General shape / class of the jet boat hull bottom ?

Discussion in 'Jet Drives' started by cyclops2, Nov 5, 2011.

  1. cyclops2
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    cyclops2 Senior Member

    Warped bottom
    Modified V
    Deep V modern 19' bow riders
    full length constant deadrise V

    Assuming a boat about 20' & V8
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Generally speaking: constant V preferred over warped, because thrust line of jet is higher than tl for a prop installation, which increases bow-down attitude. Minimum bottom angle ~10 deg helps in keeping intake free from bubbles generated at spray-root forward.

    Double chine hulls work excellent with jets, giving low resistance and soft ride. Remember NO DISTURBANCES in front of intake and within minimum 150% of intake width!
     
  3. cyclops2
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    cyclops2 Senior Member

    Where can I find some hull photographs or manufacturers that make very good hull designs ?
     
  4. speedboats
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    speedboats Senior Member

    Not always, there are ways around it...

    At 19' to 20' some of the big boat rules can be bent. Jetsprint boats run a transom deadrise angle up around 24*, I've used 19* extremely successfully with a warped bottom.

    What sort of vessel are you looking into? River, cabin, sea-going?
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    The attached perspective drawing shows you what I did on the Cherubini 20 bottom for the jet drive. The deadrise for the aft half of the hull is constant at 20 degrees, but there is a slight rocker to the keel line--it is not straight. The bow sections a convex and have higher deadrise. So, you might say that this is a combination of all of the types you list above. This shape was sized for the Mercury jet pump. It runs extremely well.

    You can see some photos and drawings on my website:
    http://sponbergyachtdesign.com/CC20.htm

    You can see Cherubini's website here:
    http://www.cherubiniyachts.com/20.html

    I have used a similar bottom for the Saetta 20, which originally had an inboard drive, and I have just reshaped the hull for a jet drive using a Hamilton jet. That is being set up as a kit now. You can see the Saetta 20 here on my website:
    http://sponbergyachtdesign.com/Saetta.htm

    And here at the Saetta website:
    http://www.saettaboats.com/

    I hope that helps.

    Eric
     

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  6. cyclops2
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    cyclops2 Senior Member

    Thank you.

    Guess it is time to show the woodie that needs a below the waterline, NON HOOKING, cleaver / vertical bow. Rouge River jet boats told me that my gut feeling of using a ..constant deadrise all the way to the transome is ONLY used in their rough river water boat. My logic of constant dead rise & vertical bow creates a non visable keel. that provides the extreme straight ahead in the worst chop & waves......Speed & efficiency mean nothing in this boat. A 5 L 220 hp inboard should easily give 40 to 45 mph. Perfect.

    It will be built as a rear 2 person, Gentle Man racer. I have cut up the full depth keel, engine stringers 23 degree triangular bottom frames. Frames are every 12" to prevent twisting of the hull. I will use the 1/2 way notched & interlocking assembly of the deep V bottom. Plywood for strength & a probable wood hull not to exceed 500 pounds.

    The boat will hopefully go THRU a wave of any height. Rather than be tossed 6 feet high. Have done that in my 19' V8 Chaparral. Teeth stopped 2" from the Aluminum windshield.

    Boat should not bow hook or become airborne unless I am stupid or showing off like a nut in a 21' X 4' boat.

    Rich
     

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  7. cyclops2
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    cyclops2 Senior Member

    Construction note,

    Obviously ALL interlocking joint will have FULL LENGTH triangular 1" x 1" soft Pine gusset blocks to lock the weakned joint securely. The same gusset blocks will be used on both sides of anything holding the marine plywood skins on. A final vacuum bagging of veneers will cover any plywood appearence.

    Plywood for twisting & pounding strength. Veneer for appearence.

    Rich
     
  8. Mat-C
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    Mat-C Senior Member

    I know this is an old thread, but I was hoping that Eric might answer a question or two regarding the bottom shape of the Cherubini that he linked earlier in the thread... (cool boat BTW :) )

    From the perspective view and the side (below) it looks like you have effectively taken a chunk out of the bottom to produce a flat for the jet intake. Is this how it is typically done in a 'deep-vee' configuration? I would have though it would create a bit of turbulence in front of the jet, as well as a possible area of 'negative pressure'.
     

