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#1
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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 |
#2
posted to rec.boats.building
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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 |
#3
posted to rec.boats.building
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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 |
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