This has been just a little tricky! We have had to drill a hole, about 30mm diameter and 600mm long, right through the keel, to take the propeller shaft from inside the boat to the stern.
We thought long and hard about how to do it, and eventually came up with a three part plan. This video gives an idea of how we went about it.
The first thing was to drill a 10mm pilot hole. We welded a standard HSS drill bit onto a meter long length of rod, and fixed up a wooden jig to hold the drill in place. The supports would ensure it stayed roughly in line, but we couldn’t guarantee that the drill bit wouldn’t wander off course a little. We did a couple of test drills, and found that it came out about 3mm off where we intended. Not bad.
Then we fabricated another drill bit. This time it was two meters long, with a couple of cutting blades welded in the middle to make a bit that would drill a hole about 20mm diameter. We fed the end of the drill through the pilot hole, and adjusted its position so that it was in place. There was enough slack in the pilot hole to allow us to shift it to correct the inaccuracy in the pilot hole. Drilling was slow progress, mostly we think due to the problem of clearing waste material from the cutting blades.
Then we fabricated a third drill bit, same design as the second, but with cutting blades the right diameter to take the stern tube. This went through much more easily, and the stern tube fitted like a glove.