This would be a challenge because the gallows is not flat and the holes must be drilled into a curved surface.
They must also be perfectly perpendicular and exactly the right distance apart, or the gallows will not fit onto the poles.
The first activity was to measure the exact distance between the support pole holes in the seatbacks.
The gallows pattern shows where the holes should be, but the chances of that corresponding to what I have built are virtually zero.
So I cut a couple of 25mm wood dowels to exactly the same length to substitute for the steel poles. We can't use the poles themselves - there is not enough headroom.
I hammered a tack into the end of each dowel, in the centre. Then I installed the dowels and laid a piece of straight stock across the boat on top of them, making sure that it was level.
It looked like this.
By tapping the piece of stock down onto the dowels I marked the exact distance between their centres.
Needless to say it was about 1/4" out from the pattern!
The pattern says the 25mm holes should be 60mm deep. I set up the pillar drill to practice on some scrap, and promptly found that it only has 55mm of travel.
So that was that - my holes would be 55mm deep.
Then I carefully set up the gallows on the drill table. This picture shows how it was done.
I scribed a horizontal line along the gallows, exactly level. This would be my datum line for checking that the gallows was horizontally level on the drill table.
I marked a line where the centre of each hole should be using the measurement obtained above, and made sure the line was perpendicular to the datum line.
I then scribed this line across the bottom face of the gallows and marked in its middle point. This would be the centre of the hole.
Then I clamped a piece of straight stock to the gallows, exactly on the datum line.
The gallows was clamped in place on the drill table with a heavy drill vice, and propped up until exactly horizontal, as measured by a spirit level resting on the datum line. As here.
The last task was to ream out the holes in the gallows a little to accept the stainless steel poles, like this.
That's it! I now have 55mm deep holes in the boom gallows in hopefully exactly the right place. We will see when we are fitting out.
That was a lot of fun!