Help me understand the limitations of a jet pump in a planing hull.

Discussion in 'Jet Drives' started by shaka, Dec 29, 2009.

  1. shaka
    Joined: Dec 2009
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    shaka Junior Member

    I am a newbie to this sight with limited knowlege of jet pumps. I ride personal watercraft and spend considerable time on a PWC forum.

    Lately many of our fastest forum members have run into a very serious SAFETY issue with their craft. Some of these riders are being ejected from their craft in as slow a speed as 75mph and as fast as the mid eighties. Needless to say some have been hurt seriously.

    It is so serious that I feel compelled to ask professionals about where and how the problem is actually originating. On this forum, many of you truly understand the hydrodynamics as well as the design of jet pumps. So I would ask of you all, where are we going wrong?

    Some of the details include the facts that the stock boat travels at approximately 62mph, and the highly modified ones are in the low 90's. The majority of the modifications include increase in horsepower and reduction of drag by paying close attention to detail. Other major modifications include increasing hull lift by manipulating trim tab and pump cover (ride plate) angle, some of these angles are in excess of 6 degrees. I know of no one who has actually modified the gullet which is part of the hull design, not a unit bolted into the hull.

    One interesting note is that one member managed to coax 77 mph out of a craft with 150hp, but only managed to get approx 80mph out of his other boat with 190hp.

    Most of the riders describe the problem as a "buck" where the stern literally lifts out of the water. Keep in mind that at these speeds, the only part of the hull in the water is the jet intake and the 3X5 inch pad directly aft of the intake. What is the "wall" that so many members are running into?

    Appreciate your input in this issue.
     
    Last edited: Jan 4, 2010
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    First; do you say that those pwc's have suddenly retarded on a straight course, throwing the driver over the stem?

    If so, we have to look for a sudden increase in drag, possibly together with a loss of thrust. At first thought, I see two possibilities:

    1/ The hull is suddenly experiencing a nose-down trim force. These vessels are balancing on a very short wetted surface, having very little longitudinal dynamic stability. A small disturbance will result in a major change of trim, dramatically increasing the wetted area and thus friction. Looking at the bottom shape of a pwc, you find that the buttocks forward are convex and the deadrise quite flat.

    When this part of the bottom is hitting the water, the streamlines have to follow the convex buttock lines, which creates a pressure reduction, ie a further increase in downforce up front. Bigger racing cats had this trouble long ago and the solution was to install a transverse step in the convex part, breaking the downward suction by ventilation.

    The waterjet has normally a high thrust line, ie it is producing a nose-down longitudinal trim moment, to which the hull is normally adapted and thus stable. Increasing power (=thrust) and drag will eat the built-in stability margin. In addition to this, the exiting jet from the pump can deviate quite drastically from horizontal.

    At high speed, the inlet area at the lip position is probably oversized, which leads to detachment of the incoming flow from the "roof" line. The flow is then reaching the impeller inlet as an isolated jet in the lower half of the impeller inlet. This flow is passing straight through the impeller vane system without much energy added. Instead all the power is added to the low speed flow in the upper half of the inlet. The stator vanes cannot handle these flows efficiently, and out of the nozzle comes a highly scewed jet with a strong downwards/transverse flow, resulting in a nose-down and yawing moment, and off comes the rider......

    2/There may also be some pump designs where a major ventilation event will choke the impeller inlet. In this case, the full inlet area acts as a drag ancor. The reason for choking may be similar to the choking phenomenon in a centrifugal compressor. In the aerated pump inlet, the mixture of water and gas will effectively reduce the speed of sound in the mixture to values below the blade velocities. This creates an effective, temporary blockage of the flow, possibly leading to a dangerous drag increase and simultaneous loss of thrust at high speed.
     
  3. yipster
    Joined: Oct 2002
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    yipster designer

    not just "a" planning hull, some of those PWC's accelerate faster than a porsche turbo :eek:
     
  4. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    The water jets used in these seem to be very inefficient. Here is an interesting discussion for you: http://www.boatdesign.net/forums/bo...neffective-pwc-converted-propeller-21381.html

    Putting a cleaver propeller to a powerful PWC increased the top speed from below 90 mph to 106 mph.

    For a outboard monohull racing boat you need less than 100hp to get 80 mph.
     
  5. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    Joakim, lets compare apples with apples here, 100hp for an outboard race boat, but what kind of boat are you considering the jet for the same 80mph? Then after you've done the excersize of similar hulls, consider the fuel consumption aspect. As fuel consumed must equal total work done (fuel consumed is energy consumed) I've found for similar huuls and similar speed the fuel consumption appears to be similar, regardless of rated hp...

