NavList:

I'm not sure. It sounds feasible as when crusing my 20 foot sail boat. I used a plastic L shaped tube with a cork in the long part  (my only means of determining speed thru water). The tube was put over the side by hand to a marking on the tube and the shorter part was held towards the direction of travel thru the water. The longer part of the tube had markings in knots (I think they went to 7 knots). It would work quite well on days with waves less than a foot or so with practice. Impossible to use in anything but the best of conditions because of having to hold it in the water at exactly the right depth. A ship would not have that issue if put thru the lower part of the hull. My concern would be any marine growth changing exact flow of the water into the tube (which could probably be cleaned with a flexible brush of some sort like a drain snake), water temperature and salinity or specific gravity. Of course they could make corrections for the latter like index error, dip, arc, parallax etc... with CN. Anyway sounds very similar to what I used for years.