I’m an admitted bowhunting tinkerer. I’m always looking to learn more. I will start by saying this not a speed thing. Speed is easy to figure. Less weight equals more speed but I’m thinking in terms of a bowhunting setup rather than a 3D, target setup. By more I mean efficiency from the bow and increased penetration. I talking in terms of the Kinetic Energy (KE) and Momentum (MO) outputs compared to draw weight and arrow weight so I started messing around with my arrow weight. For this test I used an older Bowtech (2006 Justice) set at 64 pounds. I used a variety of tip weights and arrow tubes and weight washers to hit the grain weights exactly. I used a “Chrony” brand Chronograph. I started with an IBO arrow weight of 5 grains of total arrow weight (all components) per pound of bow draw weight. This is the arrow weight that advertised speeds are derived from. I then experimented with increasing arrow weights. The following results were achieved.
Arrow wgt: 5 grains per pound of draw x 64 Lbs = 320 grain arrow.
Arrow speed (5 shot average): 277.5 FPS
KE: 54.5 ft/lbs
Efficiency factor (KE divided by draw wgt): 54.5 divided by 64 = .843 Next I tried 6 grains per pound
Arrow wgt. 6 grains per pound of draw x 64 = 384 grain arrow.
Arrow speed: 256.1 FPS
KE: 55.9 ft/lbs
Efficiency factor (KE divided by draw wgt.): 55.9 divided by 64 = .874 Next I tried 7 grains per pound of draw
Arrow wgt. 7 grains per pound of draw x 64 lbs = 420 grain arrow.
Arrow speed: 238.3 FPS
KE: 56.4 ft/lbs
Efficiency factor (KE divided by draw wgt.): 56.4 divided by 64 = .883Next I tried 8 grains per pound of draw
Arrow wgt. 8 grains per pound of draw x 64 lbs = 512 grain arrow.
Arrow speed: 226 FPS
KE: 58 ft/lbs
Efficiency factor (KE divided by draw wgt.): 58 divided by 64 = .908Next I tried 9 grains per pound of draw
Arrow wgt. 9 grains per pound of draw x 64 lbs = 576 grain arrow.
Arrow speed: 214 FPS
KE: 58.5 ft/lbs
Efficiency factor (KE divided by draw wgt.): 58.5 divided by 64 = .915Next I tried 10 grains per pound of draw
Arrow wgt. 10 grains per pound of draw x 64 lbs = 640 grain arrow.
Arrow speed: 203 FPS
KE: 58.5 ft/lbs
Efficiency factor (KE divided by draw wgt.): 58.5 divided by 64 = .915Next I tried 11 grains per pound of draw
Arrow wgt. 11 x 64 = 704 grain arrow.
Arrow speed: 190 FPS
KE: 56.4 ft/lbs
Efficiency factor (KE divided by draw wgt.): 56.4 divided by 64 = .881
Seeing I crossed the point of diminishing returns in efficiency and KE, I didn’t go any farther.
At the bottom of this you will find a graphical representation of the above data.
Allow me to ponder. Back in 1985 I bought a compound bow and used it for 21 years before buying the BowTech mentioned above. That old bow set at 70 pounds and shot a heavy aluminum arrow with a 145 grain broadhead a blazing 197 feet per second. By today’s standards, such a bow could never kill a deer but with that bow I killed countless deer as well as bear and small game and other critters and I always got a pass thru. I cannot recall a single instance where any game animal ran off with my arrow.
As far as this test goes, as expected, heavier arrows provided more power and efficiency but only to a point. It would appear that the sweet spot that can obtained from this bow comes from using a 9 grain per pound of draw weight arrow even though momentum continued to increase. Regardless of the bow you use, there is an arrow weight that will provide the maximum impact performance and efficiency. That being said, arrow lethality is not any one single factor but is a combination of several factors.
1. Shot placement (no amount of math will make up for a poorly placed shot)
2. Broadhead sharpness and design.
3. Impact force (KE and MO)
Provided you can hit your target, having all 3 will maximize arrow lethality with shot placement trumping all but in the real world of bowhunting, things happen that are beyond our control. There will be times when shot placement is not what we would have hoped for and that’s we rely on broadhead sharpness/design and impact force to bail us out of a less than perfect situation. Since we as bowhunters have control over the efficiency of our bow arrow combination and broadhead selection, we would do well to maximize those aspects of our set-up. Are you getting the most out of your bow/arrow combination? Would a heavier arrow/broadhead combination give you that edge when things don’t go exactly as planned? You no doubt paid top dollar for your hunting rig so why not get the most from your investment by squeezing every ounce of killing power out of your investment? You bought your broadheads based on a combination of hunter recommendation, hunting shows and magazine ads. You bought your arrows probably based on name and what the spine charts told you to use but neither of these are customized to you and your bow to perform at optimum bowhunting conditions.
