jet drives reconsidered.

Discussion in 'Jet Drives' started by Frosty, Aug 12, 2012.

  1. Frosty

    Frosty Previous Member

    Then designing the jet dry when not in use is the answer. An absolute cure for barnacles is not to be in the sea.

    I see a whole new world of buyers if jets could be used for non trailable boats.

    I see some jet manufacturers have what they call 'internal rakes ' to clean the intake bowl. If this were to be incorperated in the intake with a dry type jet then perfection is the result. A clean jet needing a spray of WD now and then able to achieve full perfomance no matter how lax the owner was with cleanliness.
  2. Frosty

    Frosty Previous Member

    It occurred to me that if CDK idea of flooding the unit with a hefty pump to prime the unit then the engine would/ could run without wash out of the jet.

    Then the unit could have an electric solenoid that allowed air into the inlet allowing the prime to collapse there fore returning to a no drive mode.

    These two buttons on the helm could be used like a drive selection.

    Instead of the prime being a problem it could be considered a feature that has not been seen before.

    Ide like to remind you that if the unit is kept dry when not in use than all the controls need not be water proof and fouling proof as they need to be in the conventional submerged jet. Splash proof would be sufficient meaning cheaper conrtols.

    It would appear now that the success of this dry jet would be totally reliant on the premise that a good flush of water from an external pump would be sufficient for priming.
  3. baeckmo
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    baeckmo Hydrodynamics

    Uumhum... so how would you keep the barnacles and corrosion off that hefty priming pump??
  4. Frosty

    Frosty Previous Member

    Easy--a 12 volt deck wash pump positive displacement so it can be mounted inside the boat above water line.

    Just push the button and It will prime the jet.

    But any suggestion on how hefty it would need to be?
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Don't let baeckmo discourage you Frosty.
    I once had a Chinese fire brigade pump to empty the swimming pool, a real ******* to get it primed because of the height above the water. Then I drilled a hole in the pump bowl, installed a 1/2" water hose fitting and primed it within 15 seconds using my garden hose. The pump was half the size of a Berkeley jet, but in your project the pump doesn't have to lift the water 6 ft.
    Important is that the priming flow is directed towards the air gap in the jet.
  6. sottorf
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    sottorf member

    The main reason it is not considered is because of intake losses. The longer the duct the more losses you have due to friction/boundary layer inside the duct. There are a number of craft that do use waterjet outlets above the waterline. A classic example is the Boing Jetfoil. THe difference in height between the inlet and outlet is about 2m. Efficiency of the jet is only around 55% while normally jets of that size and boat speed the efficiency is around 65-70%.

    Of course hydrofoil craft start off in the water so priming is not a problem
  7. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Very interesting thread. Could someone post a simple drawing showing what this small unit might look like? Thanks much. Would the knossel just have an upward gental curve in it to elevate it out of the water?
  8. Frosty

    Frosty Previous Member

    It would be the same but set higher.

    Ive past on my idea to a jet manufacturer, Ive not heard from them yet.

    I did offer to buy 2 units of there suggestion.
  9. speedboats
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    speedboats Senior Member

    Coming from a different angle, might it be easier to seal off both the intake and nozzle then force water out with compressed air, therefore when moored the inside of the pump would be dry but when underway you won't have priming issues, the thrust line can still be kept low, the tunnel doesn't need to be any longer so it'll minimise drag along the tunnel wall. All in all quite simple
  10. Frosty

    Frosty Previous Member

    Sure but can you think of a way to seal the intake grill that fits in place and removes quickly,--and one for the outlet. 100% water tight
  11. speedboats
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    speedboats Senior Member

    Top wall of the tunnel have a second skin with a seal around it, actuated by a ram so it closes inside the tunnel close to the bottom of the hull. Simply hinges into place. When open it would form the tunnel roof
  12. speedboats
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    speedboats Senior Member

    Or a trim / ride plate that moves forrard over the intake hole when 'sealed' then back under the pump when open, if you were clever you could make it movable so that it would trim the vessel.
  13. yellowcat
    Joined: Feb 2007
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    yellowcat Junior Member

    i dont know if that makes any sence, but why not create a water column up a mast just before shutting off. this would make available enough water to prime ? That could be done also with a human or wind powered pump.
    Still , in shallow waters, jets end up having problems with algeas and more. One needs a large intake with slow speed water intake to skim the surface and be able to eliminate debris. for the barnacles, one can look at those elect. systems. I favor the outof the water since my boat will be stationnary 95% of the time, and i am designing the boat to be completely out of the water most of the time.
  14. jonr
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    jonr Senior Member

    Sounds like you need a boat lift, either fixed or floating or inflatable. Don't raise the jet - raise the entire boat.

  15. Frosty

    Frosty Previous Member

    Do they do them for 14 tons. I dont think it will fit in the marina bay, its a squeeze now.
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