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bilge pump as propulsion
ok, Ive heard this idea brought up before and seen it poo poo'd, but can it
work to some extent? Ive got a 14' cat that i store next to the beach in sydney harbour and i like to get out for a sail when i can. Unlike most cats this size, it has heaps of flotation - 3-4 adults and still not sinking anywhere near as much as a hobie 14 with 2 POB. It has an outboard bracket on the back which was on it from the factory, but i've destroyed 2 admittedly cheap and nasty second hand outboards on it so far. I have a new honda 2hp 4 stroke on my dinghy that i dont want to put on the cat because it would probably suffer a similar fate. What happens is that because your moving so fast, the outboard gets completely drowned, as do the people sailing for that matter ;-) an outboard might work on a lake or something, but sydney harbour gets choppy. The thing is, this boat was designed for island hopping the whitsundays and the manufacturer claims enough flotation for 4 people, plus camping gear, a stove, and food and water for a week! I cant see how they get the outboard to survive... i do ok if i get the outboard started and there are no waves and i just motor round on it. a 2 or 3 is plenty to move along. its rated for a 5 at maximum. the trouble is, as soon as you sail if there's a bit of chop, that motor is getting a drink - then you'll never get it started. I want a bit of propulsion for getting into some of the finnicky bay areas rather than spending a couple of hours tacking to death, 5 minutes of propulsion would get me there. propulsion would also be nice for getting back in if the wind dies, and finally as a backup just in case something breaks. i like to go out when the wind is big, and ive already had a couple of incidents with snapping a gooseneck fitting, and destoying a couple of jib cleats. so here's what im thinking; a 1250GPH bilge pump in each hull, intake hoses going into the water, outlets hooked up to hoses facing rear. these draw 5A each and are cheap to buy. A 40-50AH sealed gel cell would give me a lot more power than i need. I cant see myself using ot for longer than about 30 minutes to an hour at a time. Is this going to move me at all? at least more than trying to paddle the boat by myself (which gets me almost nowhere!) would i get any benefit from using smaller diameter hoses to create a stronget 'jet', or am i just unnecessarily loading the pump for no real gain? am i miles off in the GPH stakes.... do i need a lot more pumping to get any effect? im not looking for a speed boat here, just *movement* even 1 knot would be something. benefits to this system are that its fully submersible, completely unobtrusive, almost silent, would cost me about $100, starts instantly when i need it. thanks, Shaun |
bilge pump as propulsion
does anyone have a good approximate way of converting flow to thrust? as an
aside, do you think you would get more meaningful motion by having, for example 4x 1000GPH pumps, or 1x4000gph pumps? Thanks, Shaun |
bilge pump as propulsion
On Feb 11, 8:58 am, "Shaun Van Poecke"
wrote: does anyone have a good approximate way of converting flow to thrust? as an aside, do you think you would get more meaningful motion by having, for example 4x 1000GPH pumps, or 1x4000gph pumps? Thanks, Shaun Hmmm.. Let's do a VERY Quick&Dirty approximation. 5 Amps at 12V = 60 Watts times 2 pumps = 120 Watts. 746 Watts per Horsepower. So About 120 / 746 = .16 horsepower. Assuming there are no losses (!) how fast will that move your boat?? (Major mechanical / hydraulic issues that I can't figure out remain)..... |
bilge pump as propulsion
wrote in message ups.com... On Feb 11, 8:58 am, "Shaun Van Poecke" wrote: does anyone have a good approximate way of converting flow to thrust? as an aside, do you think you would get more meaningful motion by having, for example 4x 1000GPH pumps, or 1x4000gph pumps? Thanks, Shaun Hmmm.. Let's do a VERY Quick&Dirty approximation. 5 Amps at 12V = 60 Watts times 2 pumps = 120 Watts. 746 Watts per Horsepower. So About 120 / 746 = .16 horsepower. Assuming there are no losses (!) how fast will that move your boat?? (Major mechanical / hydraulic issues that I can't figure out remain)..... thats a good question. here's one in return; how many hp do you think i generate with a 3 foot paddle considering that i have to move rapidly from one side of the boat across the tramp to ther other side otherwise i go round in circles? surprisingly, it does actually give me some progress. but if there is a wave going against me, i go backwards. so four pumps might give me roughly 1/4 hp? just roughly.... would there be any benefit from running the pumps in series rather than in parallel? if might give me enough thrust to put a nozzle of some sort on the end.... im open to suggestions! thanks, shaun |
bilge pump as propulsion
I suspect a water jet is not as efficient as a propellor.
