Jet pump moved forward in hull

Discussion in 'Jet Drives' started by yachtnetwork, Sep 22, 2014.

  1. cmckesson
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    cmckesson Naval Architect

    Steering?

    Nobody else has mentioned this: With the jet nozzle now so far forward, your steering lever arm is greatly reduced. Indeed, at planing speed I can even imagine that the steering action of the jet is more of a sway excitation than a yaw excitation.

    Thus I expect a lot less steering authority with the new location, plus a bit greater directional stability due to the "skeg effect" of the walls of the recess. These two influences combine to yield even further reduced maneuverability.

    Have you tried driving it yet?
     
  2. yachtnetwork
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    yachtnetwork Junior Member

    cmckesson, thanks for the input. If I am reading your post correctly you are thinking that the jet thrust will be limited in being able to turn the boat by being recessed in the hull a bit more? We actually hoped for some of this. The past boats we have done the thrust is all the way aft and with that kind of hp on a small boat the slightest movement in the steering wheel yields a huge amount of actual craft movement at higher speeds. Also at slower speeds the conventional ones we already make seem to require a lot of over steer or corrections to keep the craft going straight (idle speed)

    The recess that we build does not block the thrust of the jet nozzle. We should be able to move all the way port or starboard and not hit the sides although barely.

    Are you thinking that with the pump moved a bit forward in the hull you will get more rear end "skidding" than if the pump was on the transom line? And once its going in a straight line at speed it could be harder to break away from that line with the thrust moved forward in the hull? Or are you saying that the recess will act like a skeg and keep the boat in a straight line and stable and harder to break into another direction. Elaborate on your skeg effect if you may! Would like to know more.

    We are not far enough along to put it in the water yet. Still finishing deck molds. The pump exit is exactly 7 inches forward of the old style jet pump prior to the start of the project.
     
  3. cmckesson
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    cmckesson Naval Architect

    Your first interpretation is correct: I was thinking that the jet thrust would not produce much steering, just as if one were to put a rudder too far forward on a conventional boat. Regarding the "skeg effect" I believe that in a turn the inboard edge of the V-shaped recess will "bite a little water" and act a little like tail feathers on an arrow, encouraging the boat to go straight.

    But from your reply I infer that both of these would be GOOD changes - the the old boat was a little TOO maneuverable, maybe even "squirrely." In that case you have, let me say "accidentally" improved a feature that you weren't even targeting.
     
  4. yachtnetwork
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    yachtnetwork Junior Member


    For our sake I hope we did not improve it too much!
     
  5. Barry
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    Barry Senior Member

    Yachtnetwork
    If it is not proprietary, can you tell me the pump and engine manufacturer?
    I have considered building a small 12 foot jet, UHMW bottom sheathed for running skinny water as compared to an outboard.
    Thanks in advance
     
  6. drmiller100
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    drmiller100 Junior Member

    @barry, the high end guys run Scott Jets with an aluminum automotive engine. Some guys are running PWC with good success.
    i'd be curious how moving the pump forward worked. I wouldn't think it would turn very well.
     
  7. yachtnetwork
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    yachtnetwork Junior Member

    Sea Trial

    Ok guys so we finally got the boat in the water today down in Florida. Couple of updates and am hoping for some input. Looking for solutions not what "you" would have done. All molds are done etc so nothing changing there.

    At the start of this thread I expressed some concern about moving the pump forward as we had a shorter drive shaft package than the previous model. I received some great feedback from you guys. So here is how it was on day 1.

    I took the boat myself and ran it in small canal to get the feel of it and see how it handled. First off the boat pops right up on plane with 1 person and very minimal bow rise. The engine only needed a quick burst of 1/4-1/2 throttle to pop up and then you could back off considerable and she stayed up. The steering at idle speed is great and the large opening seems to accommodate the full range of the jet pump thrust. The reverse is fantastic and the move can be turned exactly as needed in reverse as well.

    Once you bump the throttle to a faster rate the steering is effected and seems to be more of a "yaw" as someone predicted previously. It is still controllable however it feels as though you are fighting the steering wheel and it almost feels as though you are fighting "prop torque" even thought you are not. It favors the starboard side a bit and once you really crank it up above 40mph it gets a bit squirmy. So we might limit the throttle to the 35mph range as it seems plenty fast enough for that small of a tender. My main concern is the fighting the steering issue. If you make a turn at a higher speed you can let go of the wheel and the centers itself very quickly. Very limited tail end sliding and more gripping of the water. Very different than other small jet tenders I have driven in the past. Not bad...just a different feel. Like driving a full time AWD car that seems to fight you in corners at times.

