Hull design for water jet propulsion

Discussion in 'Boat Design' started by Jet mad, Aug 28, 2022.

  1. TANSL
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    TANSL Senior Member

    @Jet mad, of course I've been on a jet boat and other types of motor boats, but I don't need to have done that at all, or many other things, to comment on the article you've shown us.
    Do you know what is the initial stability of a boat or cavitation or do you know something about hydrodynamics? Don't answer me, these are rhetorical questions. And yet, you show us an article about the suitability or otherwise of certain hull shapes and, what is even worse, it seems that you question our comments.
     
  2. Jet mad
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    Jet mad Junior Member

    Tansl, so you have been on a jet boat and other types of motor boats. The question l am asking is. The ride on the jet boat, how was the stability with the steering, did the boat travel straight or did it won't to wonder all over the place at low speed and one has to keep correcting and also how was it in a following sea?
    The steering question is one of the big problems with jet boats with the wrong hull. Directional stability is critical for jet boats.
     
  3. TANSL
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    TANSL Senior Member

    At low speeds, below planing, strakes and all that has little influence, the boat is driven by the wind and the direction of the currents. The handling when the boat is going in reverse is also not good.
    I don't understand why "Directional stability is critical for jet boats" and not for any other type of motor-powered boat.
     
  4. Jet mad
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    Jet mad Junior Member

    Directional stability is critical for jets because there is no rudder to help with direction, so you are relying on the hull. So how did the jet boat go that you were on ?
    6m centre console boat powered by an outboard on the plane, with just me. One leaves the helm walks up to the port side at the bow, looks over the side then moves across to the starboard side, looks over the side then moves down to the transom, looks over the side then too the port side, looks over the side then back to the helm. Looks back and sees that the boat has been tracking dead straight. Is that what one would call directional stability. And yes it did happen, I just wish that I had the cameras we have today back then.
     
  5. TANSL
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    TANSL Senior Member

  6. Barry
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    Barry Senior Member

    I agree with Zipper, that the strakes would provide insignificant stability at REST, which means to me, SIGNIFICANTLY resisting slow angular acceleration. Like a person walking side to side on the sole of the boat. How important is this? More than likely the hull shape
    will provide more stability.

    Not sure what comment that you are expecting me to change. If you are suggesting that lift strakes do not provide "less skid-better turning" flies in the face of reality.
    I followed (in our 21' with strakes) behind another 21' without any strakes up a skinny river in northern Canada on quite a long trip through very busy rock gardens. Same pump, each with 454's, each with 12 degree mono deadrise the only difference between the boats were the fact that he had a 454 twin turbo'd, intercooled and his 1/2 inch keel guard was on the outside of his hull and his boat might have had a bit less chine width.

    Forgot to put this in earlier, the lift -steering strakes that we built had a flat of either 5 or 6 inches with a 12 degree to horizontal down turn. The chines were the same. Sharp edges (reducing Coenda effect ) made from an extrusion with the short leg cut to create the12 degree down turn. ( as compared to the rather small strakes shown in some of the images above)

    There was little doubt in my mind, though qualitative, that our boat turned tighter corners.

    It appears that you were suggesting that lift strakes do not provide better turning. Am I wrong here?

    The sprint boats will turn tight turns due to the lift/steering strakes as well as a very flat trim angle to keep the keel and strakes engaged for more length. More than likely the result of the nozzle exit angle.
    Can the sprint boat driver or 2nd seat person adjust the nozzle on the run?


    I do have a question, if you "have been involved with jet boats since the 70's", it appears that you want to offer criticism of anyone who offers you their observation. Mind you being "involved with jet boats since the 70's" can be anything from
    quite involved, ie building many or just owning one. Perhaps a bit more information on your involvement


    I had built for personal use a zero degree, tunneled 14 foot, OUTBOARD tiller jet for running really skinny water. Obviously, lift strakes on a flat bottom would offer little lift. After building it I took it out for its first run and was constantly correcting
    for direction and it took an acre to turn the thing at planing speed. It was considered during the build but with the tunnel etc, I was looking for absolute minimum draft.
    It went back to our shop and I installed 6 full length 1 1/2 x 1 2/2 square tubing steering strakes or "directional enhancers" and the boat tracked very well. There was no alternative to stop the excessive skid as with an outboard jet
    with the intake in the tunnel was that in a skid or tight turn, the tunnel would ventilate and the jet would lose thrust

    The tunnel was about 16 - 18 inches wide, about 2 2/1 inches at the transom tapering up about 30ish inches. Late Edit, the sides of the tunnel were sides of the tunnel were parallel not a delta pad style. The taper was the height of the tunnel at the transom moving flush to the bottom of the hull at the leading edge of the tunnel. There was a small transition modification made to run the 12 degree deadrise into the relative flat width of the tunnel

    Perhaps you can post some pictures of the boats that you have built or designed?

