flat bottom with inboard jet

Discussion in 'Hydrodynamics and Aerodynamics' started by semojetman, Jul 13, 2012.

  1. semojetman
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    semojetman Junior Member

    Hi everyone, I am new to the forum
    I am in the process of building an 1856 flat bottom aluminum jet boat.

    What are all my options to slightly modify the hull to allow adequate water presher to the intake grate and avoid cavitaion.

    I know typically a boat with atleast a 5 degree hull is used, but I want to use my flat bottom.
    I have seen something like this done by building a reverse tunnel in the center rear or the hull leading to the intake.

    Thank you all in advance.

    P.s. I am using a 350 V8 GM and a Berkeley 12JE pump.
     
  2. sottorf
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    sottorf member

    my experience is if you run the boat with a good positive trim angle (> 3 deg.) then air does not tend to run under the hull. For low trim angles a lot of air gets entrained into the jet. Typically if the jet gets more than 10% air (by mass) then it will loss efficiency very fast.

    It is quite normal to have hull modifications upstream of the jet intake that allows the jet intake to be slightly deeper submerged.

    I propose you talk to your jet supplier for advice. Doen jets in Australia have a lot of experience running high-speed jet boats and can advise you well if you are still looking for a jet supplier.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Unfortunately there is no such thing as a constant "good" trim angle when operating in a seaway. This leads to bursts of aerated water migrating along the bottom, causing thrust reduction. This is why some bottom rise (minimum 5 to 10 degrees) is specified by jet suppliers.

    Now, if you for some reason cannot use the recommended bottom shape, you may install the jet with a streamlined "collar" protruding down from the bottom plating about 40 mm. This will let the jet intake to work in "green water", even if there is aerated water along the bottom surface.

    The downside of this is a reduced intake efficiency, since the energy in the boundary layer cannot be utilized in the inlet, and this consideration alone is reason enough to stick to V-shaped bottoms, if not over the entire bottom area, so at least over a beam of roughly 2 x jet inlet width.

    Also remember that the pump needs to be submerged at least up to shaft level at low speed, in order to prime correctly; this requirement sets the limit for minimum draft. Note that in addition to these opinions regarding the requirements for the pump, please observe that a flat bottom with a waterjet will have very low directional stability at displacement speeds. I have seen examples sliding around like butter in a frying pan; all because the designer was striving for minimum draft. Some had to be fitted with fins in order to make them driveable. With fins, the draft became more than would have been necessary with a "clean" V-bottom. In my opinion, a flat bottom is about the most stupid thing you can design into a boat, and to a jet boat in particular.
     
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  4. semojetman
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    semojetman Junior Member

    opinion

    I thank for your advice,
    I would have to agree that your last line is your opinion.

    In our area flat bottom aluminum boats are the top dogs.
    We have wide shallow rivers.

    There are thousands of flat bottoms on our rivers and if you have a V bottom its because it was cheap or you couldn't find a deal on a flat.
    So, I guess it just comes down to what fits your needs.
    Many people have been taking the flat bottom very popular blazer SS 1752 and installing

    SSnewer jet ski engines, and the end results have been extrordinary.
    They have been building basically a small reverse tunnel to put more water pressure on the intake.

    You would be amazed what some of these nice flat bottoms go for here and what anything with a V or semi v doesn't bring.
     
  5. sottorf
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    sottorf member

    can you post some pictures of these flat bottom hulls with waterjets. It would be interesting to see the setup - particularly the underwater parts.
     
  6. semojetman
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    semojetman Junior Member

    jet

    Yeah, ill get some pictures.

    Runnin a flat bottom with an inboard has a fine line between being a 25 mph cavitating no planing gas guzzler and being 4" deep runnin 60 mph sled.

    I will make sure i have the best hull modification design before I even start cutting.
     
  7. baeckmo
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    baeckmo Hydrodynamics

    Interesting.....could you explain or quantify the differences in performance and the general size of those boats? And what do you mean by a "small reverse tunnel"?

    The operating environment you describe reminds me of the "downhill river racing" in NZ. The boats used there are not flat bottomed as far as I understand, which makes me wonder what makes the difference in your case.
     
  8. semojetman
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    semojetman Junior Member

    It is basically a 1" drop in the middle of the hull. Approximately 12-18" wide.
    In our rivers it is very common to run in 4-6 inches of water. We also float our boats a lot. Run miles up river and float down.
    A wide flat bottom will float a lot shallower.
    I have friends with semi-vs and V bottom gators and scorpions and they just can't float in the water that flattys do.
     
  9. TKOUSA
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    TKOUSA Junior Member

    We find that optimum handling and speed can be obtained ... indeed we currently have such a design sketch on hand ... not quite ready for a blueprint yet, but the hydro dynamic projections are promising. Thomas.w.knutson@gmail.com TKOUSA
     
  10. semojetman
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    semojetman Junior Member


    Sounds good. My hull should be in anytime.
    I'm trying to get all my designs done beforehand.
    Everyone has been very helpful.

    If anyone has any designs or pictures of something along these lines I would greatly appreciate them.
     
  11. TKOUSA
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    TKOUSA Junior Member

    We posted a few pics today

    Boat Design forums., Boat Design., TKO Electric
     
  12. semojetman
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    semojetman Junior Member

    boat tunnel

    I was just watching this video on youtube of these bad@ss jet boats and they have tunnel hulls. How can they have tunnels and not have cavitation?

    I also talked to a guy today that is building a boat very similar to mine and he said his friends have already built 2 and they are full flat bottoms with no mods to the hull.
    They also have a scorpion that is a semi v but runs to a full flat in the back. No degree at all and it runs 60 mph.

    He said I was crazy for worrying about doing the reverse tunnel idea.
    I still feel that it is needed to avoid cavitation.

    Any ideas or comments??
     
  13. TKOUSA
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    TKOUSA Junior Member

    Some will call a flat bottom boat with an interior cavity the " tunnel" ?
    ... perhaps what they are referring to is an internal tunnel design?
    Also have you done any work / seen many designs using a M hull or W hull ...
    TKO
     
  14. semojetman
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    semojetman Junior Member

    I don't know what any of those things are so I deffinetly don't know how they relate to my build. M or W hull, internal cavity?
     

  15. TKOUSA
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    TKOUSA Junior Member

    The M or the W creates a V shape that may be of use to your cavity needs ...
     
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