    I am quit sure it has to do with the recess of the pump so far however and the fact we have moved the axis of the steering more forward as well instead of directly at the transom. I am also thinking that perhaps the water traveling under the boat is hitting the venturi on the pump and guiding it straight rather than displacing it around the pump as if it was transom mounted. So I am looking for any ideas or guidance to help alleviate or lesson this if possible. Some other tender company's use very small keels on the center line of the skid plates and others use them far out on the port and stbd of the transom however I would assume this it to track the boat straight. We do not seem to have this problem. We have the opposite. I have seen some small rudders attached to yamaha jet boats that stick down on each side and give a rudder effect. I think they might be for slow speed control however perhaps something like that could be a solution.

    Looking forward to your input. thanks guys.

    Also as a side note at wide open throttle the bow does dive a bit as someone predicted so as I said we are going to dial it back a bit also she planes well with 2 and 3 people in the boat. Planes well with 2 adults in the backseat and I have driven other competitors that require you to move your weight forward to get it to pop up.
     

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  8. Barry
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    Barry Senior Member

    "fighting the steering wheel"
    Do you mean that you are getting a feed back from the pump into the wheel?
    What are you using for steering, rack and pinion or something else
    There will always be a tendency in a jet for the jet stream to try to straighten a nozzle and give some feed back. When the nozzle is turned
    Different from a prop, where if you lose steering the prop will jam over to one side.
    So some straightening of the wheel feel is normal but can be minimized by the type of steering mechanism.
    There was a jet boat manufacturer who used cables off a drum on the steering wheel that hooked up to the steering arm on Hamilton jets absorbing up to 600 hp and the feed back on this was terrible. It felt like driving a truck before power steering. ( the advantage was it took about half a turn lock to lock for complete nozzle cycle)

    On the topic of squirmy? Are you saying that you are always correcting direction?

    Possibility: We had a rookie installer hook up a Teleflex cable to a Hamilton pump and found that we had a very loose steering situation that required constant correction when driving. What happened was that he had put a 3/8 bolt into the steering arm where it should have been 1/2 inch, and this 1/8 inch of looseness caused the jet nozzle to move back and forth. This very small movement caused a noticeable instability. Check your tolerances at the steering connections for tightness

    This next comment I cannot quantify but maybe someone will come to the table with the correct terminology.

    If the boat is running at 30 mph and everything is stable and you instantly push on the bow at 90 degrees to the keel, the boat will, MOMENTARILY pivot around some point
    ( a vertical axis ).
    I would suggest that it might be around the center of lift, ie if all upward forces were to be replaced by one at one location, but with the jet pump force and perhaps others occurring due to the keel design etc, it might not be at this point.
    In any case, the distance from this rotational point and the steering nozzle forces will be different in your boat as you have moved the nozzle forces closer to the rotational point. The shorter this distance is, the greater the steering instability.
     
  9. drmiller100
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    drmiller100 Junior Member

    so the complaint is it centers itself and goes straight?????

    If it pulls to one side slightly under power grind a little on the nozzle so the other side flows more water. Doesn't take much.
    If you want less centering, move the pins on the nozzle.
    As someone else asked, how many turns lock to lock with the steering wheel?
     
  10. yachtnetwork
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    yachtnetwork Junior Member

    Thanks for the input. The complaint isnt so much that it centers itself..thats great..its that its hard to break from the center as speed pics up. Lock to lock i think we use a 120 degree rotation on the wheel. So 1/3 or so a a revolution. When you say move the pins on the nozzle I am not sure what you are referring too. And grinding the nozzle is not really an option as the walls on this nozzle are pretty thing and i fear it would show terribly.
     
  11. yachtnetwork
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    yachtnetwork Junior Member

    Thanks Barry, we are going to put it back in the water today. The linkage is actually very tight with almost no slop in along the way. I used the term squirmy however its very tight. But at higher speeds takes a very strong arm to fight the feedback of the wheel. I will see if I can get some more pictures and video today to help better illustrate.
     
