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

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

  1. Thin water
    Joined: Aug 2005
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    Thin water Senior Member

    I have the plans for a class d picklefork by They do in the upper 70's. i will build it over the winter. The above contest is limited to a minimost or minimax boat with any added junk like trim tabs, fins etc... being OK.

    I think I will just run the one I am building with a 40 HP Yamaha tiller that a friend has and see how it does then call it good enough. If I can get to 40 I will be happy considering it is just two peices of plywood and cost less than $200 to build including epoxy.

    A large foil may give me enough control to get up to max speed for one good run. Then I will cut the transom down to a 16 1/2" height and just run a little 15 short shaft on it. It is set back one foot and with a water rise of 1" per foot 16 1/2" should work for a short shaft engine and still leave a little room to trim if needed.

    Thanks for the input so far.

    I will keep checking the thread until I run it then post pictures and the results. The boat will be done next week when i get some more epoxy.

  2. powerabout
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    powerabout Senior Member

    the old stock 6hp 'bathtub' racers did over 30mph and stock 25hp class did over 50mph 20 years ago so you have bit to go...

    30 and 40 years ago with non stock engines of 250 and 350cc they did 100mph on 3 pointers

    A guy I knew in Melbourne built his own rotary valve methanol injected engine, 70ish hp at 20,000 rpm from 250cc
    The part that impressed me the most about that engine was the bore was the same diameter as the throttle body on each cylinder
  3. RatliffFranklin

    RatliffFranklin Previous Member

    Stability In.jpg

    I'd think about some form of driver-controlled canard wing, such as those used on Unlimited hydroplanes.

    I'd also consider a Ken Warby T-tail.

    And I'd definitely get some good motorcyclist body armor.
  4. RatliffFranklin

    RatliffFranklin Previous Member


    The killer small displacement engine available off the shelf is the Hayabusa motorcycle engine.
  5. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    MiniMax & MiniMost

    You MiniMax & MiniMost guys should connect with Brian Cranfield at BC Seafleas . He has alot of connections to builders of these boats! He has another article on MiniMax & MiniMost boats at this site.
  6. granite
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    granite Junior Member

    I am confused about the purpose of the forward canard/wing on the design in the drawings.

    If the boat is running quickly and gets an upset so it is bow high the canard will see an increase in angle of attack and increase its lift the opposite with a bow down upset. To me the bow canard gives positive feedback to any upset destabalising the boat more than if it were not there.

    would it not be better to put the stabalising aerofoil behind the boat where it would naturally damp the oscilations?
  7. RatliffFranklin

    RatliffFranklin Previous Member


    The canards in those renderings make a lot more sense if they're driver-controlled variable pitch rather than fixed.
  8. charmc
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    charmc Senior Member

    As I understand it, most canards are now driver controlled, enabling some fine tuning of the angle of lift. Unlimited hydros all have a large, aft mounted, stabilizer wing which functions as you suggest. Rules for most other classes, including F1, prohibit a rear mounted wing.

    The primary function of all surfaces on a racing boat is to add lift, as all water contact creates enormous drag. Apart from collisions between boats, blowovers are the most frequent failures, and you're right; once a blowover starts, the lift surfaces only add to the effect. I think the current view is that allowing a large negative angle movement of the canard doesn't help in a blowover, as it's too little too late. I could see huge problems if the canards had a wider range of motion, also, mostly an increase in accidents from overcorrecting and stuffing the bows. Overall, canards seem function best as "fine tuners" allowing the driver to add or subtract a bit of lift force as needed. The definition of canard, in most racing glossaries, in fact, states that its function is to "adjust trim".
  9. RatliffFranklin

    RatliffFranklin Previous Member


    In aviation canard means a small wing located forward of the primary lifting surface.

    Canards on Unlimiteds are for blowover prevention. Using them to fine tune performance on the fly was something of an accidental byproduct.

    Light aircraft using canards for pitch control such as Rutan's Varieze have been around for decades.
  10. charmc
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    charmc Senior Member


    I stand corrected. I didn't realize the Unlimiteds' canards were developed in the hope of preventing blowovers. They are a lot bigger, so they would have more effect that way, although blowovers will probably always be a part of racing on the air-water interface.
  11. RatliffFranklin

    RatliffFranklin Previous Member


    I don't see blowovers becoming extinct anytime in the near future.
  12. kengrome
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    kengrome Senior Member

    Exactly! In fact, at the speeds these boats are designed to go it probably makes more sense to design an "airplane" rather than a boat anyways -- with control surfaces in the air and not the water. The more lift the wings generate the higher the craft rises out of the water until only the prop remains in the water for propulsion.

    Note, there's a real boat (or at least a prototype) designed to do this, it lifts up as speed increases and flies over the water with only its aft mounted propeller is in the water. I don't remember where I saw it so I cannot post a picture but some here might remember it. I think it's black and looks kinda like a dragonfly.
  13. Retired Geek
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    Retired Geek Junior Member

    the canard on our boat is a very slightly inverted section so it is generally creating downforce. It has 2 controls, one is a trim function that adjusts small angles for trim, the other is an accelerator type pedal that you can stomp on when required....which seems to me used more often than I'd desire, but so far it has prevented a few blowovers. Its also used at hump speeds to get the bow down quicker which seems to improve the hole shot accelerations a bit.
  14. clanning
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    clanning Junior Member

    The section (symmetrical, inverted or right-way-up) makes no difference -- it's all in the angle of attack (AoA). You need the 'panic pedal' to pitch the nose back down in case it threatens to rise -- and then (I'm guessing) you would run the risk of submarining when you came back down... yikes!

    Some foiling sailboats have an arm on a float (like the float in a gas tank) that controls the AoA of the main planes -- you could do something similar so that when the bow rises too far, the AoA of the canard is adjusted and the bow comes back down.

    Or maybe it just adds complication if the dead-man's lever seems to work OK.

  15. Retired Geek
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    Retired Geek Junior Member

    I think you'll find that a nervous toe works a bit faster than something that works after the boats motion has actuated the lever...what ever it is
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