Connected the depth sounder to power, and ping.... it works!
The reason I am surprised is that I couldn't fit the through hull fitting due to the angle of the hull (the shaft wasn't long enough), so instead, in forlorn hope, I fibreglassed a tube to the inside of the hull, and filled it with silicon, and plonked the sounder on top....
....and it works!
I can't test the log (speedo for landlubbers),) as I can't find the sender!..... i'm hoping that i've taken it home, as they are expensive!!!!
And the wind won't work due to my cowardly failure in reaching the masthead.