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Building Tips - Installing twin propeller shafts

One of the tricky, early tasks in building a model of a twin-screw vessel is fitting the propeller shafts and tubes. Whether you are building a kit or from scratch, it’s equally tricky because you have to locate and make two holes in a surface which is probably sloping in two directions. The holes need to be in precise locations, and of a precise shape. They’re not even circular, but are oval. To make it even more complicated, often your only reference points (the ends of the propeller shafts) are located in mid-air somewhere, and you cannot see from one to the other in a straight line. Ship modellers develop their own methods for installing and aligning twin propeller shafts. Thanks to Rowen for describing below the technique he has developed. Rowen has built three twin-screw models, so his technique has been refined the hard way.

          This method assumes you are building from a plan of a model, and the plan shows where the propeller shaft runs, and where it penetrates the hull. The steps are as follows, repeated for each shaft.
1.   From the ship plans establish, at the bulkhead closest to the shaft coupling, the height and spacing of the shaft where it would pass through. Drill pilot holes in the bulkhead at these points. If there is no suitable bulkhead at this location, make up a temporary one from scrap.
2.   Mark the outside (and inside) of the hull with the holes where each shaft tube passes through.

2a.  Picture 1 shows the technique for doing this on the outside. It requires the hull be inverted and be level both lengthwise and side to side. This is easiest using a flat table top so everything can be measured from the same datum point and remains parallel. Put a square with a stable base up against the stern of the hull. From the plan, obtain the distance from the end of the hull to where the shaft tube passes through the hull.  Clamp a steel rule to the square upright and measure horizontally to the dimension from the drawing. Project a line downwards using a small rule, or straight-edged strip of scrap material, and mark a side-to-side line on the hull. This is the axial position of the centre of the hole.  (The same method can be used to mark a line to show where the propellers will be and hence where the end of the propeller shafts will be.)
2b.  Repeat this process to get the side-to-side location of the hole. Move the square to the side of the hull, beside the line marked in step 1, and mark an axial line on the hull. The intersection of these two lines is the centre of the hole for the shaft tube.
3.  This same technique can be used either on the outside or the inside of the hull and also for establishing rudder post locations on a twin rudder hull.

4.   Set up the measuring tools to where the outer end of the propeller shaft tubes fits up to the propeller above the underside of the hull. Compare this position with the mark on the hull to check the slope of the propeller shaft between these two points.  It might be easiest to check this by taping, say, a knitting needle to the bottom of the vertical measuring ruler, and resting the other end on the hole mark on the hull.
5.    A similar technique to that described in 2a can be used to mark the propeller location.
6.    Normally, twin shafts are parallel, so the spacing between the shafts should be the same all along their length. Check on your plan.
7.   Drill pilot holes through the hull where the shaft tubes pass through.
8.   Using a long straight drill or rod pass it through the hull and bulkhead holes. Again, check the slope by measuring the propeller shaft height where the propellers fit.
9.   Open out/adjust the pilot hole positions to get the spacing, alignment and slope correct.
10.  Once the alignment is correct (check by measurements, spirit level and eye) open the pilot holes out so the shaft tubes slide through easily.
11.  Slide the shaft tubes into place. 
12.  Make up two jig plates drilled to the exact spacing and diameter of the shafts. Rowen has usually made the outer plate out of “L” shape metal.  Pictures 2 and 3.

13.  Slide one jig plate onto the outside end of the propeller shafts and one on the inside. Fit the propeller shaft hardware to ensure the shafts fit snugly into the jig plates. This is shown off the hull for clarity in Picture 4.

14.  Adjust the bulkhead and hull holes as necessary to ensure the shafts remain correctly spaced.
15.  Use a spirit level ensure the hull is level then place the level on the rear, “L” shaped jig; ensure the shafts are both level, parallel and spaced correctly. Picture 5.

16.  The shafts and hull should now be correctly aligned and they can be glued into place.