propeller selection

Discussion in 'Jet Drives' started by gharai, Dec 22, 2007.

  1. gharai
    Joined: Dec 2007
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: asia

    gharai New Member

    hi every one
    i'm a new member and i've engaged in a competition.i want to build a boat whose length is about 1.2 meters and whose width is about 25 centimeters and i have 5 lithium polymer 3.7 v each of 500mAh batteries what kind of propeller should i use for the boat to go as fast as possible?
    how can i have some graphs of comparison of different parameters of propeller?
     
  2. Guest625101138

    Guest625101138 Previous Member

    You need a bit more information.

    You need the weight estimate for the entire boat with batteries and propulsion. Lets say it is 10kg. You can make a first order approximation for drag of 1/8th of total weight if it does not use hydofoils -
    Drag = 10 * 9.8 / 8 = 12.3N.
    A flat bottom will be faster than a "V" hull but not as easy to turn. A stepped bottom could be even faster than flat.

    You need to decide how long you want to operate for or the peak power output of the batteries. Lets say peak current is 20A so peak battery power is:
    P = 5 * 20 * 3.7 = 370W
    Lets the little motor gives 80% efficiency so motor output is 300W.

    The batteries would only last about 1 minute but you did say as fast as you could.

    You now go to JavaProp:
    http://colaco.freeshell.org/mhepperle/javaprop/jp_applet.htm
    You set option for water, set foil to E193 at Re# of 300,000.

    You now kep iterating untill you get something sensible in terms of the RPM and blade shape. If it is too narrow it will flex or break. You also want to have the prop shaft horizontal. There are ways to do this without a gearbox underwater but it means you have to limit the prop diameter to a realistic size.

    I have attached a screen dump from JavaProp for the estimates above. A speed os 21m/s would mean you have to make allowance for windage on the hull and some allowance for the drag on the prop shaft.

    Rick W.
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.