     
    Last edited: Aug 30, 2022
  7. Jet mad
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    Jet mad Junior Member

    Yes jet sprint boats rely on there strakes for cornering at speed. That's why I mentioned the g-forces. If you are not strapped in in a corner the g-forces will spit you out.
    There is no trim with the nozzle while driving. The driver is concentrating on where he is going and the navigator is holding on with one hand and trying to direct with the other. 20220706_135712.jpg 1970 IMG_0678.jpg 1970 Scan0058 (2).jpg 1980 Scan0087.jpg 2000
     

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  8. Jet mad
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    Jet mad Junior Member

    Barry and Tansl do you have any photos of the boats that you have be involved with?
     
  9. TANSL
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    TANSL Senior Member

  10. Jet mad
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    Jet mad Junior Member

    What jet boats have you been involved in designing and building.
     
  11. TANSL
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    TANSL Senior Member

    You will know how to excuse me, but what is defined cannot be within its definition.
     
  12. Jet mad
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    Jet mad Junior Member

    Barry, I would like to suggest an experiment you could try. That is with your boat with the 12 degrees down turn on your lift - steering strakes and chines. Would it be possible to have the same style boat, but with out the 12 degrees down turn, but have the lift- steering strakes horizontal and the chines 5 degrees.
    At a standing start in the water, see which one is on the plane the quickest.
    Then have the boats on the plane going as fast as they can, then back the throttle right off and see which one comes off the plane first.
     
  13. Barry
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    Barry Senior Member

    We purchased a flow scan meter to determine fuel usage. Our goal was to begin with a standard shaped hull, ie 12 degree monohedron for the planing area, ie about 2/3 forward from the transom.
    The jet that that we used at the time were Berkeley 12 j, with AAA impellors, the motors were from Berkeley as well, their Pack-a-jet 454's, naturally aspirated.

    The first boat that I BOUGHT had a Berkeley 12J with a AA impellor and a 460 packajet, no strakes. ( This boat was slippery in the turns and unintended victory rolls frequent) It was NARROW and required rpm in the 4000- 4200 rpm range to lift 3 guys, 120 gallons of fuel and camping gear for 4 days.
    The fuel consumption was atrocious, getting somewhere around the 2 1/2 miles per gallon range. ( kind of a catch 22, add more fuel to get some range and suffer the increased fuel consumption due to the weight)

    Having more than a layman's understanding of fluids and reading several books about pressure distribution on hull bottoms, (the internet was not really a big thing back then) it was obvious that a wider hull provided a wider stagnation
    line than a narrow hull.

    We built a jig with an 8 foot beam and close to 7 feet on the chine giving us a wider stagnation point. Our first hull had flat chines and flat 4 inch wide horizontal strakes. We took fuel consumption readings. The next hull, same propulsion
    plant, we increased the width to 5 inch strakes and got a lower fuel consumption and then finally we turned the 5 inch strakes and chine flats down and attained even lower fuel consumption values. ( and a much desired increase in turning
    ability)

    With these changes we were able to reduce the fuel consumption to a point that we would be getting close to 4 mpg at 32 mph.

    We could have increased the chine width further but there is a trade off. ie if the chine width (facetious) was 7 feet, ie creating a flat bottom boat the ride would be harsh. So we pretty much left it at the 5 inch 12 degree down.
    The tracking was incredible and as our rivers often contained large rocks that had to be navigated around at 30 mph, this was an awesome attribute.
    So the wider the strake and chine flats, while they increase lift and steering, the resulting increase lift can increase the harshness of the ride.

    The water direction that comes off the keel on a planing hull, (mono and warped) would be back and to the chines. Turning the water downward with the strakes anddchines provides a lifting force. The larger the angle of inlet to outlet
    direction the more the lift.

    So to your question "I would like to suggest an experiment you could try." We have already done this and had taken fuel consumption wrt to rpm readings with different strake and chine options.

    I had made a comment to you about adjustable nozzles on the sprint boat. Looking at various races it appears to be obvious to me that if the nozzle is not adjustable, that the nozzle has shims installed to keep the nose down to increase
    the steering ability due to a longer engaged keel and strake length.
    We had built a custom 18 foot boat with a bored out 454 ls6, with an 1100 cfm Holley double pumper. We had a pitot style speedometer that ran off the scale somewhere around 55mph but still had some room for the needle to move
    after that, but without the scale. Due to the length of the boat at the higher speeds, we would get some porpoising so installed a "jetovator" which we could trim the nozzle on the run. Setting the nozzle down pushed the bow down, removed the problem and dramatically increased the ability to turn tight.

    Not long after that we went to the 3 stage Kem pump, a copy of the 773 Hamilton, then the 773's and then later the 212's. All axials.
     

  14. Jet mad
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    Jet mad Junior Member

    Barry we don't have the rivers like you have.
    All the boats that I have been involved in have been built too D or C survey. I have retired now, so building and testing for me is past.
    The only adjustment the crew do to the jet unit in a jet sprint boat is with the nozzle and impeller. This would be due too the consistency of the muddy water.
     
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