Water jet active stabilizers

Discussion in 'Hydrodynamics and Aerodynamics' started by jjj, May 20, 2020.

  1. jjj
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    jjj New Member

    This is probably a naive question, but active stabilization using fins is now quite common, but has the downside that the protruding fins can become a liability in a collision, ripping of an even resulting in a hole in the hull (as well as the more mundane constant additional drag forces).

    I was curious why no systems use water jets (e.g. the jet would be facing down on each side of the boat to provide a right forcing, with a pump taking water from on side and pumping to the other side to generating a righting force). Is it just impossible to get this to be efficient?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know if it is a valid comparison, but I assume some of the righting action of stabilizers is owed to the forward motion of the boat, but with water jets, it would be more akin to a helicopter versus the aeroplane, which generates lift by forward motion.
     
  3. BlueBell
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    BlueBell Ahhhhh...

    Some? More like: all.
    If it wasn't for the "forward motion of the boat" there would be no water flow over the stabilizer wing.
    No flow, no lift, positive or negative, no "righting action".

    JJJ,

    Welcome to the forum.

    Not sure what you mean here. Would you clarify?

    Your idea makes me think of rocket thrusters on space vehicles.

    What size boats are you thinking of?
    How would the waterjets be powered?
    Instant on, maybe electric?

    Trim-tabs on some smaller boats are less susceptible to damage and don't create much drag.
    Not sure if they are used in active roll stabilization though.
    This is what you're proposing is it, roll control?

    Cheers
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm happy to hear all views, but I doubt it is "all"
     
  5. gonzo
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    gonzo Senior Member

    Fins are very effective when the vessel is moving. Otherwise, gyroscope stabilizers are hard to beat. Major drawbacks to waterjets, are noise, water splashing, clogging with debris and turbulence that creates added drag.
     
  6. kapnD
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    kapnD Senior Member

    Sounds problematic, would take a lot of power to move enough water to be effective, especially in heavy seas.
    I agree with Gonzo, gyrostabilizers are the cleanest, quietest, non intrusive (non protrusive too!) solution, and are still effective when vessel is stationary.
    How about gyro controlled hydraulic trim tabs in the prop wash to counter roll?
     
  7. BlueBell
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    BlueBell Ahhhhh...

    Please help me understand where the rest of the "righting action" comes from if not from water-flow-induced lift, positive or negative, over the stabilizer wing.
    Thanking you in advance.
     
  8. Blueknarr
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    Blueknarr Senior Member

    While in morion the righting force is overwhelming provided by lift.

    While at rest there is a small stabilizing effect in the same manner that "flopper stoppers" work for smaller craft. Resistance to vertical movement.
     
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  9. BlueBell
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    BlueBell Ahhhhh...

    Ah, the penny drops, thank you.
     
  10. BMcF
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    BMcF Senior Member

    We investigated the use of waterjet nozzle vertical-plane deflection as a means of producing pitch and roll motion damping forces. Conducted some experiments with that and also with similarly vectoring outdrive legs and, in the most extrememe" version of some of our research, vertically vectored entire outboard engines mounted on servo-operated (trim angle) transom brackets. Results were mixed. While some of the motion control objectives were achieved and demonstrated, the complexity, power requirements, weight, etc were such that none of the R & D ever became a "product"..at least none that I'm aware of.

    Regardless of how your motion damping forces are being produced, they cannot be put to good use for vessel motion control if they are not predictably controllable at rates approximately four to five times times the vessel motion rates you want to dampen. So, for example, a stabilizer fin on a vessel with a 6 second roll period needs to be able to deflect its angle of attack at a rate of say 30 degrees/sec. That's pretty quick...hard to imagine any kind of pump jet that could respond like that.
     
  11. BMcF
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    BMcF Senior Member

    Active trim tab pitch and roll damping systems have been in common used since the early 90s. Some trim tabs are quite large..over 4 square meters each, for example.
    Trim tabs are also quite good for so-called "zero speed" or "on anchor" roll damping.
     

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  12. DCockey
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    DCockey Senior Member

    Do active trim tabs remain active at zero speed or do they function as static trim tabs?
     
  13. BMcF
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    BMcF Senior Member

    The control commands to trim tabs when "zero speed" mode is selected are essentially the same as those used to command fins in "zero speed" mode. When underway, the control commands are based on a mostly linear "control law" that creates fin position commands based on the processing of things like roll rate, list angle, pitch rate etc. In the zero-speed mode of operation, the tabs (or fins) are toggled rapidly through a defined range of motion...a "bang-bang" response to a very non-linear fuzzy-logic control algorithm that models the vessel motion and uses threshold detection methods and criteria to determine when to "fire" the fin or trim tab from one of its end positions to the other.

    For lightly damped mono hull vessels with longer roll periods (greater than about 7 seconds), the zero speed control can be very effective at eliminating resonant roll while drifting or on anchor. That is why nearly all stabilizer companies offer the option these days. We also employ zero-speed pitch damping control with the fins on SWATH vessels.
     
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  14. DCockey
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    DCockey Senior Member

    Thanks for the explanation of zero speed mode. Not obvious - do you know if any patents have been issued for it?
     

  15. BMcF
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    BMcF Senior Member

    It's a technology that's been "reduced to practice" for a long time, and delivered to hundreds of vessels in the last couple of decades by three or four different competing companies. If there were any patents issued, they would have been in the late 90s or early 2000s but I'm not aware of any specific one.

    That said, the patent examiners do a pretty weak version of investigating prior art or earlier patents and often approve the issuance of a patent for things that have been in service for a long time prior. That leads to "patent trolling", and we've been hit with it a few times over the last couple of decades. It's easy to defeat them but always costs money to do so..something the patent trolls are counting on a their leverage to extract a "settlement"..
     
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