Hydrofoils at low speed and high effiency

Discussion in 'Boat Design' started by WouterSU, Sep 23, 2009.

  1. WouterSU
    Joined: Sep 2009
    Posts: 7
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    Location: Holland

    WouterSU Junior Member


    I have been looking at this site for a few days now, but i couldn't find answers to all my questions.

    I have with some other people a solarr powered boat.
    We try to keep the highest efficiency as possible.

    The boat is an trimaran and weights around 300kg including the driver.
    The hull is 6m long and 0.6 m wide. With the floaters it is 2.4m wide.
    The drive is placed 0.75m in front of the backside of the hull.

    Past summer we had an maximum speed around 17km/h and an cruise speed around 12km/h.

    Very important is that our cruise speed must be gained with as less power as possible. So high efficient at the cruise speed (it does not matter that the boat has more drag at around 8 or 9 km/h)

    For next year we will try to increase the weight to 250-275kg and to get our cruise speed at 13 or 13.5 km/h.

    My question is:
    Is there a possibility to increase the drag of our boat at our cruise speed by using hydrofoils? (by creating a lifting force, so we have less drag)

    I have red a few things like:
    Use a V-foil at 4/5 in front of the axis and a T-foil at 1/5 behind the axis
    Use the Clark-Y profile

    But i cant find any case where a boat from 300kg uses hydrofoils.. at a low speed.

    Can anybody help me with this case?

    Greets Wouter
  2. Guest625101138

    Guest625101138 Previous Member

    It is almost certain hydrofoils will be lower drag at your target speed for a boat constrained to 6m length.

    A really well designed foil will achieve a lift to drag around 25 - maybe even 30.

    You can get good data on foils using JavaFoil. You need to set the options to suit water and you need to set a practical aspect ratio.

    As a first pass set the foils three cord lengths below the surface to negate wave drag from the calculation. You have to allow for strut drag.

    There will be challenges keeping the thing in stable flight but first step it to see if it has potential to lower drag.

    Rick W
  3. Guest625101138

    Guest625101138 Previous Member

  4. WouterSU
    Joined: Sep 2009
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Holland

    WouterSU Junior Member

    Rick thanks for you answers.

    I have worked a bit with JavaFoil today.

    But when I think of the amount of foils I don't see how the trimaran will lift.
    When we use 1 foil the boat will flip to one side I think.

    So we need to use more than 1 foil.
    But when we use 1 V-foil in front and T-foils under both floaters there is a possibility that the boat will lean to the left or right. (am I right?)

    And another point..
    In case there is not enough power to lift the hull complete out of the water, is there a possibility that the drag with foils is lower as without foils (by changing the angle of attack)? Or is it better to lift them out of the water?

  5. Guest625101138

    Guest625101138 Previous Member

    Most use some form of active control either automated or manually operated.

    You will need a minimum if two foils. Any foil will be more efficient if it has high aspect. I expect the best main foil would be about 3m wide. It will have some inherent roll stability and you can use dynamic control with steering once up to speed but getting up to speed could be a bit hairy.

    There are a couple of videos of a very low speed foil in operation here:
    You can see how they steer to keep it stable in roll. In pitch it is a matter of getting the foil loading right - more on the front than the stern foil.

    If you do not lift then the foils are just added drag. Typically you need to be right out of the water to get an advantage but there are foil assist that claim lower drag. These usually avoid the strut drag by having the foils extending from the hull. You have to have a design that will work for your intended distance.

    I believe foilers have proven faster in solar races when length is constrained to 6m.


  6. WouterSU
    Joined: Sep 2009
    Posts: 7
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    Location: Holland

    WouterSU Junior Member

    Thanks for the answers.

    I think I need to tell something more about the restrictions we have in the challenges.

    The boat has a height of 70cm. 17.5cm is in the water.
    The height of the saildrive is 45cm (the driveshaft at 40cm)
    With the tips of the propeller we almost reach the mud under water (at 75cm) so thats realy the maximum. (propeller has a diam. 35cm)
    We are also not alowed build wider as 240cm. (so the 3m foil is not an option)
    So I doubt if the hydrofoils are a good option for our boat. .

    For the stabilisation we can't 'just steer' to keep it right. Becouse of small waters (like 3 or 4m minimum).
    I looked at some boats like the Hydroptere, this boat has only 2 diagonal foils at the floaters, and a stern T-foil for steering.
    What I thought was that when the left foil lifts to much, the right foil will move flatter so it creates more lifting force. Is this right and is a solution to get my boat stable?

    Can you tell me a bit more about the "foil assists that claim lower drag" because I can't find real good information about that.

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