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bilge pump as propulsion
"Keith Hughes" wrote in message ... snip this seems like a way to achieve some sort of gearing to me, despite whatever losses are incurred from backpressure. It's not a matter of backpressure, it's a matter of reaction mass. It is Newtons second law of motion, paraphrased; for every action, there is an equal and opposite reaction. The 'little' stream puts a lot of force over a small area, whereas the 'big' stream puts a small amount of force over a big area. In each case, the "force/unit area x area" quantity (total Force) is the same. As long as the volume remains constant, every increase in velocity will be offset by a proportional decrease in the area over which it is applied. It's not a matter of the water stream "pushing" against the water behind the boat. Its just like how rocket thrusters work in a vacuum; you shoot out 10kg of gas at 10m/s over a 10 second period, and you'll get exactly that much "thrust" in the opposite direction. To be sure, there are lots of hydrodynamic losses and effects for the boat, but the basic properties of thrust are the same. If i understand what you're saying here, it sounds prettymuch counterintuitive. I may be mis-using some of the terms? let me give an example just to be sure that i understand what you're saying here, and bear in mind that of course the numbers im going to use are entirely made up in my head, so they'd be wrong.... lets say that you have two identical boats with the same pump on each one, running at whatever flow you like, say 5,000GPH. Boat A has a huge outlet... say 5 inches in diameter. for arguements sake, because i dont know how to calculate the speed of the water for that given outlet, lets say the speed of the water coming out the back is slow. i dont know how slow, but lets say it comes out at 3 knots. Now boat B has the same pump, but the outlet is so small, that even though its using the same pump, the water is coming out at a speed of 20 knots. what you're saying is that both boats because they have the same amount of energy put into them, and the same total force.... they'd go the same speed? is there no relationship between the speed the water comes out and the speed of the boat ie. it seems pretty obvious boat A could never go faster than 3 knots, so boat B would never go faster than 3 knots either? runing pumps in series would allow you to have a smaller outlet and still maintain the same volume of flow right? The same volume as what, a single pump with larger outlet? If you mean use a second series pump to overcome all the frictional losses to maintain flowrate, sure...but you're now powering 2 pumps. The cost of the higher velocity, at the same volume, is all the additional power you burn up in the second pump. sorry, i think i was just continuing on from something i was writing in a previous post... i was meaning to say iff you had two pumps in parallel as opposed to two pumps in series... two pumps in parallel would give you double the GPH flowing, but having two in series would allow you to have a higher velocity through a smaller outlet right? you can probably see where im going with this, but it really does hinge on the question i was just asking about the relationship between the speed of the flow and the speed of the boat.... if a higher flow speed allows a higher boat speed, then it would seem logical to me that you might get more boat speed by running two pumps in series as opposed to parallel because you could then have theoretically a much smaller outlet diameter than you could with parallel pumps, and therefore a higher speed of water being pumped.... you're probably feeling like you're banging your head against a wall here... but im sure ill get what you're saying pretty soon... all the math that you're giving me looks right, i think there may be just some basic concept that im misunderstanding? While there would obviously be a sweet spot for any given pump, having more velocity at the outlet seems like it would probably result in more real world 'thrust'. The higher the velocity *at a given volumetric flow rate* the higher the thrust. It's Newtons formula: F = m x a Where F = Force m = mass (proportional to the volumetric flow rate) a = acceleration (proportional to the velocity of the water leaving the pump versus velocity entering the pump) ok, wait i should have read this first and thought about it more.... so there is a direct relationship between water velocity and boat speed, if you can maintain the same volume of water flowing.... right? so the sweet spot would be just before the pump starts to be slowed by the backpressure? now this may be pure conjecture on all our behalfs, but assuming you could get double the pressure (which you probably cant) at the same flow rate by having pumps in series as opposed to parallel, and for the same current draw, which boat do you think would go faster, the boat with double the flow and half the speed, or the boat with double the speed and half the flow? the total numbers add up the same right, but wouldnt the boat with higher speed water jet go faster? Shaun snip Keith Hughes |
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