October '06
24. Boring the Shaft Hole

24. Boring the Shaft Hole

Boring for the propeller shaft is not as simple as boring for the rudder shaft. The acute angle between shaft and hull makes it difficult for any drill bit to start the bite into the hull without deviating. So, a starting block is used, which has a predrilled channel to guide the bit. The block can be manufactured off the boat and attached to the hull temporarily.

On the drill press a piece of suitably sized hardwood is drilled vertically with a long shank drill bit. The angle of shaft entry is then taken off the hull with a bevel gauge, if it will measure an angle as acute as this, otherwise a piece of ply cut to fit. With its blade overlying the just drilled starter hole along its length, a cut line is scribed onto the block, which will pass across the hole, leaving enough stock forward of the hole exit point to allow the block to be screwed to the hull, out of the way of the advancing bit.

If it is just as easy to tilt the drill press table accurately it could be done that way.

The block is now located properly over the shaft entry point on the bottom of the hull, and is screwed down. The long shank drill bit is again employed, this time to drill through the hull, and then block is removed. The guide hole in the hull can now be employed as a pilot for a hole saw. If a 1/4" pilot hole has been drilled, attach a long 1/4" drill bit to the arbor of the hole saw of appropriate diameter. The length of the drill bit should be sufficient that it is well housed in the pilot hole before the saw part begins to cut, otherwise the angle of cut can be distorted. My hole saw uses a set screw to attach the arbor to the drill bit, so it was necessary to machine a key slot onto the bit.

The bit on this saw is too short for the job. A
longer one has to be fitted.

It will not be possible to use a variable diameter hole saw set for this job, as the mandrel will be of larger diameter than the blade you are using. A specific diameter saw will be needed.

An alternative way of doing this was employed by Frank Meijers in his Indian Runner. He was drilling from inside the hull, and used a starter block with its end cut perpendicular to the drill, so that the hole saw could be used with a normal length pilot bit. Because there was no guidance hole in the block he positioned the drill accurately with cross braces, and used a long drill extension instead.

Boring for the shaft tube in the Frank Meijers boat.

This method would have been preferable had it not been for the fact that my shaft tube passes through bulkhead D almost as soon as it enters the hull. Therefore the pilot hole also passes through bulkhead D (and its attached floor timbers), and continues to support the hole saw's progress, even after most of the through hull hole waste has been removed.

No matter which approach is used, if you are using a hole saw you will have to back it out every few centimetres and clear the path with chisel and mallet, and remove any impacted debris from the saw itself. A slow drill speed is necessary to prevent overheating, and definitely no lubricants can be used in the perimeter of the shaft hole, because it will need to be able to bond strongly with epoxy around the tube. But if you want you can lubricate the pilot hole with something like "Silver Glide", a German, silicon-free paste which woodworkers use.

Finally, Charles Fitzharding of Solarboat also recommends a specially made log boring tool, which is described in the Paul Gartside site. I have not had experience with it, but it seems a good solution for the home tool maker.

However, for the rest of this month I am off to Italy, so no more work will be done on the boat until November.

The Doge's State Barge on the Grand Canal, Venice.

Forward to November '06

Back to Ariadne front page
Back to Monthly Progress chart

Back to January '06
Back to February '06
Back to March '06
Back to April '06
Back to May '06
Back to June '06
Back to July '06
Back to August '06
Back to September '06

Problems shows higher resolution shots as well.