Since you are evidently planning on 12v on your cat, why don't you get a small 12v trolling motor ? That would take dumkings better than a gas motor, although you would need to rinse it off. "Shaun Van Poecke" wrote in message ... ok, Ive heard this idea brought up before and seen it poo poo'd, but can it work to some extent? Ive got a 14' cat that i store next to the beach in sydney harbour and i like to get out for a sail when i can. Unlike most cats this size, it has heaps of flotation - 3-4 adults and still not sinking anywhere near as much as a hobie 14 with 2 POB. It has an outboard bracket on the back which was on it from the factory, but i've destroyed 2 admittedly cheap and nasty second hand outboards on it so far. I have a new honda 2hp 4 stroke on my dinghy that i dont want to put on the cat because it would probably suffer a similar fate. What happens is that because your moving so fast, the outboard gets completely drowned, as do the people sailing for that matter ;-) an outboard might work on a lake or something, but sydney harbour gets choppy. The thing is, this boat was designed for island hopping the whitsundays and the manufacturer claims enough flotation for 4 people, plus camping gear, a stove, and food and water for a week! I cant see how they get the outboard to survive... i do ok if i get the outboard started and there are no waves and i just motor round on it. a 2 or 3 is plenty to move along. its rated for a 5 at maximum. the trouble is, as soon as you sail if there's a bit of chop, that motor is getting a drink - then you'll never get it started. I want a bit of propulsion for getting into some of the finnicky bay areas rather than spending a couple of hours tacking to death, 5 minutes of propulsion would get me there. propulsion would also be nice for getting back in if the wind dies, and finally as a backup just in case something breaks. i like to go out when the wind is big, and ive already had a couple of incidents with snapping a gooseneck fitting, and destoying a couple of jib cleats. so here's what im thinking; a 1250GPH bilge pump in each hull, intake hoses going into the water, outlets hooked up to hoses facing rear. these draw 5A each and are cheap to buy. A 40-50AH sealed gel cell would give me a lot more power than i need. I cant see myself using ot for longer than about 30 minutes to an hour at a time. Is this going to move me at all? at least more than trying to paddle the boat by myself (which gets me almost nowhere!) would i get any benefit from using smaller diameter hoses to create a stronget 'jet', or am i just unnecessarily loading the pump for no real gain? am i miles off in the GPH stakes.... do i need a lot more pumping to get any effect? im not looking for a speed boat here, just *movement* even 1 knot would be something. benefits to this system are that its fully submersible, completely unobtrusive, almost silent, would cost me about $100, starts instantly when i need it. thanks, Shaun |
bilge pump as propulsion
"Garland Gray II" wrote in message ... I suspect a water jet is not as efficient as a propellor. Since you are evidently planning on 12v on your cat, why don't you get a small 12v trolling motor ? That would take dumkings better than a gas motor, although you would need to rinse it off. the voltage issue is pretty redundant for me as the propulsion will be the only thing powered, and i will be recharging at home, so it could be 12, 24 or 48. the trolling motor was something i originally considered, but i heard a lot of bad reports about them, and they seem expensive for what they are; the salt water ones even more so. There are no fresh water supplies when my boat is, and its stored at the beach, so washing down is pretty unlikely. The only jet boats ive been in are the ones in new zealand, aluminum hulled driven by either a single or a twin v8 engine and they sure do seem to suck the gas down, but they have performance to match. I thought at the time that they were very fuel inneficient, but compared to a similar powered sterndrive, maybe not. the 12v trolling motors have come down a lot in price, but they still seem expensive to me (in australia at least) for what they are. Shaun |
bilge pump as propulsion
Shaun Van Poecke wrote:
