You simply loosen the retaining cap that holds the knotmeter in the thru-hull, then use the ring to remove the knotmeter. Insert the blank into the thru-hull and tighten the cap. Clean the thru-hull. I have someone watch the knotmeter as I spin the paddles on the knotmeter.
To replace, unscrew the retaining cap and pull out the blank. Insert the knotmeter, ensuring that it faces the right direction (there is an arrow). Replace the cap and tighten it to hand tight.
It is no problem.
If you want me to show how to do it, I will be in Lahaina from July 27-July29 prepping a Vic Maui boat for delivery back to Vancouver.