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#2
posted to rec.boats.building
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bilge pump as propulsion
R Swarts wrote:
wrote: Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at about 3.5 mph in still water. I think this general theme (Using inboard pumps for propulsion) is worth pursuing. One interesting aspect is being able to use the same pump(s) for forward propulsion and bow-thruster/steering. I would tend to doubt that, but... Can we start with a decent idea of the efficiency of the trolling motors? How much current do some typical units draw (all 12Volts?) ?? Are they rated only in static thrust? Or also Horsepower? Horsepower can be converted to Force VS Distance VS Time. (1.0 Horsepower == 550 Foot-Pounds per second, right??) HighSchool Physics was, um, 50 years ago :-) Yes, I just see "1 horsepower [electric] = 550.221 382 975 foot pound-force/second" at http://www.onlineconversion.com/power.htm Well, yes, but HP is really irrelevant other than as a comparison between similarly configured devices. What you're really concerned with is *Force*, calculated as F=ma (i.e. force = mass x acceleration). No matter what system you employ for propulsion, it boils down to the mass of water displaced per unit time. To do a meaningful comparison, you'd need to know a lot of information. You need to have a force chart for the outboard (i.e. mass flow rate generated by the propeller over the operational range, versus amp draw for the motor). Then you could compare the amp draw of your pump versus mass flow rate. I think you'll find that the pump idea is *far* less effcient than an outboard. The propeller has no frictional losses associated with supplying water to, or discharging water from, the 'pumping' device. The frictional loss at the propeller surface is offset by the surface frictional losses at the pump impeller. Keith Hughes |
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#3
posted to rec.boats.building
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bilge pump as propulsion
On Feb 15, 5:47 pm, Keith Hughes wrote:
R Swarts wrote: Well, yes, but HP is really irrelevant other than as a comparison between similarly configured devices. What you're really concerned with is *Force*, calculated as F=ma (i.e. force = mass x acceleration). No matter what system you employ for propulsion, it boils down to the mass of water displaced per unit time. Keith, I think you're missing a factor here. "Displaced" implies a DISTANCE per unit time. That's where horsepower (Or any other POWER unit) matters. 550 FOOT - (Pounds-Force) (Per SECOND) means that you could "Gear Down" (or use other force multiplication arrangements) and lift 550 pounds at 1 foot per second, or instead lift 55 pounds at 10 feet per second. Right? A large diameter 4-blade prop on a 60 foot boat with a small pitch and a 20 HP diesel with a reduction gear can put a (lot) of FORCE on that boat and move it at 5 or 6 knots. I've seen an old 20 Hp Mercury outboard push a 3-point Hydro at over 40 MPH. We don't know enough here (yet) to say what the efficiency of an inboard pump would be in moving a small boat against it's frictional resistance at a certain speed. We all know, from experience, that moving a small boat like the Hobie mentioned at the beginning of this thread takes VERY little force at very small speeds. A gentle push by hand moves it right away from the dock. Most of us have moved a 10 meter or larger boat a few feet by leaning a little on a dockline. What we're missing is some approximation of the efficiency of a well- designed pump in converting electrical power to mechanical power to move a boat. I think we'd have to do some research and talk to some mechanical engineers who understand pumps! I have a friend who recently built a small Hydroelectric plant in New York, who did his own calculations and is using a large (Thing formerly sold as a pump) as a turbine, with excellent efficiency. He's running 2 typical homes on it.. It's a BIG pump and he's at the bottom of a 85 foot waterfall... Someone somewhere knows a lot more about propelling a boat with an inboard pump than I do! |
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#4
posted to rec.boats.building
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bilge pump as propulsion
Ive been reading up a bit on pumps, but some of the math is beyond me. i do