"Garland Gray II" wrote in message ... I suspect a water jet is not as efficient as a propellor. Since you are evidently planning on 12v on your cat, why don't you get a small 12v trolling motor ? That would take dumkings better than a gas motor, although you would need to rinse it off. the voltage issue is pretty redundant for me as the propulsion will be the only thing powered, and i will be recharging at home, so it could be 12, 24 or 48. the trolling motor was something i originally considered, but i heard a lot of bad reports about them, and they seem expensive for what they are; the salt water ones even more so. There are no fresh water supplies when my boat is, and its stored at the beach, so washing down is pretty unlikely. The only jet boats ive been in are the ones in new zealand, aluminum hulled driven by either a single or a twin v8 engine and they sure do seem to suck the gas down, but they have performance to match. I thought at the time that they were very fuel inneficient, but compared to a similar powered sterndrive, maybe not. the 12v trolling motors have come down a lot in price, but they still seem expensive to me (in australia at least) for what they are. Shaun The Minnkota series electric trolling motors can be used successfully in salt water. I do it with both a 30 lb and 50 lb model. Paint the lower unit with resin to prevent the paint flaking off. The only thing in the upper unit is a switch. If you are worried about the switch getting soaked, simply remove it and connect directly to the battery, or else move the switch elsewhere. BS |
bilge pump as propulsion
Shaun Van Poecke wrote:
"Garland Gray II" wrote in message ... I suspect a water jet is not as efficient as a propellor. Since you are evidently planning on 12v on your cat, why don't you get a small 12v trolling motor ? That would take dumkings better than a gas motor, although you would need to rinse it off. the voltage issue is pretty redundant for me as the propulsion will be the only thing powered, and i will be recharging at home, so it could be 12, 24 or 48. the trolling motor was something i originally considered, but i heard a lot of bad reports about them, and they seem expensive for what they are; the salt water ones even more so. There are no fresh water supplies when my boat is, and its stored at the beach, so washing down is pretty unlikely. The only jet boats ive been in are the ones in new zealand, aluminum hulled driven by either a single or a twin v8 engine and they sure do seem to suck the gas down, but they have performance to match. I thought at the time that they were very fuel inneficient, but compared to a similar powered sterndrive, maybe not. the 12v trolling motors have come down a lot in price, but they still seem expensive to me (in australia at least) for what they are. Shaun Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at about 3.5 mph in still water. BS |
bilge pump as propulsion
Per Shaun Van Poecke:
that i have to move rapidly from one side of the boat across the tramp to ther other side otherwise i go round in circles? Could rudder control be an issue? I've had Hobie 14's and 16's and as long as I kept steering (with a foot or by sitting on the tiller bar) paddling continuously on one side worked OK. -- PeteCresswell |
bilge pump as propulsion
Jet boats are not particularly efficient and are used in shallow water
because outdrives tend to come apart when they hit a rock. They are good on rivers but not on open water. your trolling motor will probably be less than a battery bank to run a bilge pump and remember that it is not just volume that comes through but also you want velocitiy at the outlet of the jet. Brian |
bilge pump as propulsion
i havent priced a 30lb, but off the top of my head, a no name made in china
13lb trolling motor is about AU$200 while a 50lb is about AU$500. add in battery, wiring etc and i could buy a brand new petrol outboard for those prices. I think the electric outboards are still really expensive in australia for what they are. they will probably come down in price in a year or two but for now i'd feel a bit cheated paying for one. Shaun Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at about 3.5 mph in still water. BS |
bilge pump as propulsion
Shaun Van Poecke wrote:
i havent priced a 30lb, but off the top of my head, a no name made in china 13lb trolling motor is about AU$200 while a 50lb is about AU$500. add in battery, wiring etc and i could buy a brand new petrol outboard for those prices. I think the electric outboards are still really expensive in australia for what they are. they will probably come down in price in a year or two but for now i'd feel a bit cheated paying for one. Shaun Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at about 3.5 mph in still water. BS That does seem rather high. My 30 lb was $99 and the 50 Lb $130. But they work really well, appear to be well made, and are very reliable. BS |
bilge pump as propulsion
