Designing a Water Bike

Discussion in 'Boat Design' started by malith, Jul 24, 2014.

  1. malith
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    malith Junior Member


    I'm a mechanical engineering undergraduate who is little new for boat design. I am currently engaged on a project on design and construction of a human powered Water Bike. I did some internet research about this and found lot of pictures and videos. But i couldn't find any literature on designing small hulls. There are plenty of books about ship building and boat building. But since my application is small scale i have doubt of their applicability for my project. So it will be very helpful if i can get some advises and reading recommends about small hull designing and construction.

    Thank you!!
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Malith, There are a few threads on this forum that have to do with human powered craft, specfically:

    The first thread above gets into some of the aspects of hull shape and design, as well as pedal power and propeller design.

    You will need a book on small boat design, and I can suggest "Skene's Elements of Yacht Design" by Francis S. Kinney. Look for it on the Internet, you can probably find used copies available. The 8th edition published by Dodd, Mead & Co. in 1973 is probably the best one. It is more comprehensive than the original "Skenes" by Norman Skene himself.

    Also, you can use my booklet, "The Design Ratios" which explains a lot of design terms and ratios. I uploaded a copy here.

    I hope that helps.


    Attached Files:

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  3. gonzo
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    gonzo Senior Member

    A canoe is one of the hull types with less resistance and still reasonable stability. If the crew has good balance and will use the boat in protected waters, a rowing shell will have even less resistance. If you are not required to design and build the boat, used shells can be had for cheap. Uffa Fox
  4. philSweet
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    philSweet Senior Member

    As far as basic hull shape parameters are concerned, those are determined by the propulsive power, all up weight, number and type of crew, the desired stability of the craft, and the sea conditions. So try and hang some numbers on those four and identify the sea conditions for starters. Also, is this for general recreation or competition? If the latter, please post the competition rules. We don't want to give the game away - so to speak.
  5. malith
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    malith Junior Member

    Thank you very much eric
    i appreciate your help and your document was very help full
    currently i'm planing to used a planing hull
    but i have a doubt whether it is possible to achieve planing with the limited power available
  6. Jammer Six

    Jammer Six Previous Member

    Makes me tired just thinking about it.
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Malith--a planing hull is a big mistake. A human powered craft is never going to plane, and planing hulls have a lot of drag at low speeds. You need a long narrow hull which will slip through the water quite easily. This makes it easier to propel with human power.

    I presume since you call this a bike that it will be powered by leg power on a crank. And, presumably, this will power a propeller since a propeller is the most efficient device we know of for propelling boats, particularly at low speeds. Such a propeller will likely be slow turning, so it needs to have a big diameter, and that is going to greatly affect the shape of the hull--in order to get the propeller to fit. The faster the propeller spins, the smaller the diameter it can be. So you might consider a step-up gear or sprocket system (i.e. overdrive) to get the propeller to spin faster than the spinning of the crank. These are all things to consider in a human powered water bike.

    And I'll say it again, a planing hull is a big mistake.

  8. Skyak
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    Skyak Senior Member

    Not many humans can power a hull to planing speed for very long if at all. I have seen some well designed foiling peddel craft that athletes can keep up for less than an hour. Displacement mode is what you should design for everyone and any distance.
  9. tinhorn
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    tinhorn Senior Member

    You'll want to study bicycle design a bit, too.

    My favorite "water bike" uses two inflatable pontoons and a recumbent bike-style seat. I can float around for hours without any malfunctions later.

    Attached Files:

  10. malith
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    malith Junior Member

    thank you all for your great support!!!

    after considering all the facts i thought of going for a catamaran hull instead a planing hull.

    in addition i also decided to add an electric motor to assist the pedaling.

    So how about using a planing catamaran hull???
  11. malith
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    malith Junior Member

    thanks for help!!!

    i'm making this as my final year project. so there is no design limitations.
    my idea is to make a craft for a single person which can be used in rivers and lakes.
    powering method will be human power: in addition a motor can be also installed to assist the human.
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    No, you don't even need a planing catamaran hull--the same problems apply. but catamaran hulls are long and narrow and they should have a length-to-beam ratio of at least 10. At that proportion, you'll go through the water just fine with the minimum of resistance, but you'll never get up to a planing speed.

    Which is not to say that a shallow V-bottom would not work--it will, but you'll have to make the stern end just about as pointy as the bow end. Do not make a transom stern that is squared off. Your hull should be shaped more like a canoe or kayak than a planing boat.

  13. tspeer
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    tspeer Senior Member

    When do you envision the motor will be used to assist the human? A hybrid propulsion system makes sense when there are short periods of high demand interspersed with long periods of low demand. A small energy source can provide the average power associated with the long periods, and a battery can provide the peak power to meet the short periods of high demand. With a human and a motor, do you envision the motor providing the peak power or the average power? In other words, is the motor to be used for punching through some difficult stretches, or is it to be used as a low-speed cruising mechanism to make sure the boat will eventually get back if the human gets tired?

    A motor adds weight, possibly concentrated at the stern. This will have a big impact on the resistance when using human power and on the design of the hulls. A fossil fueled motor also adds maintenance, noise, and the issue of starting the engine - does the person have to go to the stern to pull the starting rope?

    Like a hydrofoil, the dynamic lift from a planing hull comes with a drag that is inversely proportional to the square of the span (beam, in the case of a planing hull). This is one reason why planing catamaran hulls don't make a lot of sense. The narrow beam of a catamaran demi-hull means high drag due to dynamic lift. A wide monohull can plane more efficiently, but has a lot of surface area that causes skin friction drag. That's why people are advising narrow displacement hulls. It is very hard to beat a long, narrow displacement hull for low drag over a wide speed range.

    Since you're a Senior, you should have learned by now that you need to start a project with a clear statement of the requirements. You have a notion that the boat will be used on a river or lake, implying fresh water and only wind-driven chop for waves. But even chop on a large lake can be a meter high. So you need to define the environmental conditions under which it will operate, and when it will be kept on shore.

    You want it to be human powered, so you need to define how much power is going to be available for how long. You also need to decide what your target speeds will be for cruise and maximum speed. The power available, multiplied by an approximate efficiency factor, divided by the speed will give you a drag budget.

    The displacement has to equal the weight, so you need to define the heaviest crew that will operate the boat, and then look to similar craft to get an idea of how much the boat might weigh. Look at sculls, open water rowboats, sailing catamarans, etc., to get a ballpark figure. If you want a motor, you need to add in an estimate of the system's weight. This will give you a starting point for the total weight.

    Now that you know the allowable weight, the speeds, and the allowable drag, you can start to see what kinds of hulls are appropriate for your application. You can use a program like Michlet to calculate the drag of displacement hulls, but not planing hulls. For planing hulls, you'll need to use Savitsky's method.

    That will get you around the first iteration of the design spiral. Then you'll need to start doing structural design, refining weight and drag estimates, etc.

    If you do use a motor, look at model airplane propellers. They come in a variety of sizes and pitches that may be appropriate for your craft.
  14. jlconger
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    jlconger Junior Member

    Be sure to look at the drive train of the Escapade:

    The hull could probably be improved upon, but it has a very clever and simple means of converting the rotational plane of the pedals to that of the prop shaft.

  15. JSL
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    JSL Senior Member

    Whichever hull form you use make sure it has enough stability to get on board. Some years ago I fellow asked me for an evaluation of his idea of a twin hull pedal boat. One problem was the total volume of one hull would not support a person's weight (with adequate reserve buoyancy) so stepping on one hull to get on board (or off) from a dock (etc) was not possible.
    Back to the drawing board!
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