    Shaka, to answer your problem, my best guess would be what we refer to as reversion, that is that the intake consumes so much water and pressure that the impeller can't deal with it, so the intake reverses the flow and spits out some water under the hull. This creates momentary lift right at the back of what is a short and narrow hull, nose dives in creating alot of friction, and the bouyant aft end tries to overtake it. Result, voilent jet spin. Some of the race guys have solved this issue with a pressure relief valve infront of the impeller set to pressures which they determine as secret squirrel, others just block off part of the grill to reduce intake area, although this does have the down side of sacrificing a bit of hole shot, it is all trial and error. So have fun, and don't damage yourself!!!
     
  6. jbm
    Joined: Nov 2008
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    jbm Junior Member

    80mph

    i think that what maybe going on here is that at 80+ mph the water is having a hard time bending up.
    when this happens the jet will lose prime and the nose will dip down.
    konrad scott [ scott water jet ] in NZ has a test pump for just this type of testing. with the jet pump intake having such a small foot print that at these speeds it would not take much to disturb the flow. hope this helps
     
  7. speedboats
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    speedboats Senior Member

    Most jet ski's have loader bars in them to keep the top of the pump primed, it's not that the pump is losing prime, but more being overprimed. It is easy enough to tap a pressure gauge into the pump infront of the impeller, you'll need to be able to measure from 0psi (possible even 5psi vac) through to about 60psi. Most mechanical oil pressure gauges operate in this range. Graph the results at different speeds, you'll be surprised at the results. Remember you do want some pressure in front of the impeller (the pump does NOT like it if it has to suck, much the same as an EFI fuel pump). How much pressure becomes propietry information, either of racers, tuners, or manufacturers...
     
  8. shaka
    Joined: Dec 2009
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    shaka Junior Member


    Forgive my ignorance. It has taken me a while to digest what you are saying as I have limited knowlege of hydronamic jargon. I will paraphrase at times, and yes I believe that you are on the mark. Let me take the time to thank you in advance for your comments. They are eye opening.

    I have use a level lazer on a tripod to simulate the hulls wetted surface changes as the hull lifts out of the water. All I had to do was vary the lazer's height. I tried to simulate a 3 degree angle of attack. The wetted surface changes from a length to width ratio of approx. 3:1 to less than 1:1. I can see the instability. So is the fix to try to maintain a greater ratio? By changeing to a lower angle of attack to maintain a longer wetted surface, would this not change the effective size of the intake? Keep in mind that this model hull has from the factory a 5 degree up angle thrust nozzle. It seems that the designers want a much higher angle of attack.

    You are also correct in that according to my level lazer, water comes no where near the roof of the inlet area at speed. With the intake grate removed, I can tell that high pressure water would strike the impeller approx. 2-3 cm from the center of the impeller (at the bottom half of the impeller) all the way to the lip of the rake on the pump shoe. Of course the water that strikes the rake is deflected up in an aggressive angle back towards the impeller.

    With the intake grate installed, I can tell that approx 1/2 of the water entering the inlet area is gently swept up by the front wing, and this flow is diverted closer toward the center of the impeller. The remaining water would continue strikeing the bottom of the rake. From what I can tell no high pressure water hits the top half of the impeller. Once again, I am assuemeing a 3 degree angle of attack. Choosing a 3 degree angle of attack for the hull is based on pictures of the hull at speed. Would you think that the proper angle of attack would allow for high pressure water to directly strike the center of the impeller? This by itself would cause me to change my thinking and reevaluate all that I have thought.

    Assumeing that you are completely correct though, are you suggesting that any water entering the top portion of the impeller should be deflected back toward the center of the impeller? The wing on the intake grate currently in place is diverting water that would strike the bottom portion of the impeller toward the center. Would it be benificial to have another wing that would effectively divert water that would normally strike the roof of the inlet and the top portions of the impeller back towards the center of the impeller? In essence, you would have two wings creating a funnel effect toward the center of the impeller within the inlet area. Would this help to mitigate your first scenario?

    In your second scenario, I think you are describeing what many of us laymen would term "overstuffing" the pump.... a point where in your technical terms the pump can no longer process the water entering the inlet area. Many describe it as an ongoing chronic problem. You seem to describe it as a momentary phenomenon. What suggestions would you have to mitigate this scenario? Are there any impeller types that are more effective than others in mitigating this problem? In the past, has changeing impeller pitch to increase or decrease RPM's helped? Typically, these PWCs run approx. 7000 to 7400 RPM.
     
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  9. shaka
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    shaka Junior Member

    The fellow with a 106mph pwc is a forum member with me. He managed to get his boat from 62 to 75mph in a matter of months as compared to years with other forum members. He quickly changed gears and enclosed the tunnel, extended the shaft out of the hull and now uses a propeller. This 106 top speed is not even at wide open throttle, and the engine is stock (150hp). He has run into stability issues, and so the progress is slow as he is correcting these issues as they occur.