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. 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 So, IF you knew the relationship of Drag (In Pounds) VS Speed for YOUR boat, you could create a graph of Horsepower VS Speed. (This would be for "Perfect Horsepower" which certainly will not happen with real- word trolling motors and propellers, OR real-world pumps and hoses.. ) But you'd have SOME idea... If you had a 25 pound fish scale and 100 feet of line, and someone to paddle the boat OUT so you could pull it IN, you might start to get some numbers... Other Related Idea: I have thought about running a medium-large (??) pump from my inboard boat engine to bow ports for "Bow Thruster". Anyone seen something like this?? A pump could be engaged with an Air Conditioner Clutch.... So let's keep thinking about this???? |
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 |
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! |
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! |
bilge pump as propulsion
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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 |
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. |
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. |
bilge pump as propulsion
On Feb 16, 1:24 pm, Keith Hughes wrote:
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. I actually saw this being done once: Get a large cordless electric drill, mount a long shaft in it and put a trolling prop on the shaft. I saw a guy pushing an 18' canoe once this way and I nearly fell overboard watching it. For that matter, you could attach leads to power it from your 12V battery. |
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 |
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. |
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 |
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 |
bilge pump as propulsion
Shaun
snip 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.... It seems counterintuitive if you're thinking in terms of the discharge stream "pushing against the water". 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? Yes, that is the case. It's no different than if you used your hands, and applied the same force to boat A, using only one finger, as you apply to boat B, using the whole palm of your hand. It would "seem" that you're pushing much harder with the finger than you have to with the hand, but you're really not. With the finger, you have to apply a much greater force *per unit of area* than with your palm (which has a much larger surface area). For example, if you apply 10 lbf/sq.in. to one square inch on Boat A, and 1 lbf/sq.in. to 10 square inches on Boat B, the total force applied to both is the same, and the resulting acceleration would be the same (assuming the same time interval of force application). 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, It may seem obvious, but that is incorrect. The discharge water is being *accelerated* to 3 knots faster than the intake water. So there is a constant force being applied that is totally independent of boat speed. If there were no friction (and bow waves, etc.) the boat would continue to accelerate indefinitely (well...see below). This is for an axial system, where water comes in the bow, leaves the stern, the only practical way to do it. As I said earlier, if your intake isn't pointed forward, then you have Bernoulli effects, and you have pump cavitation problems that reduce the flowrate. In the axial configuration, the boat speed increases system efficiency by pressurizing the suction line and overcoming the intake line pressure drop. At some speed, you'll reach a point where the pump cannot maintain an acceleration of 3 knots (outlet vs inlet), and your thrust will drop from that point on. snip 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? Exactly. It takes more pressure (force per unit area) to get that higher velocity, so you have to do more work on the system (by the pump). That additional 'work' is then available in the form of thrust. so the sweet spot would be just before the pump starts to be slowed by the backpressure? Just before the discharge *volume* (mass flow techically) decreases. Keep in mind that shrinking the nozzle is not "free", since that creates higher pressure requirements, and thus higher load on the pump (i.e. more amp draw). 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? Nope, 'cause it's the accelerated mass of water that supplies the thrust, not the discharge water 'pushing against' anything. Does the boat you push with your finger (above) go faster than the one you push with your palm because your fingertip is applying so much more pressure (over a small area) than your palm is? No, if the total force applied is the same. It doesn't matter whether it's great force on a small area (fingertip, small water stream) or a lesser force over a greater area (palm, large water stream) if the total force is the same, the thrust is the same. Keith Hughes |
