From the comfort and dryness of the dinghy, the blade is set against the hull and guided by the top of my foot. I push the hoe as deep as I can and then draw up with force. The up stroke is where the cleaning occurs! I like to start deep and work my way towards the surface, continually cleaning and scraping until I don't see any more muck floating off in the water. At this point, I move a few inches over and begin the process over again.
This may seem time consuming, but it's not that bad. In around 20 minutes, I was able to clean from keel to water line on our 45 foot sailboat (with a 32 foot waterline, we have 13 feet of overhangs).
Later that day, I dove in with a mask and snorkel to clean the keel and rudder. The only differences were I was not dry and I didn't use my feet to guide the hoe. Instead, I would float at the waterline and extend the hoe as far as I could. When I felt the bottom of the keel, I knew I was in position and began cleaning the sides of the keel.
I know I did a poor job on the keel and rudder, mainly due to visibility issues. The water was so murky that visibility was limited to 1 foot. I have no idea how the keel looked before or after the cleaning, but I know some improvement occurred.
The reason I didn't dive under the keel with a wallpaper scraper was I could feel barnacles with the hoe near the bottom of the keel. The last thing I wanted was to puncture myself on an unseen spear of a shell in filthy water. If we were in clear waters of the Caribbean, diving under would be the ideal method of cleaning growth off of the bottom of the keel. Dangers under the hull could be located and avoided as the cleaning process takes place.
The final test came when we set sail the next day. On our sail to this creek with a dirty bottom, we were running with 15kn of wind apparent at times while only moving 4kn. Now that the bottom is "cleaner" we were able to ghost along at 3-4kn in 5-10kn of wind on a run as well. Later in the trip, we were broad reaching at 6.5 knots during stronger winds.
While we had minimal hard growth, the soft growth adds a lot of drag to the hull, slowing it's motion through the water.