Home |
Search |
Today's Posts |
#26
posted to rec.boats.building
|
|||
|
|||
bilge pump as propulsion
Shaun,
in the very simples sense though, if i had the same volume of water flowing through both a very large and a very small outlet, the speed would be much greater for the smaller outlet right? The velocity (speed) of the water stream would be greater from the smaller outlet. The resulting force, however, would be the same since you're moving the same volume of water per unit time. 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. 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. 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) I was reading a page by an RC boat builder who use a bilge pump for drive on his boat. he used a fishing scale to measure the trust produced by the boat, and found that making the nozzle on the outlet increased thust, but only to a certain point. Yes, and that certain point is where the flowrate begins to decrease as a result of the additional head pressure caused by restricting the outlet. There are other issues that arise when the outlet is sufficiently large that it represents a significant percentage of the width of the boat, which you can do with an RC boat, that just don't arise in 'real' boat applications. Keith Hughes |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Help with bilge pump installation | General | |||
Bilge Pump Upgrade | Boat Building | |||
Propane Sniffer Meltdown | Cruising | |||
Bilge Pump Switch | Cruising | |||
Bilge Pump Switch | Electronics |