Steering problem of small 2,5m, 9.5PS jet powered RIB

Discussion in 'Jet Drives' started by surfango, Jan 15, 2010.

  1. surfango
    Joined: Dec 2009
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    Location: Croatia

    surfango Junior Member

    I am completely new on the forum and my English is a bit limited.Sorry.
    My company is Surfango representative for Europe so I have the opportunity to try developing small RIB 2.5 m boat with jet drive that could be very good tender boat. My first try is with regular 2,5 meter RIB where I installed Surfango 9,5 PS engine inside boat and jet pump is mounted behind stern plate in additional custom made plastic extension. I needed to transfer my weight little in front to start planning and I reach nice planing speed.
    Problem is that in planning speed I coudnt turn unless I lean on side where I wont to turn. When I drive slowly there is no problem.
    Then I tried with Sea Doo 800cc hull and Surfango power unit. Almost the same result.
    Fact is that Surfango powered kayak act also like this, but in this case this is not problem because is more toy and it is fun to drive it like this.

    Do anybody have idea why steering on high speed is so ineffective. I think that maybe I need to change angle of pump to pump put more down - to water surface??
    Thank you in advance,
    Dan
     

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  2. shaka
    Joined: Dec 2009
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    shaka Junior Member

    First off, let me start by saying that there is no need to apologize for your English. I can promise you that your English is far better than my Croatian.

    All I can do is comment from perspective. On my personal water craft 1 of 3 things are the problem.

    The Keel in the ride plate acts to help in straight line stability as well as recovery in a very aggressive turn. In other words, it keeps your stern from sliding when you are coming out of a turn. That same keel would prevent you from turning at high speeds.... your hull wants to continue going straight. The keel must be very rounded to be effective and high and low speeds. If you have a flat plate, then this is not your problem.

    It has also been my experience that what ever the outlet diameter of your thrust nozzle is, then the steering nozzle must be a bit larger. For instance, my thrust nozzle is 87mm where as my steering nozzle is 90mm. My beliefe is that as you go faster, the water coming out of the thrust nozzle begins to take on the characteristics of a solid, and as such, it becomes more and more difficult to deflect. By loosening the tolerances, (make it sloppy), your steering nozzle is not trying to completely deflect the total flow, just enough to get your craft to turn.

    Lastly, and I must assume that you are using some type of steering cable, look at the pivot point where you are attaching the cable to the steering nozzle. Attaching the cable close to the steering nozzle will give it agressive turning characteristics, but make the craft more difficult to turn. If there are any adjustments, try moving the cable away from the nozzle.
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    To be able to change direction there must be two forces acting on the boat with a distance between them. In displacement, the bottom of the RIB resists transverse movement, so with the jet deflected, the hull turns.
    While planing, there is much less wet area, all concentrated near the transom, so the distance between the forces is very small and the hull drifts sideways instead of turning.

    A small skeg (fin) like used on surf boards, positioned at the beginning of the planing surface will improve steering considerably.
     
  4. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    Two small skegs ,one each side of the jet intake but not forward of it are normal practice ,anything forward of the jet will encourage entrained air at the intake.
     
  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Who makes Surfango? They should have some technical data, but maybe just build something that looks like a drive. If their system doesn't work, what kind of customer support can anyone expect?
     
  6. surfango
    Joined: Dec 2009
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    surfango Junior Member

    Thank you, all.
    Shaka ;I have two small keels on beck side of riding plate. Here is picture of my second test hull so you can see it . They are really small.?
    CDK and Antony.I have no drifting problems. My hull do not wont to turn (on high speed ) unless I lean my body ( shift weight ) on desired side.
    Gonzo- Surfango is producer of Kayaks and Surfboard with jet power, so what I am trying to make is not they original idea of using engine and jet unit. I just disassembled one Hawaii and I am trying to make something different.They do not sell separate power units so I cant ask them for customer support.
    Do you think that something like this can help ?http://www.cobrajetsteering.com/Jetski Photos.php .
     

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  7. anthony goodson
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    anthony goodson Senior Member

    Best picture I can find at the minute,if you look carefully you can see the fins and the sort of size you will probably need Without these some hulls behave like a hovercraft and "leaning " is a trick they use ,or so I am told.
     

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  8. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Surfango, moment is force times distance. The small "keels" are too near to the impeller to provide the torque to change direction. Remember that the boat is pushed forward by the longitudinal force acting on the impeller.
    The hull turns around a certain point, in displacement approx. half the waterline length will be approx 1 meter. That gives you enough moment to turn. Make a sketch of the waterline while planing: you'll see there is almost nothing left.

    The rudders from cobrajetsteering are helpful for low speed manoeuvring, but that's not your problem.
     
  9. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    CDK is quite correct of course extending your keels is not the best option they should be further forward ,but not far enough to interfere with the intake
    the Rib in the picture is about right
     
  10. gonzo
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    gonzo Senior Member

    Fins at the keel like on a ski boat would be very effective.
     
  11. surfango
    Joined: Dec 2009
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    surfango Junior Member

    Thank you. I need to wait spring for further testing.
     
  12. CDK
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    CDK retired engineer

    The sea is like a mirror today and there even is some sunshine in Jelsa and Gradac. What do you need spring for?
     
  13. surfango
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    surfango Junior Member

    You are lucky to be on Adriatic seaside. Unfortunately I am in Karlovac (near Zagreb) and today, morning temerature was -14 Celsius
     
  14. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    You are 100% right, I don't want to trade places with you. I even hated yesterday's night frost.
    I assumed you would be near the coast.
     

  15. surfango
    Joined: Dec 2009
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    surfango Junior Member

    Problem solved

    Hi,
    my JetTender hull is now vary good.
    Vary fast and speed planing with one person, nice speed with two persons, and stable with for persons. Now I am working on interior shell and design.
    Hope will be finished before summer for long term testing.
    You can see video on http://www.youtube.com/user/dmarko16/videos
     
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