September '06
23. Erecting a Keel Template

23. Erecting a Keel Template

Drawings from the 20' version with a plumb stem showing the keel/stem joint.

The plan does not specify the exact position of the scarf between keel and stem in relation to the length of the hull. But it can be seen above that it lies at the level of the base line, about one third of the way between bulkheads A and B. That is in the 20 foot version. In the version I am building there is a marked point on the hull which is 24 mm. above the base line, and 918 mm. towards the stern from the tip of the bow. This seems like a good point to aim for to locate the bow end of the scarf.

The locating point for the stem/keel joint. Figures have
 been blurred to preserve copyright.

This means that the stem will be approximately 1.5 metres long, plus the length of the scarf itself. 2 metres of timber should be more than adequate.

One of the problems of working from baseline measurements is that no physical baseline exists from which to measure. So, while I know that the bottom of the keel lies 300 mm. below the baseline, I do not know exactly how far that is below the hull bottom. The keel is made up of four lifts of 100 mm. but the bottom lift is shaved to take the skeg, so the distance from hull to keel bottom must be just less than 400 mm.I also know that the centre of the propeller will be half its diameter (160 mm.) plus prop clearance (32 mm.), ie. 192 mm., ignoring the angle of inclination, and that the centre of the prop shaft, where is exits the keel, is marked as being 95 mm. below base line. That all indicates that the keel, before the fitting of the skeg, is 400 mm. deep, inclined at an angle of 5.7 degrees which means it is 398 mm. below the bottom of the hull, at a point given in the plan as being 520 mm. forward of the back of bulkhead F.

So, I can now erect a dummy keel in scrap ply wood and mark the keel's dimensions directly onto it.

The first step in marking a template for the keel profile is to make a hole in the hull for the rudder stock. The centre of the rudder stock is located 70 mm. forward of the rear of bulkhead F, or 61 mm. forward of the front of that bulkhead, so a spot is marked on the bilge at that location, and a pilot hole drilled through the hull. A hole saw of the appropriate diameter is then used to enlarge the hole to fit the rudder tube.

As the original plan calls for a hog, which is absent in this boat, I felt that it would help if a block of reinforcing timber were bonded inside the hull at the position of the rudder tube hole, to strengthen this area which must stand up to considerable sideways forces from time to time. This block is also drilled with the hole saw to the diameter of the tube.

The reinforcing block for the rudder stock filleted onto the hull.

The hole is deeper than twice the cutting depth of the hole saw, so even after cutting from below and above, the final part of wood has to be removed with mallet and chisel. The hole is then reamed into a conical shape, such that the wider end is facing in towards the rudder compartment. This will allow epoxy to be packed down into the gap to reinforce the tube.

The stern tube slides into its hole.

Well, have you ever tried to buy tubing in less that a metre?

At this stage I could not resist dropping the rudder into position just for look:

A straight and strong timber batten is now fixed so that it comes through the hole in the hull from within, and runs parallel to the surface of bulkhead F. The plan ultimately calls for a block to be attached to the bulkhead which will be used to secure the top end of the rudder tube, so a temporary block can be screwed in here to align the batten. The surface of the batten then represents the centre of the rudder stock. A hole is drilled through the batten where the keel would meet it if it were to extend back that far. A string is then stretched from the hole in the batten to the rear of the stem. The string represents the bottom of the keel.

In order not to have to drill too many holes in bulkhead F, it is useful to attach this temporary block in the same position as the permanent block will be fixed. That needs to be aligned precisely with the top end of the rudder tube, so that the lip of the top bearing rests on it, and that, in turn, is about 70 mm. below the bottom of the squared end of the rudder stock. When transferring that position to the bulkhead bear in mind that the rudder blade needs a bit of clearance from the bottom of the hull, including the 25 mm. thick keel runner, so the rudder blade is lifted a little more than 25 mm. above the hull skin, as it now exists, while the block position is marked onto the bulkhead.