  12. Barry
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    Barry Senior Member

    With such a small lock to lock ratio, you do not have enough mechanical advantage within the steering system.

    What type of steering system are you using?
    Normally a hydraulic steering system will not allow significant feed back so I suspect that it is perhaps a rack and pinion style.

    I will assume that it is a rack and pinion or even a rotary. The mechanical advantage will be the ratio of the diameter of the steering wheel divided by the diameter of the
    pinion driving the rack. ( if rotary the diameter of the strg whl divided by the diameter of the rotary wheel)

    Say a 14 inch divided by 2 inch wheel, or a 6 to one MA. IF this is close and you are getting feedback, then I suspect that the EFFECTIVE distance from the pivot point of the steering pin to the point the cable rod connects to is much too short.

    The moment arm is the distance measured perpendicular from the steering rod to the pivot point.

    Because your lock to lock is so small, I suspect that this effective distance is very short.
    You might be able to lengthen the "tiller arm".

    There is a chance that the arm is long but the effective moment arm is small because rod is not at 90 degrees to the arm when the nozzle is in the straight ahead position.
    In which case you would have to reorient the angle of the rod.
    If this is the case, the moment arm length could change through the clockwise and counter clockwise rotation of the tiller arm. Unsymmetrical
     
  13. drmiller100
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    drmiller100 Junior Member

    draw an imaginary line between the two pins which the nozzle rotates around.

    Then look at the nozzle, and note the smallest diameter. This is the exit for the nozzle.

    On your nozzle the exit will be behind the imaginary line for the pivots.

    If you want the quick steering, you will have to move the nozzle exit closer to the pivot. You can do this by having the existing nozzle machined for an insert generally.
    If you can live with 1.5 turns lock to lock, you can change the steering ratio like the guy said, and make it twice as easy to turn.
     
  14. yachtnetwork
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    yachtnetwork Junior Member

    do you guys think adding a small rudder to the bottom of the venturi would solve anything or will it only make it harder to break free of that straightline plane.

    something like this but smaller and perhaps just one under the nozzle. I know it wont help the feeback but could it help otherwise?
     

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  15. Yellowjacket
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    Yellowjacket Senior Member

    No, the small rudders won't help the problem is more fundamental than that.

    The problem is basically what was said earlier about the the watejet being too far forward.

    In order to turn you must first generate some yaw angle. Then the hydrodynamics of the vehicle come into play and the boat generates lateral force and around the corner you go. In your case the waterjet was well forward of the transom in the first place and now you have moved it even further forward and this is the problem.

    As I mentioned, in order to turn you must first generate a yaw angle. Aircraft designers have a term that describes the effectiveness of a rudder and this is called tail or rudder "volume". The term is equal to the area of the rudder or tail multiplied by the length of the moment arm to the tail or rudder. Hence the term "volume" it being the a lengh x the area. But what really is, is a measure of effectiveness. A smaller surface further aft does the same thing as a bigger surface with a that has a shorter moment arm.

    Your lateral thrust to create yaw is fixed (the amount of lateral thrust is the same since the waterjet is the same), but your moment arm to create the yaw now is down to nothing because the waterjet is now mounted so far forward. This means that to create yaw you have to apply a lot more lateral force.

    Consequently the waterjet does not generate much yaw angle relative to the force being applied, and this waterjet force is toward the outside of the corner. Another way to describe the problem is that while you are generating yaw to create lateral force, the force that is creating the yaw is also pushing you toward the outside of the curve. This is why it feels like the boat is "yawing", and not really turning, that is exactly what is happening. The force that would normally create the turn are there for a given yaw angle, but your watejet is effectively pushing you toward the outside of the curve at the same time.

    This is compounded by the fact that you have what are in effect rudders (the strakes on the hull) that extend well aft of the watejet. In short, the surfaces that are generating the lateral force have a center of pressure that is not far forward of where you are pushing to generate the yaw angle.

    You might be able to reduce the size of the strakes aft of the waterjet, but since there is weight back there you may also have a problem with the boat spinning out depending on where the CG of the boat is. Something else that you could do is increase the amount of lateral force that is created for a given yaw angle. That would require deeper strakes forward or added fins close to the CG.

    While there are likely issues with the attachment of the waterjet exhaust to the walls of the tunnel that probably only happens when the steering is nearing full lock and that isn't what the real problem is.
     
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