Breastshot Wheel
Part of Hydro Generator
A water wheel design where water enters at mid-height, combining gravity and impulse forces for efficient operation on medium-head streams.
Why This Matters
The breastshot wheel occupies the middle ground between the simple but inefficient undershot wheel and the mechanically demanding overshot wheel. For rivers and streams with 4 to 10 feet of available head, the breastshot wheel is often the most practical choice — it’s more efficient than undershot (typically 50-70% versus 25-35% for undershot), requires less elaborate water management infrastructure than overshot, and can be designed to pass debris and flood flows without damage.
Breastshot wheels were the most common wheel type in English and American mills of the 17th through 19th centuries. Many mill sites with moderate head used them precisely because they balanced efficiency, constructability, and flood tolerance. Understanding the breastshot design gives you access to a proven technology for medium-gradient streams.
The key design challenge is the breast — a curved masonry or wooden wall that follows the wheel’s circumference closely from the point of water entry down to the tailrace. This breast captures the water’s weight (gravity component) and prevents it from escaping before it has done work. Getting the breast shape right is the most critical and most difficult aspect of breastshot wheel construction.
Operating Principle
Water enters the wheel at approximately the axle height (slightly above or below, classified as high breastshot vs. low breastshot). The buckets fill with water that then rides the wheel downward, its weight pressing on the bucket and turning the wheel. At the bottom, the breast ends and the water spills into the tailrace.
High breastshot (water entry above axle): More efficient because more of the potential energy (head) is captured as gravity work. Requires a more precise breast wall to contain water from entry point to discharge. Efficiency 60-75%.
Low breastshot (water entry just below axle): Simpler breast construction, tolerates debris better. Efficiency 50-65%. Also called a pitch-back wheel if water enters from behind the wheel (less common, used where space constraints dictate).
The efficiency advantage over undershot comes from using the weight of the water in the buckets (gravitational potential energy) rather than just the kinetic energy of the flowing water. The water effectively “weighs on” the buckets through a longer arc, doing more work per unit volume.
Wheel Design
Bucket shape: Unlike overshot wheels where buckets must hold water until they’re near the bottom, breastshot buckets fill at entry and ride down to discharge. Bucket shape is less critical — straight radial buckets, curved buckets, or even simple flat paddles (though flat paddles waste more water by splashing). The ideal breastshot bucket is slightly curved backward from the direction of rotation, allowing water to enter smoothly without splashing.
Wheel diameter: Typically 10-20 feet for mill applications. Larger diameter means slower rotational speed (which requires more gear ratio to reach generator speed) but also higher torque and better efficiency. Diameter should be approximately 1.1 to 1.3 times the available head to allow the buckets to fill at entry and drain before they reach the tailrace.
Wheel width (face width): Wider wheels pass more water and generate more power. Determine face width from the available flow rate and target power. Width = Flow Rate ÷ (Bucket Depth × Peripheral Speed × Fill Factor). Typical widths range from 2 to 8 feet for small hydro installations.
Number of buckets: More buckets smooth the power output. Too few buckets leave gaps in water coverage; too many are harder to build. Typical spacing: 12-24 inches of arc between buckets, giving 16-40 buckets depending on diameter. Historical mills typically used 24-36 buckets.
Axle and shaft construction: The axle must support the full weight of the wheel plus water loading, running in two radial bearings (one on each side of the wheel). Traditional wooden axles used a 12-16 inch square timber, iron-reinforced at the bearing journals. Cast iron or wrought iron axles are more durable. Modern salvage approach: use a solid steel shaft of adequate diameter — minimum 4-6 inches diameter for a 15-foot wheel.
The Breast Wall
The breast is a curved wall closely following the wheel’s circumference from the entry point to just before the tailrace. It serves several functions:
- Captures water that would otherwise splash or fall out of the buckets before completing work
- Creates a seal that forces water to stay in the buckets and push on them
- Channels the water to the tailrace efficiently
Clearance: The gap between wheel rim and breast should be about 1/2 to 1 inch — enough to prevent rubbing if the wheel deflects slightly, tight enough to minimize water leakage. Too large a gap wastes water flowing behind the buckets without doing work.
Breast materials: Traditional mills used cut stone set in lime mortar, shaped to a circular arc. This is durable and effective but labor-intensive. Alternatives: heavy timber planking (seal with tar or oil), rammed earth and stone faced with planks, or salvaged corrugated metal. Whatever the material, the surface should be smooth (not obstructing wheel rotation) and water-resistant.
Sluice gate integration: The water supply to the wheel is controlled by a sluice gate (penstock gate) at the top of the breast. This gate allows the miller to start, stop, and regulate the flow to the wheel. A simple sliding or pivoting wooden gate that seals against a wooden frame is adequate; a screw-driven gate allows fine flow control.
Managing Flow and Head
The available head is the vertical distance from the water surface above the wheel entry to the water surface below the wheel (tailrace). Measure this carefully — even a few inches of difference between calculated and actual head significantly affects power output.
Power formula: P = ρ × g × Q × H × η
Where P = power in watts, ρ = water density (1000 kg/m³), g = 9.81 m/s², Q = flow rate in m³/s, H = effective head in meters, η = efficiency (0.50-0.70 for breastshot).
A 6-foot head (1.83m) stream with 3 cubic feet per second (0.085 m³/s) flow through a 60% efficient breastshot wheel: P = 1000 × 9.81 × 0.085 × 1.83 × 0.60 = 916 watts ≈ 0.92 kW
This is enough to power 10-15 LED lights, charge a battery bank, or run a small power tool.
Tailrace management: The water must exit the wheel freely. If the tailrace floods up and the water level rises above the wheel bottom, the wheel partially runs in water — this is called “drowning” and dramatically reduces efficiency. Keep the tailrace channel clear and sized for at least twice the design flow.