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push vs pull vis a vis rudders
Thanks Jim for taking the time to experiment.
Lovely. I understand that. A plume with an enormous splay angle converging into the prop. No need to do the 'rolling downhill bit' for me. Some of the plume being interfered with by various obstacles (such as rudders and hulls). At this stage you may be interested in the behaviour of my model of smoking fag ends, bits of card and wire hinges, all mounted up stream of the heater fan suggested by Derek Rowell. First, there is a net force on the rudder, primarily exerted in the direction of the fan.It has little lateral component, but lots of fore and aft component (Those wire hinges were good for resolving things). There's strong non-linear flow when the rudder is deflected, but the net flow is an s bend zig-zagging around it, going to the fan. Approach speed dropped markedly with distance from the fan (as you comment - a cube relationship if there are no constraints) The rudder kicks hard over when it is allowed to pivot around its forward vertical axis. Within the limitations of my crude experiment, this seems to be caused mainly by the net effect of the fore and aft component of force, not a lateral component. This explains the rudder kick I've witnessed in astern in some boats, and probably explains Derek Rowell's observation that the rudder rotates when allowed to (If I understood his experiment design right). My thanks to him for suggesting the idea of an experiment. It's been great fun. So I can now understand the mechanism whereby there's rudder kick, but little or no lateral force. And I'm stuck with the revelation that the yaw effect that my old skipper demonstrated to me was wind, mirrors, inertia, prop walk and my hero worship. Ah well. But now I've got to get rid of those carpet burns before the wife comes home. JimB |
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