Create a trebuchet that is able to launch a 1ft boulder

[xXIHAYDOIXx]

I am attempting to create a trebuchet that is able to launch a 1ft boulder (approximately) 100 yds using a trebuchet mechanism.

My current idea is basically to use three 80 lbs bags of cement as the counterwieght and use a sling system at the end of the arm as the launching component.

On to the reason of my post: Do any of you have suggestions that might make this attempt easier? Mind you, this isn't a homwork project. It is slightly related to a tiny trebuchet we are making, but this is, first and foremost, a personal project. Thank you for any and all help provided.

 

[xXIHAYDOIXx]

I realized I was a little too vauge in my original post so I wish to clarify some things. First off, I realized that the cement most likely would not provide the kinetic energy necessary to launch the 1 ft boulder which I estimate to be about 120-140lbs (54kg-64kg). My new plan is to fill two (or more) 55 gallon water drums which will give me about 416 kg of weight. So, after doing some rudimentary calculations to the best of my ability, I found (or think I found) that this is not going to provide enough force for me to reach my intended goal. Can anyone confirm/refute my results? If they are confirmed (that is to say, I need more force) what can I do to increase my force besides increase the counter weight? If I do need to increase the counter weight, how much would be necessary?