Jet pump moved forward in hull

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Pretty hard to see how it can be improved, without adding some lateral plane to the hull (not too far) forward, but a fin on a jet boat is spoiling the party somewhat.
     
  2. yachtnetwork
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    yachtnetwork Junior Member


    Do you have any suggestions for adding lateral plane to the hull a bit forward of the pump? Or any ideas in theory?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Does it run fairly flat, or a bit nose up ? If you can get the bow down a little, it will move the centre of the lateral plane forward, and give you a little more steering response. But that could turn it into a wet boat, too. Then in choppy water you might get a jerky steering effect. Have you experimented with that hook in the bottom, as in increasing it incrementally ? Fitting some small tabs might be a worthwhile experiment, just to see if a flatter running attude helps steer it.
     
  4. yachtnetwork
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    yachtnetwork Junior Member

    To answer your questions.

    The boat runs very flat and true. I would not describe it as bow up. In fact when we built the plugs for the boat we knew we would be dealing with some different issues on this boat by moving the pump forward to accomidate the power plant so we added some hook in the last couple inches of the transom already. The boat pops right up on plane both light and fully loaded very quickly. In fact we were discussing taking some of the book out of the mold to allow the bow to perhaps come up a bit when on plane. At WOT is still rides flat and if not careful like to dig the bow a bit. (mind you will had not governed the throttle yet on the demo and this can be limited by dialing it back to the mid 35-37mph top speeds)
     
  5. yachtnetwork
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    yachtnetwork Junior Member

    also here are a few links to some video we shot just playing around. This is smooth even throttle to pop up on plane. Lightly loaded it starts bow hight but settles down quickly. We did not want to appear out of control as someone was shooting photos of the boat from another angle. might give you a better idea. Some is just screwing around.

    https://www.youtube.com/watch?v=ZiKHp6hMIGI

    https://www.youtube.com/watch?v=l3PnA2H0mgo
     
  6. yachtnetwork
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    yachtnetwork Junior Member

    a couple of photos of the profile along with how its riding underway in the water.
     

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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, if it really can't accomodate the trim angle being less, and you say it is already quite flat, which is pretty normal anyway for jet boats, there isn't anything prospective in that idea. The only other angle that comes to mind is a vane(s) or rudder(s) that acts on the ejected water after it leaves the nozzle, so you get two bites at the cherry, one at the nozzle, and another when the water is deflected again. But isn't in play, slowing the boat down, in the straight line running. Just an idea, may be impracticable, but I haven't been thinking about this problem as long as you, obviously.
     
  8. Yellowjacket
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    Yellowjacket Senior Member

    In this situation and with the waterjet where it is, the more nose down the boat rides the easier it will be to turn. But obviously as you go to higher speeds you risk instability if you have too much forward hull in the water.

    Your hull is difficult to analyze because so much of the rear end of it is missing. Normally that would be good because you seem to have a forward CG because of the forward engine placement, and the two would, from a trim standpoint, help to offset one another. With a more normal hull you would want the CG to be between 26 and 32% forward of the transom. Because your hull is really truncated in the front it might makes sense to be a bit more forward than that, but with the notch cut out it's hard to make any sense out of what you have. Because that material is cut away, as the hull goes faster more of the V lifts out of the water and this changes the trim angle relative by reducing lift aft relative to the available planing surface. The hook would help offset this somewhat so in this case it might be helping to keep the trim angle under control.

    Removing the hook will lift the nose and that will make steering more difficult at lower planing speeds because the nose will be higher and the back of the hull will be deeper in the water and this will make the hull more stable.

    The vertical walls come into play as the yaw angle increases. Once you get to the point where the yaw angle reaches the angle of the vertical walls, they become rudders and you most likely aren't going to yaw any more than that no matter how much you try to steer the whatejet. Before that they probably don't come into play.

    I would get rid of the strakes aft the watejet and lose some of the hook. Those would allow the boat to turn at lower speed and might get more of the nose out of the water as the speed increases.

    At this point you're pretty much in a "cut and try" test effort. The reluctance to turn is because you essentially have fixed "rudders" (the strakes aft of the steering input) and that is preventing the turning normally. As you go faster probably too much of the nose is wet and this probably causing instability at higher speed.

    You are pretty much going to have to tune this by adding bondo to the hull to get rid of the hook and trim back the strakes as a start, and then make sure the hull performs as you want in both empty and loaded configurations.
     
  9. drmiller100
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    drmiller100 Junior Member

    Lift the nose more, and you'll get the jet intake out of the water.

    The more power you apply, with the jet forward, the more it sucks the nose of the boat down as the "lifting" the water in the gullet causes downforce on the boat.

    >>>>With a more normal hull you would want the CG to be between 26 and 32% forward of the transom.

    Jets under 25 feet almost universally have the CG well aft of this. Keeping the jet "under water" is critical in chop for a Jet, so they tend to let the nose of the boat work up and down more and keep the transom (and intake) planted.
     
  10. yachtnetwork
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    yachtnetwork Junior Member

    all good stuff guys. Thanks for the inputs. I am thinking we might grind a bit of the hook out of the hull as well as some of the strakes aft of the ride plate if possible and see how it goes after that. We laid it up pretty thick on this hull so we have some material to work with.
     
  11. Yellowjacket
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    Yellowjacket Senior Member

    As with any cut and try testing program I strongly suggest that you do only one change at a time and evaluate what that does before you do a second thing.

    The reason is that cutting back the strakes will not only reduce the rudder effect, but that will also reduce lift at the back of the hull and change the trim angle at the same time. If you take the hook out at the same time you may get more trim change than you want or need, and you have no way of knowing what modification had what effect and if a second change in the same direction will do more or less.

    For that reason one change at a time is the only way to do this.
     
  12. yachtnetwork
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    yachtnetwork Junior Member


    point taken. Believe me..we will do as little as possible to make it work :)
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Thinking a little more, I came up with this idea, apologies for the rough "artist's" rendering. This is looking down on the stern area, with the water from the jet exiting at the nozzle at "A", that flow represented by the broad arrow shape, "B" an "C" are curved, fixed deflector plates situated either side of the "jet exhaust", but not intercepting it in the straight-ahead steering position. These plates would need to be shallow so they don't interact with water flowing past the hull, but only contact the jet stream, or even only the upper half of it. The plates would likely need a flange to stop spray shooting upwards. It ought to create a strong steering effect, but would need some fine tuning so it is reasonably progressive, and not sudden, which could cause problems. Just an idea.
     

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  14. Barry
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    Barry Senior Member

    Is it possible on this pump to install a wedge ahead of the nozzle to change the nozzle up direction?
    You need some bow up to generate lift and the Picts show some bow up attitude
    From a set of tables on the net, you should be able to find a table that gives an optimum bow up angle for your dead rise though it will be out a bit due to your stakes, hook, jet intake etc
    We tried to get to about 5 degrees up with 12 degree dead rise
    If you google jet sprint boat races YouTube, they often show flat running boats, pushed down with an adjustable nozzle, turn on a dime but often swap ends a little violently
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    I'm sure that works with someone who knows exactly what they are doing, a skilled drive in a sprint boat can do amazing things, but this hull needs to work in both the power off and power on positions when an idiot doing the driving.

    If for instance you use the jet to hold the bow up under power, and that lets you get to a higher speed than the hull is naturally stable, what happens when someone quickly chops the power and the nose goes down and the hull becomes unstable??? In that case you've got a lawsuit on your hands the first time it happens...

    Just say'n....
     
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