UNDERSTANDING ABOUT BOAT TAIL AND REBATE BOAT TAIL PROJECTILES

 

UNDERSTANDING ABOUT BOAT TAIL AND REBATE BOAT TAIL PROJECTILES

A.    What is Boat Tail Projectiles?

Boat Tail bullets are a specific type of bullet design commonly used in long-range shooting due to their aerodynamic properties. The term Boat Tail refers to the tapered shape at the rear end of the projectile, which resembles the stern of a boat. This design is intended to reduce drag as the bullet travels through the air, improving its ballistic performance.

  Design and Structure:

·        Shape: The boat tail bullet has a tapered base, unlike flat-base bullets that have a perpendicular rear end. The tapering usually starts near the base of the bullet and gradually narrows toward the very end, forming a sleek, pointed shape.

The angle of the taper can vary, but it typically ranges from 7 to 12 degrees. This tapered shape reduces the amount of turbulence created as the bullet moves through air, leading to more stable and predictable flight.

·        Materials: Typically, boat tail bullets are made from lead or a lead alloy encased in copper or brass jacket. The jacket serves multiple purposes that is it protects the core, prevents excessive fouling in the barrel, and contributes the overall structural integrity of the bullet.

 

C.    Aerodynamic and Ballistic Advantages:

·        Reduced Drag: The tapered rear of the boat tail bullet reduces air resistance (drag) as it flies through the air. This streamlined design minimizes the turbulent wake behind the bullet, leading to a flatter trajectory and less loss of velocity over a distance.

·        Increased Ballistic Co-efficient: The ballistic coefficient is a measure of bullets ability to overcome air resistance. Boat tail bullets generally have a higher BC compared to flat base bullets, means they retain velocity better, resist wind drift more effectively, and deliver more energy on target at long ranges.

·   Improved Stability: The boat tail design helps the bullet maintain stability in flight, reducing the chances of tumbling or yawing, which can reduce accuracy. Well-designed boat tail bullet is less likely to wobble or tumble, ensuring a straight and consistent flight path.

·    Energy Retention: Due to reduced drag, boat tail bullets retain more of their kinetic energy as they travel downrange. This means they deliver more energy on impact, which is particularly important in hunting or tactical application where penetration and stopping power are critical.

·     Pressure Distribution: The tapered shape of the boat tail allows for a smoother transition of airflow from the body of the projectile to the surrounding air, reducing the wake behind the projectile. This reduction in wake leads to decrease in pressure drag.

·   Improved Streamlining: By tapering the rear of the projectile, the boat tail design minimizes the turbulent wake region, allowing the projectile to travel through the air with less resistance.

·   Center of Pressure vs Center of Gravity: The boat tail design shifts the center of pressure (the point where aerodynamic forces are concentrated) rearward, closer to the center of gravity. This alignment reduces the moment arm between these two points, enhancing the stability of the projectile in flight.

Center of Gravity: The center of gravity is the point at which the total weight of the projectiles is considered to be concentrated. It is the balance point where the projectile would balance perfectly if it were suspended.

·        Location: For most projectiles, including boat tail designs, the CG is located towards the front part of the projectile but not at the very tip. In a boat tail projectile, the CG is typically closer to the middle or slightly forward of the midpoint of the projectile length.

·        Role in Stability: The CG affects how the projectiles handle forces during flight. A properly positioned CG helps ensure that the projectile remains stable, reducing yaw and pitching motions. In boat tail projectiles, the CG is designed to be in a position that complements the aerodynamic stability provide by the boat tail.

Center of Pressure: The center of pressure is the point where the total aerodynamic forces acting on the projectile are considered to be concentrated. It is essentially the average location of all the aerodynamic forces such as lift and drag exerted on the projectile.

·        Location: The CP is generally located behind the CG. In boat tail projectile, the CP moves rearward due to the tapered design, which reduces drag and creates a more stable flight path. The boat tail design helps ensure that the CP remains behind the CG during flight.

·        Role in Stability: For stable flight, the CP must be behind the CG. When the CP is behind the CG, aerodynamic forces help to keep the projectile alighted with it flight path. This alignment prevents excessive yaw an ensures that the projectile flies in a straight line.

Stability Dynamics: The distance between the CG and CP affects the stability of the projectile. A larger distance between these points typically means greater stability, as aerodynamic forces exerted at the CP help to correct deviations and maintain a straight trajectory.

Effect of Boat Tail Design: The boat tail design shifts the CP further back compared to a flat base projectile. This rearward shift enhances stability by increasing the distance between the CG and CP, helping to keep the projectile oriented correctly during flight.

·      Yaw Reduction: A more stable projectile experiences less yaw side to side motion, which improves its accuracy and consistency over longer distances.

·      Maintained Velocity: Due to the reduced drag boat tail projectile can maintain a higher velocity over a given distance compared to a flat base projectile. This results in a flatter trajectory, less time of flight, and reduced wind drift.

·      Trajectory: The boat tail design contributes to a fatter trajectory, meaning the projectile drops less over a given distance. This flatter trajectory improves the precision of the shot, particularly at extended ranges.

·     Reduced Wind Drift: With its higher velocity, retention and improved stability, a boat tail projectile is less affected by crosswinds, making it more accurate in varying environmental conditions.

·    Extended Effective Range: The combination of reduced drag, improved stability, and higher velocity retention allows boat tail projectiles to be effective at longer ranges, making them ideal for long range shooting applications.

·        Kinetic Energy: Due to better velocity retention, a boat tail projectile retains more of its kinetic energy over distance, which can translate to better penetration and impact effectiveness at long ranges.

·     Terminal Ballistics: While boat tail projectiles are primarily designed for improved flight characteristics, the retained energy can also contribute to improved terminal performance such as penetration, expansion depending on the projectiles design.

 

D.    How the Boat Tail Projectile Works in its Flight:

When a projectile moves through the air, the air flows over and around its surface. As the air encounters the front of the projectile, it is compressed, creating high pressure region. The air moves along the side and rear, it expands and form a wake behind the projectile. Flow separation occurs when the air can no longer follow the contour of the projectiles surface and detaches, creating a turbulent wake. This separation typically happens more abruptly in projectiles with a blunt or flat base, resulting in increased drag.

The boat tail shape gradually narrows towards the rear, reducing the cross-sectional area of the projectile as the air flows around it. This gradual tapering helps the air maintain contact with the surface for a longer distance, reducing the likelihood of early separation and the smooth conical taper of the boat tail provides a more gradual transition for the air flowing along the surface. This smoother transition allows the airflow to decelerate and adjust to the lower pressure behind the projectile more gradually, rather than abruptly separating and causing a large, turbulent wake.

By tapering the rear of the projectile, the boat tail design helps delay the point at which the airflow separates from the surface. The air stays attached to the surface longer, following the contour of the taper. This reduces the size of the low-pressure wake behind the projectile. When the air flow eventually does separate, the smoother transition results in a smaller and less turbulent wake. This means that the low-pressure area behind the projectile is reduced, leading to less pressure drag.

The primary source of drag in a projectile is pressure drag, which is caused by the difference in pressure between the front and rear end (in front high pressure and rear end low pressure) of the projectile. By reducing the size and intensity of the wake, the boat tail shape tapering reduces the pressure difference, hence low drag. The tapered design ensures that the airflow is better managed, with less energy lost to turbulence and separation. This leads to a more aerodynamically efficient projectile that can maintain higher speeds and stability over longer distance.

 

The above figure shows the air flow separation and base drag reduction, A properly angled boat tail allows the airflow to remain attached to the surface until it reaches the reduced diameter base. As a result, the low-pressure suction will apply to smaller area at the base of the bullet and result in less base drag.

 

The above figure shows the flat base bullet, the air flow simply separates from the bullet body and the low pressure is applied to the entire base of the bullet resulting in maximum base drag.

 

If the boat tail is too steep, the air flow will not remain attached and will separate off the bullet body it is similar to the flat base. No base drag reduction is accomplished with a boat tail that has too steep base or more than about 20 degrees.

 

If the drag coefficient is higher, it means that projectile will experience more resistance as it moves through air.

If the drag coefficient is lower, it means that projectile will experience less resistance as it moves through air.

 

The below figure shows the graphical representation of the Drag Coefficient and the Boat Tail angle.

 

E.     What is Rebated Boat Tail Projectile?

A Rebated boat tail projectile (RBT) is a type of projectile design to used primary in long range shooting and precision ammunition. It is the hybrid of Boat tail and Rebate.

The rear end of the bullet tapers to a smaller diameter, resembling the shape of a boats hull. This design reduces air resistance (drag) and helps the bullet maintain velocity over longer distances, improving its ballistic efficiency.

The rebate refers to a step at the starting the boat tail, the term rebated means that a section of the projectiles base has been reduced in diameter. In RBT projectile, the diameter of the boat tail (the tapered rear portion) is smaller than the diameter of the bullets main cylindrical body, creating a step at the transition between the main body and the boat tail. Rebated boat tail bullets have a number of technical advantages over both flat base and standard boat tail bullet. There is approximately 15% improvement in the group size, on average with identical bullets fired from the same gun, in favor RBT over the standard boattail. This is brought about by the improved gas flow pattern at the muzzle, as the muzzle gas impinges on the angled rebate and is blown off in a different direction depending on the angle of the Rebate.

Bore life is generally longer when shooting RBT base bullets as compared to conventional BT bullets, because of the superior gas seal made by the rebate or step as compared to the angle on the junction of the boat tail with the shank.

 

F.     How the Rebated Boat tail bullet works:

When a bullet moves through the air, the air pressure around it changes. High pressure builds up in front of the bullet, while a low-pressure zone forms behind it. The transition between these high and low-pressure areas can cause the airflow to separate from the bullets surface, creating a turbulent wake. The size and intensity of this wake contribute to the bullets drag, the larger and more turbulent the wake, the greater the drag, which slows the bullet down and can also lead to instability.

The boat tail shape already helps reduce the wake by streamlining the bullets profile, allowing the airflow to stay attached longer and reducing the abruptness to flow separation. This alone reduces the wake size and lowers drag. The rebate refers to the reduction in diameter at the very end of the projectiles base. By making the base diameter smaller than the main body of the bullet, the rebated design creates a less abrupt transition between the bullet and the low pressure wake behind it. This less abrupt transition allows the air to fill in behind the bullet more smoothly, resulting in a smaller, less turbulent wake.

The smaller base area provided by the rebate means there is less surface for the low-pressure zone to pull against. With a smaller base, there is less air disruption as the bullet moves through the air, which minimizes the formation of a large turbulent wake and the step created by the rebated base softens the transition from the bullets body to the open air. This smooth transition reduces the intensity of the low-pressure zone, leading to calmer and more controlled wake.

By reducing the abruptness to flow separation at the bullets base, the rebated design diminishes the turbulence that typically makes up the wake. This means the wake that does form is smaller and less energetic, resulting in lower air drag.

 

As a bullet travels through the air, it moves at high speeds, displacing the air in front of it. The bullet pushes against the air, creating a high-pressure zone at its nose. The air in front of the bullet gets compressed due to the bullets motion, increasing pressure at the font. As the bullet passes through this compressed air, it forces the air to move around its surface. As the air flows along the surface of the bullet, it eventually reaches the rear end, where the bullets shape changes abruptly. For a flat based bullet, the air must suddenly move away from the surface at the rear, causing the airflow to separate from the bullets surface. This separation of airflow is where things start to change in terms of pressure.

 

When the air separates from the bullet at the rear, it leaves behind a void or a region of much lower air density. The air does not immediately rush in to fill this void due to the high speed of the bullet and the inertia of the surrounding air. This creates a lower pressure area, or wake directly behind the bullet. In the wake region, the air expands to fill the space left behind the bullet. However, this expansion happens rapidly and chaotically, leading to a drop in pressure. Essentially, the air molecules in the wake are more spread out compared to the high-pressure region in front of and around the bullet. The different pressure between the high-pressure front and the low-pressure rear creates a pressure gradient. This gradient causes air from the surrounding higher-pressure areas to move toward the low-pressure region as the bullets rear, but it does so in a turbulent and chaotic manner, further sustaining the low-pressure wake.

 

·        Flat Based Bullets: With flat based bullets, the abrupt end creates sharp separation points for the airflow, leading to a larger and more pronounced low-pressure zone behind the bullet. This can result in a large, turbulent wake that generates significant drag.

·        Boat Tail Bullets: The boat tail design helps mitigate this by tapering the rear of the bullet, allowing the airflow to stay attached longer before separating. This reduces the size of the low-pressure zone and the resulting wake.

·        Rebated Boat Tail Bullets: The rebated boat tail projectile design further optimizes this by creating a less abrupt transition from the projectiles body to the wake, further minimizing the low-pressure area and reducing drag. The turbulence is mitigated by tapering the rear of the bullet, and the Rebated reduced diameter at the very base, allowing the airflow to stay attached longer before separating. This reduces the size of the low-pressure zone and the resulting wake.

·        Larger Base: A bigger base at the back of the bullet creates more turbulence and increases drag, slowing the bullet down more.

·        Small Base: A smaller base creates less turbulence and drag, helping the bullet fly faster and more smoothly and smaller base leads to a more efficient and stable flight.

 

G.    Understanding the Abrupt Transitions:

In a flat-based bullet, the rear end has a sharp, 90 % edge where the bullets cylindrical body suddenly ends. As air flows along the bullets surface, it hits this sharp edge and abruptly separates from the bullet. This sudden separation creates a large, turbulent wake with a significant drop in pressure behind the bullet, increasing drag.

 

In a boat tail design, the rear of the bullet is tapered, creating a more gradual transition from the bullets surface to the surrounding air. This taper reduces the angel at which the air must separate from the bullet, leading to a smaller and less turbulent wake.

 

The rebate on an RBT bullet refers to the slight reduction in diameter at the bullets base, where the boat tail tapers down to a smaller diameter than the main body of the bullet. This reduction in diameter creates a step at the base of the bullet.

The rebated base makes the transition between the bullets surface and the wake less abrupt because the air is not forced to detach from the full diameter of the bullets body all at once. Instead, the air gradually detaches as it reaches the reduced diameter of the rebated section, creating a more gradual separation.

By reducing the diameter of the bullets base, the rebate decreases the effective surface area that interacts with the low-pressure wake. This means there is less area for the low-pressure zone to pull against, reducing the overall drag.

The smaller, rebated base allows the air to flow more smoothly into the low-pressure region behind the bullet, reducing the intensity of the turbulence in the wake. This results in a smaller, more controlled wake, which in turn reduces drag.

 

H.    Advantages Compared to traditional Boat Tail:

·        Improved Aerodynamics: The RBT often results in a more streamlined flow of air around the projectile and reducing drag more effectively than traditional boat tails.

·        Reduced Drag: By better managing the air flow and reducing turbulent wake, a rebated boat tail can lower the drag coefficient, leading to improved fuel efficiency and better overall performance.

·        Increased Stability: The rebated design can help maintain stability by minimizing turbulence and ensuring a smoother air flow, which is particularly beneficial for long range projectiles or high-speed vehicles.

·        Enhanced Performance in Specific Conditions: Rebated boat tail can be optimized for different speed ranges or environmental conditions, offering tailored performance improvements over traditional designs.