Ask the Editor- Downrange Arrow Energy
Posted on April 23, 2013
Hi Joe. Great mag! I have four different bowhunting magazine subscriptions, and I honestly feel that I learn more reading yours than I do in the other three combined. Here’s my question: I am in the market for new arrows. I’m looking at Easton Axis 340s and Easton Flatline 340s. Here are the respective performance numbers: Axis 428-grain arrow at 279 fps=74 foot-pounds/kinetic energy; Flatline 381-grain arrow at 305 fps=73 foot-pounds/kinetic energy. These specs are straight out of the bow. Arrows are cut at 29.5 inches and shot from a Hoyt Maxxis 35 with 30-inch draw at 65 pounds.
Can you tell me how much kinetic energy each arrow has at 20, 30, 40, 50, 60, and 70 yards without shooting through a chronograph at each distance? I hunt turkey, deer and bear in Massachusetts but plan on elk hunting soon out West. I don’t want to have to change my setup. Thank you.
J.N. via e-mail
A: Thanks for the compliments and for writing in. Refer to this online calculator and find out your arrow’s energy values: http://peteward.com/ballistic.calc.htm.
Also, here’s a note about momentum from Darin Cooper, Hoyt’s ex-senior engineer. It is an excerpt from my book (Technical Bowhunting). It’s something to consider when measuring the advantages and disadvantages of arrow weight. Hope it helps.––Joe Bell
“I definitely think archers shooting less than 400 grains on an elk or even mule deer and northern whitetails are asking for trouble regardless of arrow velocity,” says Cooper. “I’ve done some engineering calculations that indicate how critical mass is to maximizing penetration. My calculations show that 50 grains of arrow weight is equivalent to shooting a 10-pound heavier bow… In other words, a 500-grain arrow shot from a 60-pound bow will achieve roughly the same penetration as a 70-pound bow with a 450-grain arrow. The key is momentum and not kinetic energy.”
“We got handed down kinetic energy as a measurement from the firearms industry––they’re concerned about knock-down power, kinetic shockwaves, and bullet expansion for a projectile that’s designed to stop in the animal,” says Cooper. Hydraulic shock and energy transfer is highly velocity dependent. “Our killing efficiency is determined by how far the object will penetrate and cut tissue. The laws of physics dictate that when comparing two identically shaped objects in motion with equivalent momentum, one being heavier and the other faster, the heavier object will always travel farther (out-penetrate) because it has derived more of its momentum from mass.” Momentum = (mass x velocity) ÷ 225,218. The units of momentum are (pounds-force x second)