter cold and weather. If you have ports like these:
…you can easily make and fit storm windows for them too! I don’t take credit for this idea – it came from Drew, 6 years ago.
But in any case, it couldn’t be simpler. First you need to remove the screens from your ports (we don’t have ours installed – no bugs to speak of in the PNW… :^) ). It is easy to do this. The rubber gasket that traps the screens in place is not glued in – it is just wedged into a slot on the port frame: