Watching someone draw a 45-pound recurve against a 900-pound moose on subarctic terrain brings physics into brutal focus. When you drop high-energy compound cams, 80-pound draw weights, and flat-shooting light shafts from the equation, you eliminate your margin for mechanical error. On survival expeditions like the History Channel's Alone, or during deep wilderness pack-ins, hunters are limited to traditional gear pulled by cold, calorically depleted muscles.
Under these constraints, the standard marketing metrics pushed by modern bow shops fall apart. Achieving ethical, lethal pass-throughs on heavy-boned animals with a stickbow requires strict adherence to terminal ballistics: momentum, structural integrity, mechanical advantage, and rotational torque.
Here is how the physics of arrow penetration actually works when your power stroke is small and the game is large.
The Kinetic Energy Fallacy vs. Momentum
For decades, archery manufacturers marketed kinetic energy ($KE$) as the ultimate benchmark of killing power. This made commercial sense; modern compounds launch ultralight carbon shafts at extreme velocities, and because velocity is squared in the kinetic energy equation ($KE = \frac{1}{2}mv^2$), speed inflates the number.
Kinetic energy measures the capacity to do work, but in terminal ballistics, it primarily dictates how violently an arrow expends energy upon initial impact. High-velocity, low-mass arrows dump their energy immediately on the surface, fracturing broadhead blades, flexing shafts erratically, and stalling out against heavy hide and muscle.
Momentum ($p = mv$), on the other hand, measures an object's resistance to stopping once set in motion. Unlike kinetic energy, mass and velocity scale linearly in the momentum equation. In fluid dynamics and tissue penetration, momentum determines how far a projectile travels through resistance before friction and drag sap its forward travel.
When shooting a 45- to 50-pound traditional bow, you cannot generate high velocity. A 500-grain arrow might leave the shelf at an honest 155 to 165 feet per second. Because you cannot drastically elevate $v$, the only variable you can scale to preserve penetration momentum is mass ($m$).
- Kinetic energy determines how loud the impact sounds and how hard the arrow hits the surface.
- Momentum dictates whether that arrow slides through three feet of dense vitals or stops halfway through the offside rib cage.
The Ashby Threshold and Total Arrow Mass
Much of our modern understanding of heavy-arrow mechanics stems from the extensive field research conducted by Dr. Ed Ashby for the Natal Parks Board. Ashby dissected hundreds of terminal impacts on Cape buffalo, giraffe, and heavy African plains game, documenting what stopped arrows and what pushed them through.
His primary finding remains incontrovertible: total arrow system weight must be balanced against the game species, but for large-boned game, ultra-heavy arrows drastically reduce structural failures. Ashby identified a critical baseline known as the heavy arrow threshold, generally set at 650 grains or more.
For traditional bowhunters pulling moderate poundages, building a 600- to 700-grain arrow requires running a high grains-per-pound (GPP) ratio. Instead of the standard 5 or 6 GPP common in the compound world, traditional setups hunting big game operate best between 10 and 14 GPP.
Running high GPP yields two distinct physical advantages:
- Elevated Bow Efficiency: Traditional bows convert potential energy into kinetic energy far more efficiently with heavier arrows. A recurve shooting a 350-grain arrow might run at 60% efficiency, with the remaining 40% absorbed into hand shock and limb vibration. Up that mass to 650 grains, and efficiency jumps toward 75–80%. The bow shoots quieter, feels smoother, and transfers more available energy into the shaft.
- In-Flight Inertia: Heavy arrows drift less in crosswinds, shedding less momentum downrange compared to light, high-surface-area arrows.
Mechanical Advantage and Broadhead Geometry
An arrow does not kill through hydraulic shock or cavitation like a high-velocity rifle bullet; it kills strictly through hemorrhage and tissue destruction. Friction is the enemy of penetration. Broadhead geometry governs how cleanly an arrow slips through tissue without bleeding off momentum.
Mechanical broadheads and wide-cutting three-blade heads demand massive amounts of kinetic energy to deploy and push through hide. On a 45-pound longbow, deploying a mechanical head on a moose is an immediate failure of woodcraft.
The gold standard for low-energy penetration is a cut-on-contact, two-blade fixed broadhead featuring a high mechanical advantage ratio.
The Math of Mechanical Advantage
Mechanical advantage (MA) in a broadhead is calculated by dividing the length of the cutting blade by the total width of the cut.
- A broadhead that is 3 inches long and 1 inch wide has an MA of 3.0.
- A short, stubby head that is 1.25 inches long and 1.25 inches wide has an MA of 1.0.
A broadhead with an MA above 2.5 acts as a sharp, progressive wedge. It parts tissue slowly and requires minimal energy to displace muscle and organs as it moves forward. Steep-angled heads push tissue out of the way violently, transferring resistance straight back down the arrow shaft and bleeding momentum instantly.
Single-Bevel Physics and Bone Clearance
Ribs are designed to protect the thoracic cavity of an ungulate, and on an adult elk or moose, those ribs are thick, hard, and overlapping depending on the animal's quartering angle. If your arrow strikes bone dead-center, penetration often ends right there unless the head can split it.
This is where blade bevel geometry becomes decisive.
Double-Bevel vs. Single-Bevel
A conventional double-bevel blade is ground on both sides of the cutting edge. When it strikes heavy bone, it cuts straight in until the bone pinches the blade and stops the arrow's forward progress, often wedging itself firmly in place.
A single-bevel broadhead is ground flat on one face with an aggressive angle (typically 25 to 30 degrees) ground on the reverse. When a single-bevel head encounters resistance in tissue or bone, the unequal pressure surfaces force the head—and the arrow behind it—to rotate.
This rotation creates significant mechanical torque:
- The head begins to cut on a helical path.
- As the flat face and angled face push through bone, they induce high-pressure lateral shear forces.
- Instead of jamming, the broadhead acts as an aggressive splitting wedge, fracturing the bone along its structural grain lines.
- Once the bone splits open, the broadhead and arrow clear the obstacle with minimal loss of structural momentum, continuing forward into the pleural cavity.
For a low-poundage archer, a single-bevel head built from tool-grade steel (like A2, S30V, or CPM-3V) hardened to 58–60 Rockwell is not an aesthetic luxury; it is mechanical insurance against a terminal shoulder or rib strike.
Forward of Center (FOC) Dynamics
Arrow penetration is not merely about head design and overall weight; it is also determined by how that mass is distributed along the shaft. Forward of Center (FOC) refers to the percentage of total arrow weight located in the front half of the projectile.
A standard hunting arrow sits between 8% and 12% FOC. Extreme Forward of Center (EFOC) setups sit between 19% and 30%. Building an EFOC arrow involves using heavy steel inserts, brass weights, and dense broadheads paired with lightweight, high-strength carbon shafts.
EFOC aids low-energy setups through three critical mechanisms:
- Flex Recovery: Every arrow paradoxes—flexing violently left to right—upon release from a traditional bow. An arrow with high front-end mass stabilizes and dampens this flex far faster in flight. A shaft that reaches stable, straight flight quickly enters game with its full kinetic profile aligned behind the point of impact.
- Tracking and Leverage: When an arrow impacts an animal at a non-perpendicular angle, the shaft wants to kick sideways, levering against the entry hole and scrubbing off momentum. A heavy point pulls the back of the arrow through the wound channel behind it, maintaining an uninterrupted line of force.
- Reduced Shaft Drag: If the broadhead point leads with sufficient mass and creates a hole slightly larger than the shaft diameter, an arrow with high FOC tracks cleanly down the center of the wound channel without slapping or rubbing against the channel walls.
System Tuning and Practical Application
You cannot simply glue 300 grains of steel to the end of an arbitrary carbon shaft and expect it to fly. Adding weight to the front severely weakens the dynamic spine of the arrow. If the arrow is improperly spined, it will leave the bow crooked, fly sideways, hit at an angle, and shatter or bounce.
To apply these principles in the field:
- Bare-Shaft Tune First: Strip your fletchings and shoot at 15 to 20 yards. The bare shaft must enter the target dead straight. If it kicks nock-left or nock-right, adjust your point weight, strike plate thickness, or shaft spine until entry is perfectly perpendicular. Fletchings stabilize an arrow, but they should never be used as a band-aid to mask an out-of-tune system.
- Match Shaft to Broadhead Ferrule: Ensure the outside diameter of the broadhead ferrule is slightly larger than the outside diameter of the arrow shaft. If the shaft is wider than the head, the carbon will catch on the edges of the cut bone and tissue, instantly stopping forward travel.
- Hone to Hair-Popping Sharpness: A dull blade, regardless of its mechanical advantage, pushes blood vessels and elastic lung tissue out of the way rather than severing them. Finish your edges down to a mirror polish with ceramic stones and leather strops.
Bottom Line
When survival or an ethical harvest hangs on a single shot from a traditional bow, you cannot rely on velocity to compensate for poor gear choices. Speed is fragile; mass is resilient.
By building an arrow system focused on structural integrity, a total weight over 600 grains, high FOC, and a sharp single-bevel broadhead with a high mechanical advantage, you maximize momentum and minimize tissue resistance. That is how low-energy tackle routinely passes clean through the largest game on the continent.