I'm replacing my knotmeter with a new unit. The hitch is that the
old sensor required a 1 3/4" hole in the hull, but the new sensor
requires a 2" hole.

Ever try to use a hole saw without something for the pilot drill to
bite into? Very difficult, and certain to make gouge marks where
you'd prefer there be none..

My solution was to nest a 1 3/4" hole saw with the 2" hole saw.
The smaller hole saw now becomes the "guide" for the larger.
After well started, the inner hole saw is removed. Worked perfectly.

I bet I'm the only person in the world who didn't know this trick, but
just in case not, I thought I'd pass it along.
Norm B