I was keen to not damage the transom when cutting out the slot for the tiller, inside or outside the boat.
So I fixed a piece of scrap ply on the inside face of the transom where I would be drilling access holes for the jig saw, to prevent tear-out by the drill bit. Here it is.
And here is a temporary screw through the transom, holding the scrap in place.
Next I pencilled in the holes in the corners of the slot, as here.
I wanted to drill holes of the same diameter as the bearing on the bearing guided round over cutter which I intended to use in my router so that the cutter would glide smoothly around the corners of the slot, creating an even round over.
It required a 13mm hole, which I marked up using this fantastic device given to me by my son at Christmas. It's called an Iris drawing compass, and is used to accurately draw any diameter circle up to 70mm.
I've set it to 13mm and marked in the drill holes in each corner. Thanks Nick!
I used the big DeWalt drill to make the holes. Here it is with the hefty drill bit.
The holes were drilled easily enough. Here they are.
Then I used my jig saw to cut out the waste, and cleaned up the edges with a rasp. This is what it looked like.
Next I used the router fitted with a 1/2" round over cutter to shape the inside edges. Here is the router.
And here is the partly shaped slot.
The transom skirt prevents the router from shaping the top of the slot - it's in the way. The piece of blue tape marks the limit of travel. The top section has to be finished by hand.
So next I had to mark in the rest of the round over on the sides and top of the slot. Like this.
Rasps, files and sandpaper were used to finish off the slot. Here it is.