    But enough off topic. I'm glad you see that this member seems to have reached the point of deminishing returns, and throwing horsepower at the problem is not the solution. It comes back to the pump. Is the pump getting to much water that it cannot process as some suggest, or is it not getting enough? Stator vanes have been distroyed in a matter of a few hours by some of the faster riders, presumeably by cavitation....lack of water?

    These boats use a small amount 1-3% of the water which enters the pump to cool the engine, then dump this water overboard. Would it be benificial to recycle this water back into the pump inlet? Granted, the water has no velocity, but it does ad volume. Would this help to solve the problems described, or increase them?

    On the other hand, the pump shoe can be milled at the mounting bosses to effectively reduce the size of the inlet directly in front of the impeller. Total volume of water entering the pump at any given speed would be reduced.

    From a hydrodynamic point of view, which direction would you suggest?
     
  10. shaka
    Joined: Dec 2009
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    shaka Junior Member

    I have read that some have reported spikes in pressure. What I do not understand with this mod is that it is installed on the wear ring of the pump close to the trailing edge of the blades. This makes no sense to me. It's almost a black art thing going on. A few have reported a loss of the "buck", there seems to be little performance gains to it though.

    If allowing to much water into the inlet is truely the problem, then blocking off a portion of the intake or reducing the intake size makes perfect sense, and the safety factor more than compensates for a loss in bottom end.

    I will hook up a pressure gauge to my pwc in the inlet. My set up allows my craft to accellerate extreemly well from 30 to 65 with the last few mph increasing progressively slower. Would you believe this would be the target pressure this pump operates most efficiently? If not, what would you conclude?
     
  11. TollyWally
    Joined: Mar 2005
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    TollyWally Senior Member

    I like your approach and desire and hope you keep us posted on your developements now and in the future.

    What is the location of the forum where you and Av8r reside? I loved that guy! Very hands on, very pragmatic and focused. Glad to hear he is still among us. When you lose track of guy who is operating past the edge of the envelope... :)
     
  12. shaka
    Joined: Dec 2009
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    shaka Junior Member

    www.greenhulk.net

    He has not posted recently, but when he does it is in the Yamaha 2 stroke section.
     
  13. TollyWally
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    TollyWally Senior Member

    Thanks, I've bookmarked that site. I'm very interested in what you guys figure out and how you do it. You may or may not be aware of these guys.

    http://www.performanceboats.com/html/forums/forumdisplay.php?f=37

    They are into bigger hulls obviously but are very sharp about building engines for the specific needs of jets.
     
  14. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    I was not talking about water jets generally, only about the water jets found in jet skis. I don't know any details about them, but just looking at the top speed vs. power, size and weight they seem very inefficient. They seem to need about double the hp compared to a good similar size/weight outboard racer.

    And this 106 mph was on jet ski hull, just with the change from water jet to cleaver. I'm quite certain it could reach 80 mph with less than 100 hp.

    I know that a well optimized water jet can be as (or even more) efficient as a propeller, but that doesn't seem to be the case for jet skis and also most other small boats commercially available for freetime use.
     

  15. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    the pressure regulator needs to be before the impellor to do it's job. I see you refer to the phenomenon as over-stuffing. (We are unfortunately seperated by a common language...lol). Placing anything after the impellor will alter the nozzle pressure, and we are interested in the other end. Beware of some publishings about jetboats using an American turbine or Berkeley type mix flow pumps, these pumps had their wear ring at the front of the impellor which seals on the impellor in quite a different manner to the ID/ OD you are most likely used to.

    Same goes with the water take-off for motor coolant, as it is down-stream of the impellor it won't be contributing to the over-stuffing/ reversion problem.

    Loader bars we've put in full sized boats haven't required to be full length to the impellor, but merely long enough that you are able to overcome the need for seperation on the top of the tunnel. There is a paper published about it out of Canterbury University here in NZ. They even used diffusers in a localised area to disturb the boundry layer in much the effect as a golf ball, this made the intake water overcome seperation at the radius on the roof of the tunnel. With the amount of debris that the guys can often pump, we find it easier to use the loader to hold pressure in the affected area.

    I'd recommend placing the pressure sensors along the top of the tunnel, one at teh top, another just above the mainshaft (but high enough to be away from the turbulance), and another a little in from the hull. I'd expect to see pressure in the 15 to 30psi range in front of the impellor. The top is where you are most likely to lose pressure so it is unlikely to be necessary to test up the side of the tunnel...
     
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