bilge pump as propulsion
On Feb 16, 2:43 pm, "Shaun Van Poecke"
wrote: 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? I was afraid someone would ask that. :) So I'll admit that I am going on what I have been told by jet boat and jet ski people. If anyone knows better about jet pumps, please correct me. The efficiency of the jet pump is based on the mass of water it discharges. Any back pressure and turbulence at the outlet only reduces the velocity of the flow and therefor the rate of the flow. The discharge ports are below the water line but only when the craft is not yet up to speed. I think there is a benefit to having a higher outlet pressure during start up or the "hole shot". Jet pumps also depend on the design of their intake ports, because at top speed the forward motion of the craft and the shape of the intake actually assist in directing the water flow into the pump, much like an air intake scoop on a dragster. You might milk another 2 or 3 oz of thrust from that bilge pump if you put a scoop on it. :) Then again that would really jack with the drag on a sail boat hull. I'd go with an old used cheep trolling motor with a busted speed controller. Clean it, replace the brushes and mount it on one of the transoms with a hinge that lets it flip down into the water and then steer with the rudders. Add a simple on/off switch and avoid the variable speed controller or any other electronics that will just present other points of potential failure. Best Regards Doug www.submarineboat.com |
bilge pump as propulsion
Keith Said:
.... 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. THAT makes a lot of sense, (a 3 or 4 to 1 ratio) and gives us some real-world numbers to think about... And the following implies that a decent experiment could be done by using a maximum-outlet-diameter smooth hose to the outlet, and then fitting different experimental nozzles: 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. Let me try an approximation based on the above, looking at at my idea of running a large? inboard pump connected to my existing marine engine thru an air-conditioning compressor clutch, and piping it thru a control valve to vary bow thrust port-to-starboard. 30 Amps at 12 V gave maybe 7 pounds thrust. That's about 1/2 horsepower. Say I can use 2 HP (What I understand a car air- conditioner uses) to a pump with the same losses as the example Keith showed. So maybe that's 28 pounds thrust. That sounds like plenty for a 25 foot boat... Question: How much thrust do typical electric bow-thrusters give in the smaller sizes?? (We'd expect them to be more efficient).. BTW, they are expected to be used at close-to-zero hull speed, so the thrust measurement is reasonable here. Maybe I can try some of this out this Summer on Lake Champlain (Vermont) . (Now I'm boatless :-( on the Med this year, but moving to the shore of the South China Sea for the next 2 or 3 years where I WILL Mess With Boats!). Interesting discussion! Regards, Terry King ...On The Mediterranean in Carthage |
bilge pump as propulsion
"Doug J" wrote in message
ups.com... On Feb 16, 2:43 pm, "Shaun Van Poecke" I'd go with an old used cheep trolling motor with a busted speed controller. Clean it, replace the brushes and mount it on one of the transoms with a hinge that lets it flip down into the water and then steer with the rudders. Add a simple on/off switch and avoid the variable speed controller or any other electronics that will just present other points of potential failure. Best Regards Doug www.submarineboat.com im with you there doug, that would suit my budget and my temperament perfectly. even used trolling motors on ebay in australia attract quite a premium... Ive seen second hand 40lb motors going for up to AU$250! since ill probably chop it anyway, buying new is not a big concern of mine, and ill only want full speed, so drect wiring seems the way to go. what is the general thought on the life of a trolling motor? are replacement bushes readily available? Shaun |
bilge pump as propulsion
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bilge pump as propulsion
Shaun Van Poecke wrote:
ok, Ive heard this idea brought up before and seen it poo poo'd, but can it work to some extent? Ive got a 14' cat that i store next to the beach in sydney harbour and i like to get out for a sail when i can. Unlike most cats this size, it has heaps of flotation - 3-4 adults and still not sinking anywhere near as much as a hobie 14 with 2 POB. It has an outboard bracket on the back which was on it from the factory, but i've destroyed 2 admittedly cheap and nasty second hand outboards on it so far. I have a new honda 2hp 4 stroke on my dinghy that i dont want to put on the cat because it would probably suffer a similar fate. What happens is that because your moving so fast, the outboard gets completely drowned, as do the people sailing for that matter ;-) an outboard might work on a lake or something, but sydney harbour gets choppy. The thing is, this boat was designed for island hopping the whitsundays and the manufacturer claims enough flotation for 4 people, plus camping gear, a stove, and food and water for a week! I cant see how they get the outboard to survive... i do ok if i get the outboard started and there are no waves and i just motor round on it. a 2 or 3 is plenty to move along. its rated for a 5 at maximum. the trouble is, as soon as you sail if there's a bit of chop, that motor is getting a drink - then you'll never get it started. I want a bit of propulsion for getting into some of the finnicky bay areas rather than spending a couple of hours tacking to death, 5 minutes of propulsion would get me there. propulsion would also be nice for getting back in if the wind dies, and finally as a backup just in case something breaks. i like to go out when the wind is big, and ive already had a couple of incidents with snapping a gooseneck fitting, and destoying a couple of jib cleats. so here's what im thinking; a 1250GPH bilge pump in each hull, intake hoses going into the water, outlets hooked up to hoses facing rear. these draw 5A each and are cheap to buy. A 40-50AH sealed gel cell would give me a lot more power than i need. I cant see myself using ot for longer than about 30 minutes to an hour at a time. Is this going to move me at all? at least more than trying to paddle the boat by myself (which gets me almost nowhere!) would i get any benefit from using smaller diameter hoses to create a stronget 'jet', or am i just unnecessarily loading the pump for no real gain? am i miles off in the GPH stakes.... do i need a lot more pumping to get any effect? im not looking for a speed boat here, just *movement* even 1 knot would be something. benefits to this system are that its fully submersible, completely unobtrusive, almost silent, would cost me about $100, starts instantly when i need it. thanks, Shaun Much bigger than what you all are talking about, but this is electric propulsion done right: http://www.asmomarine.com/2005/asmo_uk/00.shtml There are a number of others selling electically propelled boats too. Just google up electric boat BS |
bilge pump as propulsion
Doug J wrote:
On Feb 16, 2:43 pm, "Shaun Van Poecke" wrote: 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? I was afraid someone would ask that. :) So I'll admit that I am going on what I have been told by jet boat and jet ski people. If anyone knows better about jet pumps, please correct me. The efficiency of the jet pump is based on the mass of water it discharges. Any back pressure and turbulence at the outlet only reduces the velocity of the flow and therefor the rate of the flow. Yep, the discharge stream has to displace water already behind the boat, and that requires work (seen as higher backpressure at the discharge nozzle, reducing the mass flow rate). Where you could gain efficiency would be in having a directionally adjustable discharge nozzle (primarily with a planing hull) so you could optimize the discharge vector, for maximum thrust in the direction of boat travel, for different bow angles. For e.g., as the bow rises, the discharge angle, for a fixed nozzle, rotates downward. The thrust is now a vector addition of the upward + forward thrusts, with the upward thrust component being wasted energy. Seems that angling the stream back to horizontal (and letting the hull do the lifting) would increase the forward thrust. The discharge ports are below the water line but only when the craft is not yet up to speed. I think there is a benefit to having a higher outlet pressure during start up or the "hole shot". Jet pumps also depend on the design of their intake ports, because at top speed the forward motion of the craft and the shape of the intake actually assist in directing the water flow into the pump, much like an air intake scoop on a dragster. You might milk another 2 or 3 oz of thrust from that bilge pump if you put a scoop on it. :) Then again that would really jack with the drag on a sail boat hull. Details, details... :-) I'd go with an old used cheep trolling motor with a busted speed controller. Clean it, replace the brushes and mount it on one of the transoms with a hinge that lets it flip down into the water and then steer with the rudders. Add a simple on/off switch and avoid the variable speed controller or any other electronics that will just present other points of potential failure. Sounds like the best plan to me. Keith Hughes |
bilge pump as propulsion