The rudder is chocked above the hull skin while the block position in marked on the bulkhead.

The block and batten are attached to the bulkhead and a hole is bored into the batten at the line of the keel bottom.

The string is stretched between the batten and the stem to outline the keel bottom.

Once the block and string line are attached, and preferably with the aid of an assistant, a piece or two of scrap plywood is placed over the hull and scribed to the longitudinal shape of the keel flat. Naturally, the ply needs to be a bit bigger than the area defined by the string. After the scribing line is cut out and the ply refitted to the hull satisfactorily, the bottom of the keel, ie. the string line, can be transferred to this template.

Scrap ply attached to batten (stern) and stem, ready for scribing.

The rear piece of ply template (above) is scribed for cutting, and the front piece is already cut and fitted to the hull.

Before the second piece of the template is shaped, the position of their overlap is marked onto the first, so that the exact relative positions can be found again easily.

Both pieces fully fitted to the hull.

After the template is fitted to the hull, the string line is transferred onto it, and the excess is trimmed away.

Finally, the keel shape is fully defined on the template.

It has been helpful to have the template in two pieces up to this point, because it makes their handling more manageable, but before doing any more work on it the two pieces have to be permanently attached to each other. A glued joint is held tight with small bolts while the glue dries.

You now have the shape of the full keel drawn on the ply template, which can be further marked with other details, such as lift dimensions, cut-outs, etc.

Here, the rear of the keel and the position of the propeller are marked to the left, and the
rear end of the cut out is shown right. The continuation of the keel runner is extending
sternwards of the keel to the rudder, where it will need to be widened. Dots marked 1 to 3
show the lines of the four lifts.

Here, the cut out is shown in full. Note also the small wedge, 10 mm. maximum thickness,
 under the back of lift no. 4. This is to ensure adequate propeller clearance for my
oversized prop.

The dotted lines in the above photo show the outline of the prop. shaft tube. Where the solid line between them (the shaft itself) meets the bottom of the template is where the shaft enters the hull bottom, and it's location is transferred to the hull.

The arrow marks the centre of the shaft at the point
where it enters the hull.

All that now remains is to confirm the practicality of this position for the hole. To estimate that, I need to know where this shaft line will intersect bulkhead B. I locate the position of bulkhead B on the outside of the hull, by running a measure forward the appropriate distance from the through-hull batten representing the shaft tube, and transfer that mark to the keel template. (Similar marks can be made for the locations of bulkheads C,D and E, and vertical extensions from them can be drawn on the template, all running parallel to the shaft tube batten. The position of bulkhead E coincides with the rear end of the keel cut out.) Then, a strut is attached at that point to the template to represent the bulkhead.

The arrow marks the location of the back of bulkhead B on the keel template, which is then removed
 from the hull, and a strut is attached to it which represents the bulkhead.

It is known that the engine bearers in the 20' version must eventually meet the rear of bulkhead B 255 mm. above the baseline, but the 22' 9" version I am building has a longer passenger compartment. The extra length means that the propeller shaft angle is less than in the 20' version, and since the position of the shaft is fixed at the rear, it must be significantly altered at the front. Indeed, projecting a string line from the prop. position on the template to the bulkhead B strut shows an intersection 155 mm. above the base line, rather than 255 mm.

A string is stretched between the prop. shaft
position, overlying the marking on the template,
and the strut. It measures 155 mm. above base.

Fashioning engine bearers at this height will not alter the working of the engine or propeller shaft significantly. In addition, there is a lot of leeway here: the electric engine is much smaller than the diesel, and can be positioned with flexibility, as it uses a toothed or V grooved rubber belt to drive the propeller shaft. The major consideration is whether there is enough room for the sprocket on the end of the shaft to fit. As they usually have a diameter of 100 mm., in this case there is enough, but it would cause a major headache if the diesel engine were being used, because the whole keel would have to be redesigned to accommodate it. I discuss this more fully in the Problems page under the heading "Keel Shape Adaptations".

