Any better ideas?

I use a little self-powered portable knotmeter that is rather like an
electronic update on a Walker log. It's sold by Speedtech (
http://www.speedtech.com/asp/prodtype.asp?prodtype=35 ). Its life
isn't as long as I'd like (wore the first one out in four seasons) but
it works well, is portable and is only expensive (as opposed to REALLY
expensive). The big advantage for me is that it involves no
installation (other than throwing the trailing wire over the side) and
no holes in the boat.

Giles Morris