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  9. Eric Sponberg
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    Eric Sponberg Senior Member

    Mat-C: You deduce the shape correctly--it is a chunk taken out of the bottom of the boat. I did that on the advice of the engineers at Hamilton Jet. They said that in their experience, this type of shape works extremely well. The trick is that the slope of the chiseled front forward of the inlet flat has to be a small incline, and what I show passed muster with Hamilton. A gentler slope might be better for the jet, but worse for the ride--it would cut into the V-bottom too far forward. The boat likes to ride on a V-bottom, not on a flat. So my shape was the best compromise. The slope is gentle enough that, considering the running trim angle and hull depth when running, there is still positive hydrostatic and dynamic pressure into the jet inlet.

    When we built the hull molds for this design, the hull plug was carved out of foam at DLBA Robotics in Virginia, and it had the full V, not cut away. In the mold, we created an insert for the jet pump flat and ramp so that we could build the boat either way--as a jet with the flat or as an inboard or an I/O with the full V-bottom. In fact, the jet version was not that popular; rather the I/O is, and so that is the style that Cherubini typically builds. If an owner came along that wanted a customized inboard or a jet, I am sure they could build it. I don't know if they have built a 20'er in a while, but they do have a new model that they transformed themselves from the 20'er to a 25'er (a really long stretch of the hull lines, by the way), put a top on it, and call it the CC 255 Sport Cruiser. Looks and runs pretty nice, actually, but I did not have anything to do with that modification.

    I hope that fills in some details.

    Eric
     
  10. speedboats
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    speedboats Senior Member

    Had you considered the merits of running the flat area (which we refer to as the 'delta', not sure of the origins but it could be due to the triangular shape) from the transom to just forrard of amidships? The flat area forrard becomes quite narrow so as to minimise the effect of 'slamming' etc that you suggest. Although it does rotate the jet giving a 'trim up' effect, depending on the pump you use they have a 5* down angle on them anyway (to allow sump clearance for the motor to the bottom of the hull.

    [​IMG]
     
    Last edited: Apr 10, 2013
  11. Mat-C
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    Mat-C Senior Member

    Thanks Eric...can't argue with the boys at Hamilton I guess...they've made a few jets in their time!
    As I said, I never really looked critically at a deep-vee jet installation before now... I suppose I always assumed it would be done as a horizontal version of what speedboats has pictured, as in the pic below. That would give a flat 'pad' for the boat to ride on - quite common in many faster boats I believe - but avoid any potential turbulence problems. It would also avoid having the bottom effectively 'sweeping up' towards the stern as is the result in speedboats pic...
     

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  12. Eric Sponberg
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    Eric Sponberg Senior Member

    To both Speedboats and Mat-C,

    Yes, in fact, I did go through a rendition of the hull lines with a flat on the bottom exactly as Mat-C describes, not inclined like Speedboat's photo, and got all the way through general arrangement drawings. The flat was about 3/4s the length of the boat. And after studying that shape in relation to other boats that had been so designed, I learned that they did not handle very well--they pounded in waves and were slippery in turns--they'd skid rather than dig in. I redesigned the V-bottom and consulted with Hamilton on the jet pump flat. In the end, while I designed a diesel jet pump version with Hamilton's smallest jet, we elected to use the Mercury jet pump which is a really neat compact little unit, although gasoline instead of diesel. The market did not accept it that well, and finally Cherubini got rid of the flat and went with the I/O propulsion with either gas or diesel, Mercury or Yanmar.

    If I had to do it over again to provide a jet design in a speedboat, I would do it the same way with the bottom shape that I have developed, which I have used on a number of boat designs--the boats trim out and handle very well. The jet version of Cherubini's 20'er handled extremely well--I was very impressed.

    Eric
     
  13. Mat-C
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    Mat-C Senior Member

    Thanks Eric
    Here is a pic of a Haines Hunter 1800S, many of which were built in the '80's. They are still very highly regarded for their handling and rough water performance. Most were outboards, though some were also fitted with sterndrives and I think even a few jets. I post it only because it has the kind of 'pad' that I was referring to....

    Thanks again for the insight :)
     

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  14. Eric Sponberg
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    Eric Sponberg Senior Member

    You're welcome.

    Eric
     

  15. speedboats
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    speedboats Senior Member

    That is unfortunate, the Sportjet (which I am assuming you used as it is drawn in your previous drawing) would have struggled with performance due to the diameter of the pump. Yes they are a nice compact setup and lightweight, simple installation for a production boat (particuarly for guys used to outboard installs), but infortunatley lack any significant torque to quickly move a larger vessel. American Turbine's SD312 or Hamilton Jet's HJ212 or HJ213 would have been better matched
     
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