know that its possilb eo hook must pumps up either in series, or in parallel. in parallel you quite logically get a doubling of flow in gallons per hour or whatever, while in series you combine the 'heads' whatever that means. i think it means head pressure? i know a lot of the losses in small pumps are from pumping 'up'. most small pumps are rated by how high they can pump water, and the rating for flow goes down as the height increases. installed in a boat, i would try to keep the whole thing on the level with the shortest hose runs possible. on a beach cat, i would have a thru hull on the side of the hull with maybe 6 inches of hose going to the pump, then another foot of hose going to the outlet. i think youd have to start with two pumps in each hull, both running off a common larger diameter inlest, and through a Y joiner to a common outlet. this would give you some options. you could run the pumps in parallel, or in series. then you would have to experiement with various reductions in the outlet to see what the smallest diameter nozzle you could use without losing flow would be. this is probably how you would use 'gearing'. if you used too large of a diameter nozzle, you really wouldnt get any force at all. Shaun Keith, I think you're missing a factor here. "Displaced" implies a DISTANCE per unit time. That's where horsepower (Or any other POWER unit) matters. 550 FOOT - (Pounds-Force) (Per SECOND) means that you could "Gear Down" (or use other force multiplication arrangements) and lift 550 pounds at 1 foot per second, or instead lift 55 pounds at 10 feet per second. Right? A large diameter 4-blade prop on a 60 foot boat with a small pitch and a 20 HP diesel with a reduction gear can put a (lot) of FORCE on that boat and move it at 5 or 6 knots. I've seen an old 20 Hp Mercury outboard push a 3-point Hydro at over 40 MPH. We don't know enough here (yet) to say what the efficiency of an inboard pump would be in moving a small boat against it's frictional resistance at a certain speed. We all know, from experience, that moving a small boat like the Hobie mentioned at the beginning of this thread takes VERY little force at very small speeds. A gentle push by hand moves it right away from the dock. Most of us have moved a 10 meter or larger boat a few feet by leaning a little on a dockline. What we're missing is some approximation of the efficiency of a well- designed pump in converting electrical power to mechanical power to move a boat. I think we'd have to do some research and talk to some mechanical engineers who understand pumps! I have a friend who recently built a small Hydroelectric plant in New York, who did his own calculations and is using a large (Thing formerly sold as a pump) as a turbine, with excellent efficiency. He's running 2 typical homes on it.. It's a BIG pump and he's at the bottom of a 85 foot waterfall... Someone somewhere knows a lot more about propelling a boat with an inboard pump than I do! |
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#5
posted to rec.boats.building
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bilge pump as propulsion
Hi Shaun,
Ive been reading up a bit on pumps, but some of the math is beyond me. i do know that its possilb eo hook must pumps up either in series, or in parallel. in parallel you quite logically get a doubling of flow in gallons per hour or whatever, while in series you combine the 'heads' whatever that means. i think it means head pressure? Yep, head is pressure. Basically, you have one pound/sq.inch for each 27.68" of water column (height). i know a lot of the losses in small pumps are from pumping 'up'. That's kind of a misconception regarding 'head'. Pumping up, down, or horizontal, the flowrate is dependent on the total backpressure on the discharge line (but of course, 10' of vertical pipe does have more total backpressure than 10' of horizontal pipe - of the same size). most small pumps are rated by how high they can pump water, and the rating for flow goes down as the height increases. installed in a boat, i would try to keep the whole thing on the level with the shortest hose runs possible. on a beach cat, i would have a thru hull on the side of the hull with maybe 6 inches of hose going to the pump, then another foot of hose going to the outlet. Keeping the tubing runs as short as possible is certainly the right approach to reduce frictional losses. One problem with the inlet on the side of the hull (or any hull surface tangential to the water flow) is that you get Bernoulli effects as the boat speed increases, that tends to create a vacuum in the suction line (the same concept that makes paint sprayers - the kind that use air hoses - or end-of-hose garden sprayers work. The high speed stream across the diptube end creates suction to raise the paint/roundup into the discharge stream). i think youd have to start with two pumps in each hull, both running off a common larger diameter inlest, and through a Y joiner to a common outlet. this would give you some options. you could run the pumps in parallel, or in series. then you would have to experiement with various reductions in the outlet to see what the smallest diameter nozzle you could use without losing flow would be. this is probably how you would use 'gearing'. if you used too large of a diameter nozzle, you really wouldnt get any force at all. Don't confuse "velocity" with "Force". Just like with a garden hose where you have, say 80psig, you can pinch the end to get a higher velocity stream, but you get less flow (i.e. less mass). Since the force = mass x acceleration, the force however is the same (you only have 80psig to start with). The same is true for pumps, as you note above, when you create more backpressure (pinching the hose), the flowrate goes down. If you move 100gpm of water through the system, the force is the same whether the discharge is 1" or 3", only the velocity of the dischage changes. Remember, PSI is pounds per square inch (i.e. force per unit area), so the 1" discharge stream may be at 10 times the pressure of the 3" stream, but the 3" stream has 10 times the cross-sectional area of the 1" stream. Keith Hughes |
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#6
posted to rec.boats.building
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bilge pump as propulsion
The following was posted by Cliff on the psubs.org group. Personal
submarine and ROV builders consider bilge pumps frequently. They are often used in ROV's because they are easy, but bilge pumps or jet propulsion is rarely used in Subs because they are inefficient. If you do go with a pump, be sure to place the discharge just above the water line to increase the efficiently. --Doug www.submarineboat.com Below is a derivation of thrust that can be developed from a axial flow pump in terms of volumetric flowrate. The thrust due to accelerating fluid through a pump can be written as F=M(V1-V0) Where M is the mass flow rate, V0 is the free stream velocity upstream of the pump and V1 is the velocity exiting the pump. But the mass flow rate M can be related to the volumetric flow rate Q as M=Density*Q Substituting, the thrust in terms of volumetric flow rate is F=Density*Q(V1-V0) But the volumetric flow rate Q is related to velocity in the pump duct ID as Q=V1*A=V1*Pi*D^2/4 Where D is the duct ID. Solving for V1, and substituting, the thrust can be written as F=Density*Q(Q/(Pi*D^2)-V0) For a thruster oriented approximately normal to the direction of flow, the inlet velocity can be assumed to be zero. The thrust then reduces to F = 4*Density*Q^2/(Pi*D^2) Or F= 0.001766*(q/d)^2 for freshwater where, F = Thrust, lbf q = pump volumetric flow rate in gpm d = pump outlet duct inside diameter in inches As an example, a pump with a capacity of 200 gpm flowing through a 2" duct would develop 17.7 lbf of thrust. |
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#7
posted to rec.boats.building
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bilge pump as propulsion
Doug,
Thanks for the derivation - I was too lazy to look up the volumetric flow/velocity relationship. Looking at the RULE site, their largest bilge pump is 8000gph, or 133gpm. That calculates to 7.8 Lbf thrust, with a 31 amp draw at 12VDC. Comparing that to a Minn Kota Endura 30, with 30 Lbf thrust, at 30A/12VDC gives a good comparison of the relative efficiencies. Nice site, BTW. Looks like someone's got a lot of time on their hands...or a buttload more motivation than I have :-) Keith Hughes Doug J wrote: The following was posted by Cliff on the psubs.org group. Personal submarine and ROV builders consider bilge pumps frequently. They are often used in ROV's because they are easy, but bilge pumps or jet propulsion is rarely used in Subs because they are inefficient. If you do go with a pump, be sure to place the discharge just above the water line to increase the efficiently. --Doug www.submarineboat.com Below is a derivation of thrust that can be developed from a axial flow pump in terms of volumetric flowrate. The thrust due to accelerating fluid through a pump can be written as F=M(V1-V0) Where M is the mass flow rate, V0 is the free stream velocity upstream of the pump and V1 is the velocity exiting the pump. But the mass flow rate M can be related to the volumetric flow rate Q as M=Density*Q Substituting, the thrust in terms of volumetric flow rate is F=Density*Q(V1-V0) But the volumetric flow rate Q is related to velocity in the pump duct ID as Q=V1*A=V1*Pi*D^2/4 Where D is the duct ID. Solving for V1, and substituting, the thrust can be written as F=Density*Q(Q/(Pi*D^2)-V0) For a thruster oriented approximately normal to the direction of flow, the inlet velocity can be assumed to be zero. The thrust then reduces to F = 4*Density*Q^2/(Pi*D^2) Or F= 0.001766*(q/d)^2 for freshwater where, F = Thrust, lbf q = pump volumetric flow rate in gpm d = pump outlet duct inside diameter in inches As an example, a pump with a capacity of 200 gpm flowing through a 2" duct would develop 17.7 lbf of thrust. |
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#8
posted to rec.boats.building
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bilge pump as propulsion
does having the outlet above the water line really increase the efficiency?
