a question on momentum

[kal]

So I have been doing allot of testing with some chronographs and a variety of arrow weights. The results are fun to look at and compare but I am having a little trouble. When it comes to energy I have always looked to kenetic energy as a primary measurement. The more I have researched the more I am paying attention to the momentum side. The question I have is what it's a good number to have? I know that for deer you want to be over 40-50 in the ke and I know 80ke it's a lot of energy. That being said when I see .53 momentum or .68 momentum I don't really know how much power that is. I really hope this makes some sense so someone could give me some examples.

 

[BRICKBURN]

Where did you pull the .68 momentum numbers from?

 

[kal]

Brickburn I use the exact number .68 is just an example. To get the numbers I used an online calculator that was posted on another post. It seemed right on with ke when I compared it to the standard methods. I have also seen momentum written as 68.2 for example.

 

[Norwegianwoods]

Momentum alone is not enough. You need to look at the KE, arrow weight or speed too.
But the momentum is by far the most important when it comes to penetration.
The problem with using momentum alone, is that it says nothing about the speed of the arrow. The arrow can be extremely heavy and go very very slow and you will have a very high momentum.

When it comes to the numbers.
A good minimum for Cape Buffalo is 80 KE from a 750 grain arrow. This you will get if the arrow is launched at 219.4 fps and that setup will give you 0.73 momentum.
Personally I shoot a set up with a 615 grain arrow that goes 260 fps. This gives me 92 KE and 0.71 momentum.
I am not sure the 0.02 and a bit in momentum difference matters much, but I know I will get about the same KE(slightly higher) and more momentum(0.78-0.79) if I use a 750 grain arrow from the same bow.
My next bow will be with 80 lbs limbs and with the bow I have in mind and my draw length I will launch a 750 grain arrow at about 260 fps. That will give me 0.86 momentum and 112 KE. 18% more momentum than the minimum for Cape Buffalo.

Some think 0.4 momentum is enough for Whitetails. I think it is in most situations, but I am a person that prefers to be on the safe side and prepare for the not so perfect situations. I think that 0.4 momentum limits you to much. Both in choice of broad heads, but also when it comes to shooting angles and distances.
I would have 0.5 or more in momentum for Whitetails.

But let us say that 0.4 momentum is a sound absolute minimum for Whitetails and 0.73 momentum is the absolute minimum for Cape buffalo.
Both demanding a high quality super sharp fixed broad heads. With a double blade single bevel for the buff.

When you just look at the numbers, an increase from 0.40 to 0.45 momentum doesn't look like much, but it is in fact more than a 10% increase and can make a big difference in penetration.

 

[kal]

But let us say that 0.4 momentum is a sound absolute minimum for Whitetails and 0.73 momentum is the absolute minimum for Cape .

Thanks norwegianwoods, this was the kind of comparison I was looking for.

 

[Primo661]

If you're so inclined to read up on the science and experimental results on such things, you might want to take a look at the work done by Dr Ashby posted here. Looking at it from an engineers point of view that physics and mechanics of arrow penetration relies on more than momentum or kinetic energy. Sectional density and impulse matter a great deal and as Dr Ashbys research implies sometimes arrow mass trumps all others for reliable penetration on bone. On his study on penetration on buffalo he found that, independent on the type of bow it was fired from and arrow velocity counted for much less than you might think. At some point which was found to be arrows of a mass of below 750grains produced a more or less linear relationship between penetration and arrow mass. At 750 grains and above, there was a non linear jump in penetration(through buffalo rib bone if memory serves me correctly).

Of course, few will be hunting buffalo with a bow but we can all learn from this. If we want reliable penetration through bone, chasing kinetic energy will not necessarily be the best tactic. For my bow atleast, I get 87ft/lbs of kinetic energy from an arrow of between 450 and 500 grains and only 66ft/lbs from an arrow of the 750 grain minimum arrow mass suggested by Dr Ashby for use on buffalo. This is because energy is proportional to the mass of the arrow but also the square of the velocity. This means that velocity has a greater effect on energy than arrow mass. Since arrow mass has been found to be the greatest factor in penetration through bone, kinetic energy alone is not a reliable predictor of penetration.