December '06
26. The Dummy Shaft

26. The Dummy Shaft

The propeller shaft tube mock up has to pass through five obstructions:

• the hull itself
• the complex comprising bulkhead D and the floor timbers attached to its forward and aft surfaces
• the complex comprising bulkhead C and its floor timbers
• the intermediate floor timber located between bulkheads D and C
• the intermediate floor timber located forward of the bulkhead C complex which serves to support the foot wells, if such a floor becomes a reality.
  

Showing the obstructions to the shaft whose path is close
to the chine line (the diagonal).

As yet there is no floor located forward of bulkhead C in the boat's current stage of construction. I have been waiting to see if the foot wells are actually necessary, or whether there will be enough foot room without them. But is seems likely that they may be built, for comfort perhaps, rather than absolute necessity. Clearly, it will be easier to bore a hole through the entire bulkhead C complex rather than through its unassembled components, so the floor attached to its forward surface has to be made and fastened prior to the shaft being drilled at this level.

Detail of the bulkhead/floor complexes, intermediate floors
and foot well. Yellow dots mark points through which the
shaft must pass.

Had I thought this far ahead when the boat was still upright I would have attached the floor then, but it can still be done, albeit with some flowery language, inside an inverted hull.

The driver's compartment (left) showing the floor attached behind bulkhead C, and the engine compartment (right)
shot from beneath, showing the forward surface of bulkhead C with no floor and no fillet.

Firstly, small drill holes are made at the level of the top of the aft-side floor through the bulkhead, to find the right level for the top of the forward floor. Then the forward floor is shaped to the bulkhead profile and fitted so that it is (hopefully) exactly level with the aft floor, such that the sole will be able to pass across both floors without encountering a bump or dip. The forward floor is attached to the bulkhead only temporarily, as it may yet be necessary to plane it down or fill it up to the true level of the aft floor, once the cut out in the bulkhead is made for the foot wells.

Inside the engine compartment, the floor has been screwed to bulkhead C ready for drilling. Blue dots mark the location of four
 bamboo skewers (barely visible in the photo) which have been threaded through the bulkhead to locate the floor's vertical level.

The boat is now ready for drilling, and the drill guide is screwed onto the hull at the point of penetration from the outside. A pilot hole is drilled through the guide and into the hull as deeply as it will penetrate.

The drilling jig is marked at the point of emergence of the bit, and this point is aligned with the appropriate position on the hull.

The jig is centred and the angle of inclination of the drill bit is parallel to the shaft line (dotted on the template).

The guide is then shortened, and the pilot hole extended. This is repeated until the guide can be removed altogether, and the drill emerges through the forward surface of the bulkhead D complex.

Before shortening the jig the long bit only just makes contact with the hull. Afterwards, when the jig is removed, the pilothole can be seen,
centred on the required entry point. The screw holes will later be removed by the hole saw.

Before proceeding with the hole saw to cut through the hull, I set up a laser level, using the keel template to align its beam at the correct angle, such that it passes through the pilot hole and projects forward to the intermediate floor between bulkheads D and C. The beam's intersection with the back of the floor is marked, and it's athwartships dimensions are checked to ensure that it is truly in the midline. Corrections are made, and a punch is used to locate the point. A shorter drill bit is now employed and, using the punch hole and the laser spot, another pilot hole is drilled through the intermediate floor. In this case, the laser spot can be located on the back of the drill at a point directly in line of the bit. Keeping it so aligned during the drilling ensures the correct drill angle.

An arrow on the hull marks the position of entry of the propeller shaft. Darkened photo (right) shows the laser beam.

This process is continued now up the the bulkhead C complex, locating the laser beam, checking for accuracy athwartships and pilot drilling. As the intermediate floor between bulkheads C and B is not yet fitted, the laser beam should now be able to reach through as far as the back of bulkhead B.

With the pilot holes all aligned it is now time to start the boring, so the hole saw is attached to the long bit again. The first bite into the hull is an anxious time, as it seems that it only wants to skim along the surface. Here again, the laser beam can be used against the back of the drill to reassure you that it is going at the right angle.

Laser beam falling on the back of the drill (left), and the hole saw set to begin the bore into the hull (right).

Soon enough the saw stops progressing because it is full of waste. It needs to be cleared, and so does some of the waste in the hull. You will need to back out the saw to clear both it and the hole quite frequently.

Here, the shortened drill guide is reattached to the hull to start the hole saw cut. It can then be removed for the
remainder. The first lot of waste has just been removed from the hull.

Fortunately, the long pilot drill bit enters the bulkhead D complex before the hole saw has cut through the hull completely, and this helps keep the drill aligned. But before the hole saw has fully emerged through the hull to the inside, the drill motor comes up against the hull on the outside. So an extension to the hole saw/drill bit is needed. There are snap on extensions on the market designed for hex head spade bits, but the hole saw I have is too big to fit into them. An extension has to be acquired which is less than the diameter of the hole saw, but strong enough to withstand the torque. These things are not readily available in Australia, but I managed to track one down on Ebay. It will give me an extra 300 mm. of clearance from the drill, and should be enough to clear the hull.

Extension bit for spade drills(left) and hole saw arbors (right).

Admittedly, there is a certain amount of inherent instability about these extension sets, but in this case the saw is being firmly supported by the pilot hole. In addition, as the hole saw I am using is the same diameter as the stern tube, it is certain that the holes will need to be enlarged a bit for perfect alignment, so even if there a little bit of wobble in the extended hole saw that will not be a problem.

I should point out here, that in order to use the extension bit I had to acquire an arbor with a 7/16" fit. The diameter of that arbor is 35 mm. or just over 13/8". As the arbor has to follow the hole saw through the hole, the minimum diameter of hole saw which could be used was 7/16" which is 36.5125 mm. In the end, it seemed not to matter, as there would need to be some enlargement of the hole anyway for a 15/16" stern tube to fit through, taking into account the slight irregularities in alignment of the holes.

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