Wonderful story, Tony! Often, the "obvious" theoretical explanation is just shy of the mark...
> So much for theory! :-) > > I'm reminded of making toy paddle boats with children of the sort where a > rectangular notch is cut at the back of a plywood 'boat' and a cruciform > paddle is driven in the gap by twisted elastic. The experiment was to > find the most efficient paddle shape. > Plane square blades worked OK but made a lot of splash so curved tinplate > 'scoop' blades were substituted to give an action like the crawl stroke > in swimming. Much to everyone's surprise the boat hardly moved but lots > of water was thrown upwards and the stern pushed down. AND THEN *by > accident* the same paddle was put in backwards resulting in the opposite > of 'the crawl'. Wow! did that boat move! and with no splashing! > Everyone was so fixated on the expected efficiency of the 'crawl' blades > they would never have given the successful idea a thought - unless they'd > first looked at a centrifugal impeller perhaps! > > > Tony Moss I've also observed the hose/nozzle phenomenon, and am convinced there is a mass-flow/reaction basis to it, but can't put my finger on it. There is a related physics puzzle: Take an ordinary "whirlygig" garden sprinkler. Run water through it, and observe the direction the jets spin, let's say clockwise. Now, immerse the sprinkler in the swimming pool, and connect it to a suction pump. What happens? Why? I never connected the two puzzles, but there is obviously a relationship. Now I can see what would happen if you pumped water through the immersed sprinkler in the normal direction! Dave
