The only knot meters I'm familiar with require a through hull fitting to accommodate the paddle wheel

I use a portable knot meter sold by Speedtech. Mine appears to be obsolete, but the current model is shown at http://www.speedtech.com/asp/product.asp?recorprod=1&product=206&cat=35& ph=&keywords=&recor=&SearchFor=&PT_ID= Seems to work well, and I was sufficiently happy with it to buy a second one when the first wore out after a few seasons of abuse. Giles Morris