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#1
posted to rec.boats.building
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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 |
#2
posted to rec.boats.building
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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 |
#3
posted to rec.boats.building
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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)..... |
#4
posted to rec.boats.building
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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 |
#5
posted to rec.boats.building
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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 |
#6
posted to rec.boats.building
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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 |
#7
posted to rec.boats.building
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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 |
#8
posted to rec.boats.building
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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 |
#9
posted to rec.boats.building
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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 |
#10
posted to rec.boats.building
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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 |
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