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bilge pump as propulsion
Jets (be they axial like real waterjets or centrifugal like a bilge
pump) are more efficient than larger diameter propellers ONLY when the boat is moving fast enough that the drag from propeller strut/shaft/and rudder becomes a significant part of the drag values. In the real world that works out to be around 25-30 knots. A bilge pump will never beat a small trolling motor propeller at displacement speeds. I've seen Cal 20's pushed by a big trolling motor for a sailing school. Works o.k. in calm water but in any sort of breeze it's not enough thrust. The Cal 20 with a 3.5 HP outboard was a LOT faster and would end up towing the ones with a trolling motor. For a Thunderbird, a typical 5-8 HP long shaft outboard is the only solution that makes sense. Evan Gatehouse |
bilge pump as propulsion
"Evan Gatehouse2" wrote in message ... Jets (be they axial like real waterjets or centrifugal like a bilge pump) are more efficient than larger diameter propellers ONLY when the boat is moving fast enough that the drag from propeller strut/shaft/and rudder becomes a significant part of the drag values. In the real world that works out to be around 25-30 knots. A bilge pump will never beat a small trolling motor propeller at displacement speeds. I've seen Cal 20's pushed by a big trolling motor for a sailing school. Works o.k. in calm water but in any sort of breeze it's not enough thrust. The Cal 20 with a 3.5 HP outboard was a LOT faster and would end up towing the ones with a trolling motor. For a Thunderbird, a typical 5-8 HP long shaft outboard is the only solution that makes sense. Evan Gatehouse hi Evan, there may have been some crossed wires here.... the bilge pump/trolling motor solution was for a 14 foot beach cat. Shaun |
bilge pump as propulsion
Keith Said:
OK, I'm confused. Are you talking about *just* a bow thruster operation? **Yes, ONLY Bowthruster, maybe also piped to SternThruster ?? Just for smooth docking/undocking or dead-slow movement in calm water.. I can and do bring my 22' Inboard CuddyCabin to shore / rock points with the canoe paddle. But having a fully-controllable couple of horsepower should allow total maneuverability. The marine engine would be in neutral, just running the pump... Question: How much thrust do typical electric bow-thrusters give in the smaller sizes?? (We'd expect them to be more efficient).. BTW, they are expected to be used at close-to-zero hull speed, so the thrust measurement is reasonable here. My understanding is that they are considerably more efficient than typical outboards, since the "ring" around the prop eliminates a significant amount of prop slip relative to having an open prop. Keith Hughes Right! Apparently this is a significant factor in dead-slow operations. There are huge (Kort Nozzles ?) on big tugboat propeller installations. Interesting idea... We've had some excellent information in several above posts. What I'm getting from all this is: - Inboard pumps to various outlet nozzles can be effective for maneuvering and very slow speeds. - Efficient forward-motion propulsion up to 3 or 4 knots is much more efficient using external propeller (such as 'trolling motor' types). My idea is a different one: I have conventional marine propulsion for running underway, but I wish I could have good fine-control maneuvering / docking / dead-slow sightseeing based on an inboard pump driven from my regular marine engine. Especially with a conventional fixed-propeller / rudder type boat that steers for c*** in reverse, and is very difficult to turn in a short radius, this would be "Nice". Regards, Terry King ...On The Mediterranean in Carthage |
bilge pump as propulsion
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bilge pump as propulsion
On Sun, 11 Feb 2007 07:33:28 -0800, R Swarts
wrote: Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at about 3.5 mph in still water. BS It's always the maker's claimed thrust that raises questions in my mind. They take the thrust reading stationary, as far as I know. Anyway: if you maintained 30 lb thrust at 3.4 MPH that would need 203 watts or more i.e. 17 amps at 12 volts to provide about 0.27 HP. Brian Whatcott Altus OK |
bilge pump as propulsion
On Sun, 11 Feb 2007 09:53:07 GMT, "Shaun Van Poecke"
wrote: .... thats a good question. here's one in return; how many hp do you think i generate with a 3 foot paddle considering that i have to move rapidly from one side of the boat across the tramp to ther other side otherwise i go round in circles? shaun I'm late to the party. A paddler is probably putting less than 60 watts into it. Less than a tenth HP. Brian Whatcott Altus OK |
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