propeller selection

[gharai]

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?

[Guest625101138]

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/mhepperl.../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.