How fast can an 8' boat go and not wreck?

Discussion in 'Stability' started by Thin water, Sep 2, 2007.

  1. Thin water
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    Thin water Senior Member

    I have started a little contest between several builders of the minimax and minimost type hydroplanes to see who can make the fastest boat and still keep the 4 x 8' hull size of the original design. I have made the sides higher so it won't take on water with the extra weight or a bigger motor. I am reenforcing the boat with fiberglass and some kevlar over the plywood. I will have the strength but I need stability. The last one I built would porpoise when you broke 30 mph (with an 87 lb 15 hp outboard and my 230 lb's in it). A small fin on the motor fixed most of the problem. I made a bracket to mount the motor 1' behind the transom on my new build and plan to make a pair of fairly long and wide trim tabs to increase the effective length of the hull. I am planning to mount one small fin near the rear of each trim tab so it will turn and to help keep it stable (I think)

    Any ideas? I will wear a heavy duty ski vest and helmet when I run it an a glass smooth lake. I don't want to blow it over or turn it into a submarine.

    HP will start at 35 and go up from there if it works.

    Below is my last one, it is still going strong.

    Any ideas are welcome.

    JIM

    [​IMG]
     
  2. Retired Geek
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    Retired Geek Junior Member

    this is our 3rd version and already a bit past 30 mph on 8 hp....in NZ the class is called Max 8 and quite popular. This version is building now, will let you know if its faster than the previous ones.
    RG
     

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  3. Retired Geek
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    Retired Geek Junior Member

    oops....forgot....its easier to use a wing forward and less drag than putting something in the water
    RG
     
  4. waikikin
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    waikikin Senior Member

    Looks cool RG, like something from the "thunderbirds", I'm gunna google max 8 to check out some more! from Jeff.
     
  5. Retired Geek
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    Retired Geek Junior Member

    not sure you'll find it as the page is disabled right now....boat I did is for dave with the tohatsu....url for pics is: http://www.kinloch.net.nz/max8/boats/

    enjoy
    RG
     
  6. Retired Geek
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    Retired Geek Junior Member

    Unlike what you might expect, these boats employ almost every idea in current offshore powerboat racing you have seen and then some, most are amateur designs or adapted from other designs, some are fine in flat water, some do better in chop, but over all most are unstable at some point in their performance envelope....hulls for the most part are under 15 kg and can be built by anyone with a few months of free evenings...perhaps 3-4 weeks for those who have done it before
    RG
     
  7. Thin water
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    Thin water Senior Member

    That Max8 site is very cool. That is they type of playing I usually do but I use a little more power because I am 6'05" 230 lbs. These boats are much more fun with 8 - 9.9 hp motors because you can run them wide open and not become unstable.

    I know it is silly to push one of these to high speeds but I have to do it and see how fast I can make it go.

    JIM
     
  8. Guillermo
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    Guillermo Ingeniero Naval

    Jim, you could try this one, too... :D
    (Sorry, I couldn't avoid joking!)
     

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  9. Retired Geek
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    Retired Geek Junior Member

    Good one Guillermo...just add some safety lines around the legs and your offshore legal <grin>

    Jim... even though they use lower horsepower, most of them are riding on a waterplane that's less than 200mm long, it doesn't take much to upset them and make them unstable, hence the foil at the front of ours...nice long arm and not a lot of force required to make it stable...not a lot of drag either.

    When designing it I got the tunnel boat design program (by aeromarine) which is good for classic tunnel boats but found it wasn't that great for my approach....none the less it did confirm most aspects of what I was after. I wouldn't look at your height or weight as an impediment, rather as an advantage as mass in the right place can accomplish a lot with little or no penalty when designed for correctly.

    Mostly I like the Max 8 class because its totally open and you can try really innovative ideas without penalty...a pity not all of racing, sail or powered isn't like that.
    RG
     
    Last edited: Sep 4, 2007
  10. waikikin
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    waikikin Senior Member

    RG, thanks for the link thing, especially liked the paint & look of "Madness", regards from Jeff.
     
  11. Thin water
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    Thin water Senior Member

    I ran the above pictured Minimost today with a stock 15 Johnson. it reved out and only went 27 mph. I need a bigger prop.

    Still looking for any additional ideas for more stability at speed. i like the fin in front in the air to keep it down.

    JIM
     
  12. Retired Geek
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    Retired Geek Junior Member

    Jim, we use a torq shifting prop so you get more pitch as the speed increases. Last I heard the factory that made them burnt down a year or so back, but we managed to find one on ebay that we reverse engineered and then had one made to our own design (the blades that is, we kept their hub), Im sure you could find something to fit your 15hp on ebay without to much hassle. 27mph on 15 hp sounds like you have a huge ways to go in the speed dept yet :)
    RG
     
  13. Thin water
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    Thin water Senior Member

    I hope to obtain speed the old fashioned way, excessive power :)

    JIM
     
  14. messabout
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    messabout Senior Member

    Your original boat looks good for its purpose. It looks like a single plane runabout from the 50's. If you want to go faster build an APBA style three point or 4 point hydro. Maybe a pickle fork hydro. There a plans around for that sort of weapon.

    Be warned that overpowering any small boat may be hazardous to your health. Flipping a boat at 60+ MPH is an experience you can do without. Been there, done that! An eighty MPH blowover can be fatal. I've seen it happen. Not an entirely pleasant memory.
     

  15. Thunderhead19
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    Thunderhead19 Senior Member

    One of the options that I have used is to get a outboard with counterrotating props and a big aftermarket foil.- get the hull completely out of the water. Things can get a bit hairy as messabout stated because the control you have is unpredictable in terms of what will happen and how agressive a reaction your movement will cause.
     
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