Help redesigning hull of custom jet ski jet boat powered by supercharged 572 jet propelled by Scott

Discussion in 'Boat Design' started by Kkuzmicki14, Aug 19, 2018.

  1. Kkuzmicki14
    Joined: Aug 2018
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    Location: Chicago il

    Kkuzmicki14 Junior Member

    Hi there, so I redesigned a v hull and changed it to a delta pad bottom. The reason for this is that the original boat was at one point a jet ski and now that it's been largened its running a Scott 852 jet pump which likes the bottom of the boat to be a delta pad.... The question I have is how it should have been done..... Assuming the original shape of boat has a constant dead rise of say 19 degrees should nothing be touched and than a long delta pad (In a sense a long triangle) be cut out and that's it? My thought process with that is if everything else is untouched by doing this in a sense the bottom of the boat is getting higher and higher as it goes towards the back.... Which doesn't seem right.... So the next thought would to put a delta pad onto the bottom of the boat leaving the front to back height of the hull the same and than run from the side edge of the delta pad to the chines.... The problem with this would be as the delta pad gets narrower the distance from the outer chine to delta increases there by making the dead rise of the hull (or at the very least the angle from delta to chine) change as you move from the back of the hull and go forward (theoretical angle if you measure from exact centerline of hull to chine is still the same the whole time as the chine height and height of center of the bottom hasn't changed)
    Last option of course would be to just go at the same angle from the delta pad but that would be in a sense the same thing has taking an angle against which way the water is traveling so that definitely doesn't feel right....

    My gut is saying to stick with just 'cut a delta pad out of the v pottom currently without altering anything else?

    Thoughts? Any feedback is greatly appreciated!
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the width of the jet intake orifice, and how far from the back of the boat is it ?
     
  3. Kkuzmicki14
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    Kkuzmicki14 Junior Member

    It is 13 inches wide(or will be the pad) in front of the jet intake, that's what the pump manufacturer wants at least and it will taper down to 0 inches as you go forward right before the hull starts to rise where it starts becoming the bow..... That point from the back of the hull is about three feet (21inches of intake area plus 24 inches of essentially like a ride plate that's also a pad(similar set up to a jet ski) the overall length of the boat is 17.5 feet....

    Thanks in advance!
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You will lose a lot of angle of attack slicing a delta pad with sufficient width to accommodate that intake, if there is a relatively short distance from the forefoot to the stern, as seemingly there is, in this case. Not to mention, that is major surgery on the hull. Member "baeckmo" here knows jets and their requirements, he would be able to advise whether such a loss of angle of attack, even momentary negative attack angle, is a critical issue. But a severe discrepancy between the angle of attack of the pad, and the rest of the hull, would be a likely cause of porpoising, in a prop driven boat, a jet I am less sure about.
     
  5. Kkuzmicki14
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    Location: Chicago il

    Kkuzmicki14 Junior Member

    So that definitely makes sense.... Slicing in the delta pad especially to that great a width does create a significant amount difference in angle of attack of delta pad VS the rest of they hull.... What if I did as I mentioned for the second option.... Meaning put a delta pad in top of the bottom of the hull and worked the rest of the hull to it? Of. Course that's where the issue of variable dead rise angle comes in?

    What I should add is I already have something done and I'm trying to fix it
    .... What was done originally was I put a delta pad on it but it was basically a straight pad thirteen inches wide the whole time and tapered from 13 to 0 in the last three feet going forward..... And I didn't really Rework the sides of the hull but instead just basically built a four inch 'wall' from the side of the delta pad to meet the original hull at let's say almost a 60 degree dead rise angle.....

    The boat does fine except once at high speeds about 50 and higher if I turn slightly it leans properly but to get it to lean the other way or straighten on I don't have to just Straigjten my steering but have to go the other way a little extra in order to force it up again.... This is because my current 'pad' is protruding down significantly and the boat is in a sense falling from side to side and is also not being able to 'slide' as easily because of the shape.

    I have zero porpoising but I have had that in the past so I'm definitely being careful in order not to run into that ahain.

    Bottom line I want to make the transition of the pad the whole way from the jet intake to the front and not just the last three feet and at the same time I was going to eliminate the wall I created and run from actual delta pad side to chine at that angle the whole time thus in a sense increasing the angle more than before.... I'm just trying to figure out what it is actually supposed to be.... Meaning all these sprint jet hulls they run in Australia which are done in 14-15 foot boats..... How are they shaped for example?

    And yes I do realize it is major hull and fiberglass work but it is what it is at. This point.... Just want to do it right
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I can visualize what you have done there, it will have had the effect of lifting the boat a lot, on to what is really a wide
    runner plank. Much more so than a delta pad. So it is really starting to "chine walk", because it is balanced on that central "plank". I'd be inclined to look at some strakes mid-way between the centreline and the chine, as stabilizers. No pix of the bottom, as is ?
     
  7. Kkuzmicki14
    Joined: Aug 2018
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    Location: Chicago il

    Kkuzmicki14 Junior Member

    That is a good visual.... I will attach what I have from the back of the boat which will show the Shape of the bottom as it is exactly that going forward the whole time.... The aluminum plate shows the shape of the delta pad going forward.....

    Last option I'm considering is just having the delta pad taper the whole time moving forward past the intake which will in a sense have the 'walls' of the delta pad get smaller and smaller as well being that as the delta pad get smaller there is more time for the hull sides to get closer to the bottom.
     
  8. Kkuzmicki14
    Joined: Aug 2018
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    Location: Chicago il

    Kkuzmicki14 Junior Member

    Here is a pic of the back
     

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  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    can't tell much from the picture, unfortunately, but is does seem as if your need a stabilizing strake to stop what sounds like chine-walking. The placement would be a matter of nice judgement, however. I guess you could experiment with temporary ones.
     
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