The toothed belt and V grooved belt versions of the electric motor and shaft assembly.

If the shaft position is acceptable all that needs to be done now is drill a hole in the hull for the shaft tube.

The hole is then enlarged sufficiently to allow a laser beam to be projected down the line of the shaft and through the hull. Further holes have to be made in any obstructions, such as bulkheads D and C, until finally the laser spot can be seen on bulkhead B. Measurements are checked to ensure that there will be enough clearance for the chosen stuffing box, motor, etc. and adjustments made if necessary. Finally, the holes can all be increased to the size of the shaft tube.

24. The Propeller Shaft

The plans call for a 3/4 inch prop shaft, which is supported by two water cooled cutless bearings in the keel, as well as the thrust bearing at the engine end and another flanged bearing, along with the shaft gland, at the forward end of the shaft log. These four support points roughly quadrisect the length of the shaft between prop and engine. However, because of the altered shaft angle in the 22'9" version, and the tight fit for the shaft sprocket, the motor end of the shaft itself will need to come forward towards the back of bulkhead B, which makes it about 3.5 metres from prop. to motor. Charles Fitzhardinge of Solarboat, who is a wealth of technical know-how, has pointed out that that is a lot of length for a small diameter shaft, and that it could cause problems with whipping unless it is sufficiently supported by extra bearings.

Charles Fitzhardinge in one of his Solarboats.

Possible solutions include increasing the shaft diameter, or including more bearings along the shaft tube. It has already become apparent that there will be not enough keel thickness to allow for keel bolts to be placed around the shaft, as the shaft and its tube will require up to 40 mm. of the keel's 60 mm. dimension. So increasing the shaft's diameter is difficult. If the tube is to be fibreglass, incorporating extra bearings in its length will make it impossible to replace them at a later date without risking tube fracture, as they would need to be reamed out. So a stainless steel tube seems to be the best idea, with bearings positioned into it at the time of manufacture. The metal option also has the advantage of being able to be of a smaller diameter than the glass. For a 3/4" propeller shaft a stainless steel shaft tube has only to be 21/16" (33 mm.) OD. That will leave 1/2" of wood on either side of the shaft tube. That is not the only advantage.

With a fibreglass tube, the forward end flanged bearing would attach to a wooden shaft log, and the tube itself would need to be strong enough to allow the attachment of the shaft seal. (The recommended one is a flexible greased gland). However, a stainless steel tube would be strong enough to support itself without a surrounding log, and would not need reinforcement to take a shaft seal. An in-tube bearing could be used at its forward end. Furthermore, I am thinking of using a more modern, drip free packless shaft seal. It could attach to the tube immediately it enters the hull, because the bearing would be inside the tube. Then, provided the length of prop shaft to motor coupling is not so long as to require another intermediate bearing, the final thrust bearing would be all that was required. One place to get shaft tubes already fitted with bearings in Sydney is D H Porter of Parramatta. They are also agents for PSS seals but can provide the old style glands as well.

The packless shaft seal attaches directly to the shaft tube, and, although a bronze shaft log is shown here, does not need one.

The old style packing gland: the flanged stern tube bearing is shown here attached
to a bulkhead, but in Ariadne would need to be attached to the shaft log.

The question of which seal to use is covered more fully in Problems under the heading "Shaft Seals".

Finally, one more consideration is the actual propeller size. As mine will be a "square" one, ie. diameter and pitch the same, in this case 320 mm., the rotation speed will need to be increased a little from the speed required for the preferred size of 375 mm. for electric motors. Luckily, this means that the sprocket on the prop shaft will have to be smaller again than the 100 mm. used for larger props, if the same size motor shaft sprocket is to be used. That may allow me to position the motor/shaft assembly a little more sternward, and thus reduce the length of the prop shaft.

But the shaft itself will need to be keyed at both ends for the prop and the sprocket, so its exact length must be known for machining. Clearly I am going to have to insert a dummy shaft and sprocket and measure them before I can order the real one. That is the next task.

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