I had previously marked in the position of the openings for the Dorade box ventilators using masking tape and a compass. As here.
So now I drilled a centre hole and removed the tape, and then marked in the position of the opening on the deck with a compass using the centre hole as a guide. Like this.
When I test fitted the portlights I found that the easiest way to create an opening was to drill multiple access holes and then cut out the waste with a keyhole saw.
So here I have drilled plenty of access holes in the top of the starboard Dorade box.
And here I am cutting out the opening with the Japanese keyhole saw. Much quicker and easier than a jig saw.
Finally I carefully rounded out the opening to the pencil line with a Spiraband abrasive cylinder in the drill, like this.
I used my corded drill for this. It has plenty of power for prolonged grinding activity.
The ventilators consist of three parts, illustrated in this pic.
The outer flange (top middle) is fastened to the deck with three machine screws. The inner flange (bottom left) is threaded and screws into the outer flange to open or close the ventilator. A flexible and adjustable cowl (bottom right) clips over a lip on the outer flange.
It's a bit tricky to do a test fit without a helper inside the boat. So I taped the inner flange in place and fitted the outer flange over it, tightening down fully. As here.
Lastly I marked in and drilled the holes for the machine screws, like this.
I didn't try to fit the cowls because they do not look as if they will detach easily once in place, which is of course as you would wish. So that will wait until final installation.
So that's it. Test fit complete!