All the jet boats I''ve seen have the outlet below the waterline, but i could be wrong... is this what all the RC boat builders do? Ill have to have a beer or two before i try to get my head around the numbers, but thanks for the information! Shaun "Doug J" wrote in message ups.com... The following was posted by Cliff on the psubs.org group. Personal submarine and ROV builders consider bilge pumps frequently. They are often used in ROV's because they are easy, but bilge pumps or jet propulsion is rarely used in Subs because they are inefficient. If you do go with a pump, be sure to place the discharge just above the water line to increase the efficiently. --Doug www.submarineboat.com Below is a derivation of thrust that can be developed from a axial flow pump in terms of volumetric flowrate. The thrust due to accelerating fluid through a pump can be written as F=M(V1-V0) Where M is the mass flow rate, V0 is the free stream velocity upstream of the pump and V1 is the velocity exiting the pump. But the mass flow rate M can be related to the volumetric flow rate Q as M=Density*Q Substituting, the thrust in terms of volumetric flow rate is F=Density*Q(V1-V0) But the volumetric flow rate Q is related to velocity in the pump duct ID as Q=V1*A=V1*Pi*D^2/4 Where D is the duct ID. Solving for V1, and substituting, the thrust can be written as F=Density*Q(Q/(Pi*D^2)-V0) For a thruster oriented approximately normal to the direction of flow, the inlet velocity can be assumed to be zero. The thrust then reduces to F = 4*Density*Q^2/(Pi*D^2) Or F= 0.001766*(q/d)^2 for freshwater where, F = Thrust, lbf q = pump volumetric flow rate in gpm d = pump outlet duct inside diameter in inches As an example, a pump with a capacity of 200 gpm flowing through a 2" duct would develop 17.7 lbf of thrust. |
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#9
posted to rec.boats.building
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bilge pump as propulsion
"Keith Hughes" wrote in message ... Hi Shaun, Ive been reading up a bit on pumps, but some of the math is beyond me. i do know that its possilb eo hook must pumps up either in series, or in parallel. in parallel you quite logically get a doubling of flow in gallons per hour or whatever, while in series you combine the 'heads' whatever that means. i think it means head pressure? Yep, head is pressure. Basically, you have one pound/sq.inch for each 27.68" of water column (height). i know a lot of the losses in small pumps are from pumping 'up'. That's kind of a misconception regarding 'head'. Pumping up, down, or horizontal, the flowrate is dependent on the total backpressure on the discharge line (but of course, 10' of vertical pipe does have more total backpressure than 10' of horizontal pipe - of the same size). most small pumps are rated by how high they can pump water, and the rating for flow goes down as the height increases. installed in a boat, i would try to keep the whole thing on the level with the shortest hose runs possible. on a beach cat, i would have a thru hull on the side of the hull with maybe 6 inches of hose going to the pump, then another foot of hose going to the outlet. Keeping the tubing runs as short as possible is certainly the right approach to reduce frictional losses. One problem with the inlet on the side of the hull (or any hull surface tangential to the water flow) is that you get Bernoulli effects as the boat speed increases, that tends to create a vacuum in the suction line (the same concept that makes paint sprayers - the kind that use air hoses - or end-of-hose garden sprayers work. The high speed stream across the diptube end creates suction to raise the paint/roundup into the discharge stream). i think youd have to start with two pumps in each hull, both running off a common larger diameter inlest, and through a Y joiner to a common outlet. this would give you some options. you could run the pumps in parallel, or in series. then you would have to experiement with various reductions in the outlet to see what the smallest diameter nozzle you could use without losing flow would be. this is probably how you would use 'gearing'. if you used too large of a diameter nozzle, you really wouldnt get any force at all. Don't confuse "velocity" with "Force". Just like with a garden hose where you have, say 80psig, you can pinch the end to get a higher velocity stream, but you get less flow (i.e. less mass). Since the force = mass x acceleration, the force however is the same (you only have 80psig to start with). The same is true for pumps, as you note above, when you create more backpressure (pinching the hose), the flowrate goes down. If you move 100gpm of water through the system, the force is the same whether the discharge is 1" or 3", only the velocity of the dischage changes. Remember, PSI is pounds per square inch (i.e. force per unit area), so the 1" discharge stream may be at 10 times the pressure of the 3" stream, but the 3" stream has 10 times the cross-sectional area of the 1" stream. Keith Hughes 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? this seems like a way to achieve some sort of gearing to me, despite whatever losses are incurred from backpressure. runing pumps in series would allow you to have a smaller outlet and still maintain the same volume of flow right? 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'. 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. Shaun |
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#10
posted to rec.boats.building
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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 |
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