Looking for some hull design help.

Discussion in 'Boat Design' started by FeedbackForce, Sep 28, 2011.

  1. FeedbackForce
    Joined: Sep 2011
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    FeedbackForce New Member

    I am currently trying to design from scratch a >19ft boat able to carry around 400lbs. I am powered this boat off solar panels and batteries so my priority is efficiency. The issue I am having is that I want to reach around 8 mph average while my max length has a hull speed of around 6.7mph if I am being generous. At the same time, the speed is slow enough that I don't think I will be able to achieve any planing. Currently I have been looking at the ideas of hydrofoils to maybe make the low speed planing possible; or a multi-hull boat, as I have heard they alter the hull speed, though I cant imagine how. The thing with these ideas is I am afraid that the drag will be too great and I wont even be able to reach the ideal 8 mph or even the hull speed of a normal displacement boat. There is also a sprint aspect I want will need to deal with, but I would first like to see some opinions about this endurance issue.
  2. alan white
    Joined: Mar 2007
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    alan white Senior Member

    It would be better if you could stay within the hull speed as it's going to take a lot more power to get up to 8 mph. A canoe or kayak form is probably the most efficient. Bear in mind, longer waterline length (19 ft) translates to lower efficiency at speeds that may be ideal for both human and electric inputs.
    That is to say, an important aspect of small boat design is designing the hull to be most efficient at an energy input level that is easiest to maintain, say 200 watts as an example. A long hull will require a higher energy input than a shorter one at low speeds while a short hull will require a higher energy input to go fast. Short lengths favor less wetted area, the extreme example being a perfect bowl shape, which requires far less effort to row at extremely low speeds than a long, needle-like hull.
    While a perfectly semi-cylindrical midsection is most efficient at any speed, the next design consideration is to modify that perfect midsection to accomodate a human passenger. The most efficient midsection is unfortunately very tippy so some submerged area will have to be added to create a flatter bottom. How flat depends on the skills and requirements of the operator.
    Try out some different canoe/boat hulls for stability and use their shape to influence your midsection design.
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    The "hull speed" concept is just a term which describes a huge increase in resistance of hulls with a low "slenderness ratio", defined as SR=Length/(Displacement^0.3333).
    Take a look at the graphs Ad Hoc has posted in this thread: http://www.boatdesign.net/forums/hy...laning-hull-low-speed-35799-3.html#post426942

    The first graph shows the Resistance/Displacement ratio (R/D) at increasing Froude numbers Fr, defined as Fr=Speed/sqrt(g*Displacement^0.3333), for various SR hulls.
    You can immediately notice that a hull with SR=4 shows a high drag rise for 0.5<Fr<1.3, with a peak resistance at Fr=1.5 . This zone of the resistance curve is referred to as "the resistance hump". If the hull is not properly shaped and the installed power is not huge enough, the boat will not be able to overcome this increasing resistance and reach high speeds. That's where the term "hull speed" comes from - it's a maximum speed which a low-SR hull can attain if not adequately shaped and powered.

    Now see what happens at the right-most curve in that graph. It shows the R/D ratio for a hull with SR=10. No hump there! The curve is climbing in an almost linear way as the speed (or Fr) increases, so the maximum speed which this hull can attain is nearly directly proportional to the thrust applied by the prop (and hence the power is roughly proportional to the square of the speed).

    The conclusion is: if you want to go high speed with limited power, you have to use a high Slenderness Ratio hull. Rowing shells are a classic example of this concept:


    Now, there's a problem with such slender hulls - they have a very little latteral stability, meaning that they are tippy. It means that they either have to be stabilized with a small lateral ama (the boat is then called "a proa"):


    or with two small amas:


    When amas grow up, the boat becomes a trimaran:


    Finally, you can simply take two slender hulls and let them work side-by-side - in which case you have just invented a catamaran:


    These are the possible strategies - the final choice will depend on various other design constraints.

    I invite you to carefully read these excellent research papers by Leo Lazauskas, about low-resistance hulls:


  4. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    You are right to be concerned about the drag of hydrofoils. The drag due to lift of a hydrofoil is inversely proportional to the square of the speed, so trying to fly a low speed creates a great deal of drag. The only way to reduce this drag is to give the hydrofoils great span.

    At 8 kt, a slender displacement hull will have less drag than a hydrofoil. For efficiency in this speed range, a long slender hull is really your only option. The question then becomes just how many long, slender hulls to use to meet the other design requirements (stability, loading, transportation over land, etc.)
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