As the Edon has a thick, polyethylene foam sandwich, hull the impeller sensor has to go into either the front or rear compartments. To ensure watertight integrity I decided to install it using a Blue Sea clam fitting. I also needed a longer T bracket to install the unit just below the rigger. Here's a couple of pics. It's all working nicely. I just have to decide on the final wiring routing so it's out of the way. More later. 

The unit mounted to the back of the foot stretcher. There's a large gap for the heart rate sensor plug (not shown) to fit without stressing the cable

The sensor wire going into the front compartment. I decided on mounting the impeller at the front (on the side actually) so it wouldn't get knocked off when I pull the boat out of the water at the beach or onto the carpet I have canal-side.

and I thought I was actually going quite fast! Ha... the awful truth is revealed  by a calibrated Speed Coach - I'm old, unfit and have terrible technique..