Propeller shaft and keel bottom not parallel.
The small size of the electric motor in my boat has been a very useful factor in determining whether to alter the keel to suit the longer boat than the 20' one for which it was designed. Because I was planning to use the double routed trench method to bury the shaft tube, it was necessary to locate the through hull tube wherever the keel lifts dictated that it should be. That, in turn, was dictated by the overall profile of the keel bottom, since all lifts had to be parallel to that, and so, therefore, did the propeller shaft.
The point of exit of the shaft from the rear of the keel had to be sufficiently distant from the hull to allow a clearance of 10 to 12 percent of the prop diameter, but no further than that. To increase the clearance any more would require a larger rudder, because the prop would be lower in the water. So only a small deviation from the plan was possible at the rear end of the keel/prop./rudder complex.
In other words, both the through hull hole and the rear of the prop. shaft are in fixed positions. Hence the engine had to adapt to the shaft, rather than vice versa. Being so small, and not in a line with the shaft, but suspended above it, the electric motor easily accepted a change in position from the one stated in the plans, which related to a diesel engine.
However, if the diesel is going to be used, the keel will have to be adapted somehow, to allow the shaft to enter the engine compartment at the engine manufacturer's specified angle and height. In order to allow this to happen, the shaft would have to be out of parallel with the keel bottom, as is shown in this sketch.
Propeller
shaft and keel bottom not parallel.
Compare that with the parallel set up being used in Ariadne.
Keel
lifts and propeller shaft in parallel,
I think that the easiest way to arrange a non-parallel alignment (assuming a long bored hole to be impractical) would be to cut the opposing surfaces of lifts 2 and 3, (the ones enclosing the shaft tube), such that they are not parallel to 1 and 4. The inner lifts would then be parts of a triangle. That way, the height of the engine beds and the depth of the prop shaft at the rear of the keel could be fixed, and only the through hull hole would be variable.
An alternative would be to bury the shaft entirely in one lift, not in half of each of two. If it were buried in lift 3 (where lift 1 is the bottom of the keel) it would enter the hull at a more sternward point, and would meet bulkhead B at a higher level. But it would also reduce the propeller clearance from the hull bottom. This is not a good option for an efficient electric drive system.
Another option worth considering with the electric drive is what is called a "Power Pod" by the people at Solarboat. It is basically the bottom part of an electric outboard, mounted to the keel or bottom of the hull, which dispenses with the need for a conventional propeller shaft completely.
The
Power Pod, used here in conjunction with a rudder.
The pod can be mounted in front of a conventional rudder, or can be steerable as above, thereby dispensing with the need for a rudder altogether. In the case of a slipper launch, the electric connections could be via a vertical rudder stock type hole.
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