None of this will come as a shock to traditional bowhunters using a stick and string. They have long known the advantages of high arrow weight to draw weight for penetration but the modern compound bow market is stuck in the speed fad and all advertising seems to be based on feet per second as if that were the end all and be all of bow performance. Speed is only one part of the equation. But the current fad is speed and from a bowhunters perspective we consider speed when thinking in terms of things like yardage estimation but rangefinders have all but negated that. Another consideration for arrow speed is the effect of speed in regards to a deer’s ability to jump the string but please consider the following. If your bow provides the maximum in performance with a heaver arrow at 250 FPS vs 300 FPS you are still better off with a more efficient performing bow/arrow because a few FPS means nothing to a deer’s ears since sound travels from your bow to a deer at 1,126 feet per second. A few feet per second of bow speed pales in comparison to the speed of sound. Even with the fastest bows on the market at 360 FPS, the sound of your intentions will reach the deer more than 3 times faster than your arrow will.
Having bowhunted for 35 years and knowing many many bowhunters I can say with great confidence that bowhunters are some of the cheapest hunters on the planet. We are always trying to do the most by spending the least but is saving a few bucks robbing you of a few big bucks or does or antelope or bear? Arrows are not cheap but weight tubes and broadhead weight collars are so you have the ability to maximize (or at least improve) your bow’s performance without breaking the bank. We tend to spend a boat load of money on a bow but scrimp on the things that kill game (arrows and broadheads) Perhaps after spending all that money on the broadhead delivery system, there is little left for the one piece of gear that actually kills the deer. After experimenting with the performance of your bow you have to ask yourself some questions. Is the 100 grain broadhead market where you should be shopping? Will your rig perform better with a 125 or even 220 grain head? Are 7.8 grain per inch arrows better penetration or would 9 grains per inch perform better? The off season is long but it gives bowhunters the opportunity to refine and experiment to put the odds in their favor.
So how does the hunting archer test and evaluate and experiment to get increased performance for his bow? You will need a chronograph. If you don’t own one, perhaps your local archery club does (another good reason to be affiliated with a local club). You can also visit a local archery dealership and for a few dollars and with an assortment of field tip weights, weight washers, weight tubes, weed trimmer line and even masking tape to get you to the weight you want, you can find out the sweet spot that will give you better performance from your bowhunting gear. Perhaps you can maximize performance with a few weight collars and/or a weight tube inside your arrow. Using weed trimmer line to boost your arrow weight which reminds me of a story. I was contacted by a whitetail bowhunting friend who planned to go out west on an American bison hunt and he was looking to optimize the output from his 70 pound compound. He knew his 375 grain whitetail arrow was not going to cut it for such a big animal. After a little experimenting we ended up with brass inserts, a 200 grain broadhead and 4 lengths of weed trimmer line inside his arrow which brought him to a hefty 700 grain arrow (10 grains per pound of draw). The performance and penetrating ability of his arrow ended the life of a 1,700 Bison at 35 yards. His arrow passed through both sides of the chest of the roughly 3 foot wide beast. This might be a bit extreme for thin skinned whitetails but hopefully you get my point. Maximizing a bows output puts the odds in your favor for making a quick, clean, good penetrating and ethical kill regardless of the size of your prey. Personally I find it objectionable for a game animal to run off carrying an arrow not to mention the increased difficulty in blood trailing a deer with no exit hole. If that happens, there was a breakdown in at least one of the three points of arrow lethality mentioned above.
If speed is your thing, the answer is easy, shoot an IBO weight arrow weight but I caution using an arrow lighter than that since less than 5 grains per pound of draw risks voiding your bow’s warranty so be aware that too light an arrow comes with some risks. There were other things I noticed as I increased arrow weight. Bow noise and vibration felt in the hand dropped as arrow weight increased. Both are a good thing for bowhunters. Also, there was a noticeable increase in penetration into my target as I reached 11 grains per pound of draw due to the ever increasing momentum. This pondering turned out much lengthier than I originally anticipated, thanks for listening. I hope you were able to pull something of value from it.