Carl Scholer wrote: |
Thank you for the thoughtful reply. It helps explain how earlier, low draw weight, composite crossbows were competitive against other bows.
I think your numbers are a little off though with regards to the velocities and weights of the bolts involved in your tests. In this test you use a 175 lb steel prod crossbow to propel a 467 grain bolt to a stated velocity of 181fps. http://www.uccrossbows.com/2006testingplate.html In the test you just mentioned you used a 450lbs steel prod crossbow to propel a 445 grain bolt to 190 fps. To sum up your 175lbs bow is putting out bolts with 46 joules of KE and your 450lbs bow is putting out bolts with 48 joules of KE. If these numbers are accurate then you are getting practically nothing extra out of your 450lbs bow compared to your 175lbs bow. If this is really what is happening then you might try experimenting with heavier bolts to see if you are getting all the power you can out of your heavier crossbows. |
I tend to agree here just because of the math: The only thing that make a difference for momentum is mass times velocity, or Momentum= Mass X Velocity. We could complicate things making the comparison to Kinetic energy instead which is Ke=MV2 or mass X velocity squared. Just high school physics here and believe me past basic algebra I' m lost.
At these low velocities, and very close velocities, Ke won't be an important factor compared to Momentum differences.
A bolt at 2X velocity will have 4X the Ke but only 2X the Momentum at equal weight of bolt.
A bolt 2X the mass and the same velocity as a lighter bolt will have 2X the Momentum also, BUT only 2X the Ke.
So high Ke numbers of light projectiles at much higher velocity give impressive Ke numbers but can have very low Momentum numbers. With high velocity bullets, high kinetic energy can show an explosive effect and make very and fast bullets effective. Also a lot of Kinetic energy is transformed to heat on impact as opposed to momentum that has more effect on penetration. ( The old fast / slow bullet argument read a million times in Gun magazines i.e. .45 versus 9mm )
The only way for a bolt of equal weight to show dramatically more effect on target, the velocity has to be substantially greater. Or, at the same velocity you are shooting a much heavier bolt. The draw weight only makes a difference if the projectile ends up going faster or transfers more Ke or Momentum to the bolt.
In real life a small loss of velocity can be more than compensated by a big increase in mass of projectile.
Two projectiles can have identical Ke but very different Momentum: A very light projectile at a very high velocity with the same Ke as a heavy much slower projectile, the light projectile can ( Will ? ) have much less Momentum.
So, I don't dispute the effects your are seeing on target but if the bolts are very close in weight and the velocity is also close, there should be no observed difference on target !? So, all I can see here is that one of your measurements is off somehow ???