Jet power on warped bottom hull?

Discussion in 'Boat Design' started by nbehlman, Oct 18, 2011.

  1. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    I am planning to build a 14 foot runabout in the style of the early 30's and 40's chris crafts. The power plant is a 70hp TigerShark jet ski motor and pump. I am designing the hull from scratch and I wanted to see what the experts on this forum have to say. If you have any comments or advice on what types of design features I should incorporate into my hull, please share them! Here is some basic info:

    http://nbehlman.com/hullprofile.png
    length: 14ft
    beam: 4.8ft
    weight: 650lb (jet ski weighs 476lb)
    deadrise: 6.5 deg
    draft: 6.5 inches (one person aboard)

    Are there any special considerations I should take into account when designing a hull for a jet drive? These hulls are meant to take an inboard prop. I have noticed that jet skis have a much more rounded hull, while my boat will have sharper chines and a low deadrise vee bottom.

    I have looked around on the internet for similar projects and there seem to be quite a few. Most of them are simple "what happens if I do this?" type projects. I came across one site that shows a very strange profile below the waterline:

    http://www.sponbergyachtdesign.com/Designs/IC20 Profile single small.jpg

    It looks to me like it would create massive amounts of pressure drag because of the keel line near the intake.

    I found a video of a chris craft converted to a jet drive:

    http://www.youtube.com/watch?v=uBUUeNfGjTg

    At the 10 second mark, you see it go into a drift turn characteristic of jet boats. I am concerned that these kinds of turns could cause the hull to "trip" over the nearly square chines.

    I expect that my hull will provide a great deal more lift than the jet ski hull, but will of course weigh more. I have done quite a bit of work with the Savitzky power prediction method, but I suspect it is not very accurate for jet boats since the kind of suction onto the water. My target speed is 30 mph. The jet ski currently does 40 mph.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Because jet driven boats don't have any rudder or drive leg at the stern in the water, the stern may want to overtake the bow in following seas, particularly if the transom deadrise is shallow and there is a pronounced forefoot up front. Also, the line of the drive thrust is higher, which further increases this tendency.
     
  3. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Nbehlman,

    The weird keel profile on Eric Sponberg's IC20 is so that it can accommodate the jet intake and still have a reasonably sharp entry. If I recall correctly, the deadrise of that boat is pretty close to constant from transom to midships, but the aft two or three feet of the keel are cut back slightly to make a flat surface for the intake grate.

    Quite a lot of successful jet boats, including most PWCs, share this same basic shape- a constant deadrise V hull with a slight cutaway or nacelle for the intake. Simply put, it just works.

    Warped bottoms (deadrise near 0 deg at the transom, 10 deg or so at midships) often don't fare quite as well with jets. Post #2 mentions a possible tendency to spin out- a very real concern with such a hull. This shape also puts the pump higher, which can increase the chances of sucking air into the pump; also, if the drive shaft is above the waterline, some pumps may suffer excess bearing wear on startup.

    The pump suction is generally not negligible, but it won't appreciably change your Savitsky results in the speed range you're talking about. At higher speeds, it requires quite a bit more thought.

    I'd suggest sticking to a simple constant-deadrise V, tweaked as necessary to get the pump to fit.
     
  4. Frog4
    Joined: Oct 2011
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    Location: Arizona desert

    Frog4 Proletariat

    would like to see a full set of plans for Sponberg's IC20 in a 12' OAL
     
  5. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    No dramas, this is what all my boats are based on. Yes they can have a tendancy to spin in a turn or if you come off the gas abruptly, but you can successfully compensate for this by creating more lift at the front of the vessel.
     
  6. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    I thought the idea of a jet boat is that it could be spun in a turn ......
     
  7. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    From reading around on this forum it sounds like it is best to minimize warp in the aft planing surfaces. This is a consideration for all planing hulls. In my case the lack of stabilizing surfaces will be even less stable than a constant deadrise PWC hull. I will try to keep warp down in the aft section.

    How would I add lift up forward? Shallower vee? What about lifting strakes?

    As far as the hull design near the intake grate... I will try to make a smooth transition from the vee into a slight fairing just befor the inlet. I am still wary of pressure drag associated with this feature. I will keep it small and localized. It might be ok if I keep the curve shallower than what the free wake profile would be in that region.
     
  8. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    A jet boat usually has a ride plate about 1/3 the width of the transom at the rear ..you will be riding on the back few feet of the hull if you put enough power in it ...have a look at www.ScreamandFly.com The website for power and jet boats
     

  9. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    Really?

    My boats have a chine width of 1680mm and delta pad at the transom has been as narrow as 220mm and generally no wider than 320mm depending on the pump employed. Perhaps I'm doing something wrong?

    Yes, it is desirable for commercial adventure tourism vessels to be able to perform manouvers such as slides, spins, nose-offs, etc. However a waterjets advantage are shallow water navigation, superior manouverability particularly at lower speeds, less mechanical complexity meaning less wear and maintenance costs.

    Different hull forms will yield different performance characteristics in much the same way as any other propulsion method. Better lateral adhesion can be designed into the hull, as can better lift.
     
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