A Water Wheel (form 6) is a vertically-mounted gravity wheel with shaped buckets around its rim that captures water delivered from above and converts the falling weight into rotary shaft torque. Water enters near top-dead-centre, fills each bucket, and the unbalanced mass column on the descending side rotates the wheel until the buckets discharge near the bottom. Mills, tanneries, and small hydro sites use this form to harvest steady low-head streams — a 4 m diameter wheel running 8 RPM at 60 L/s reliably delivers 2-3 kW of usable shaft power.
Water Wheel Form 6 Interactive Calculator
Vary wheel diameter, flow rate, and RPM to see estimated gravity-wheel power, torque, rim force, and tip speed.
Equation Used
The calculator uses the gravity water-wheel estimate P = rho g Q H, with effective head H approximated by wheel diameter D. Shaft torque is then T = P / omega, where omega = 2 pi rpm / 60.
- Water density is 1000 kg/m3.
- Effective head is approximated by wheel diameter.
- Ideal gross gravity conversion is used to match the worked example power range.
- Friction, leakage, splash loss, and tailrace drag are not included.
The Water Wheel (form 6) in Action
Form 6 is a gravity wheel — almost all of its torque comes from the weight of water sitting in the buckets, not from the velocity of the water hitting the rim. You feed it through a launder (a wooden or steel chute) that drops water into the buckets just past top-dead-centre. Each bucket fills to roughly 60-70% of its theoretical volume, hangs heavy on the descending side, and discharges as the bucket inverts past bottom-dead-centre. The shaft turns slowly — 4 to 12 RPM is typical — but with serious torque, which is exactly what an old mill needs to drive stones, hammers, or tanning drums through a simple gear train.
The geometry matters more than people expect. The bucket entry angle, the depth of the shroud, and the launder's exit position together decide how much water actually stays in the bucket on the way down. Get the launder too far past TDC and water shoots straight over the bucket lip and falls free — pure energy loss. Get it too close to TDC and the bucket hasn't opened to the launder yet, so water spills onto the rim and runs down the outside. The sweet spot is usually 8-15° past TDC for a wheel of 3-5 m diameter, and the launder lip should sit 25-50 mm above the bucket lip at the moment of fill.
If tolerances drift you see it immediately. A wheel with worn shroud-to-rim seals leaks 15-20% of its bucket volume before bottom-dead-centre and loses torque proportionally. Bearings running dry will stall the wheel under full load even when the hydraulic input is correct — we have seen restored mills where the owner blamed the millrace and the real problem was a glazed bronze bushing. Backwatering, where tailrace level rises above the bottom of the wheel, is the other classic failure: the wheel literally drags through standing water and efficiency falls off a cliff.
Key Components
- Shaft (Axle): The central rotating member, typically 150-300 mm diameter for wheels in the 3-5 m range. Cast iron or forged steel for restoration work, with journals running in bronze or lignum vitae bushings. Shaft deflection must stay under L/1000 across the span or the wheel face wobbles and shroud seals open up.
- Spokes (Arms): Connect hub to rim and carry the unbalanced bucket load. Traditional designs use 6-8 wooden arms; modern restorations often use steel angle or square tube. Each arm carries roughly the weight of one full bucket plus dynamic load — for a 4 m wheel that's 80-150 kg per arm at peak.
- Rim (Felloes): The outer ring that the buckets bolt to. Built up from segments — 6, 8, or 12 felloes per ring depending on diameter. Joints must be staggered between the two rim rings or the wheel develops a flat spot under cyclic load within a few thousand revolutions.
- Buckets (Floats): Curved compartments around the rim that hold the falling water. Bucket pitch (spacing) is normally 250-400 mm on the rim circumference. Bucket depth runs 200-300 mm radially. The leading face is curved or angled 15-25° to scoop and retain water rather than splash it out.
- Shrouds: The two side disks that close off the buckets at the wheel faces. Critical for retaining water — even a 5 mm gap between shroud and rim leaks enough volume to drop wheel power 10-15%. Hardwood or steel plate.
- Launder (Penstock Chute): Delivers controlled flow from the headrace to the buckets. Outlet positioned 8-15° past TDC, with a sluice gate to throttle flow. Cross-section sized so exit velocity matches rim velocity within ±15%, otherwise water either splashes back or undershoots the buckets.
- Tailrace: The channel that carries discharge water away from the bottom of the wheel. Must sit at least 100-200 mm below bucket discharge point to prevent backwatering. A blocked or silted tailrace is the single most common cause of lost power on restored mills.
Where the Water Wheel (form 6) Is Used
Form 6 wheels turn up wherever a site has steady head between 2 m and 8 m and a flow that rules out turbines as too expensive or too fussy. The wheel itself is forgiving — debris, leaves, even small fish pass through without damage. That's why it survived as the workhorse of pre-industrial Europe and why it keeps showing up in modern heritage restorations and off-grid micro-hydro builds. Practitioners pick it over an undershot or a Pelton turbine when the flow is moderate, the head is real but modest, and reliability matters more than peak efficiency.
- Heritage Milling: Daniels Mill in Shropshire runs a 11.5 m overshot — one of the largest working wheels in England — driving stone-ground flour production for the on-site bakery.
- Craft Distilling: Annandale Distillery in Scotland uses a restored water wheel as a working historical feature alongside its production line, pumping cooling water during peak distillation.
- Off-Grid Power: PowerSpout and Hugh Piggott-style micro-hydro builds use compact 1.5-2.5 m form 6 wheels driving permanent-magnet alternators for cabin systems in the 500-1500 W range.
- Tannery Operations: The restored Healeys Cyder Farm in Cornwall and several Spanish leather workshops in La Rioja use water wheels to drive tanning drums at 6-10 RPM directly, no gearbox needed.
- Sawmilling: The Hopewell Furnace National Historic Site in Pennsylvania operates a working water-powered sawmill with a form 6 wheel driving the headsaw and carriage.
- Education and Public Demonstration: Beamish Museum in County Durham operates several working water wheels on rotating engineering demonstrations for visiting school programmes.
- Textile Heritage: Quarry Bank Mill in Cheshire runs a restored 7.4 m breast/overshot composite wheel that drives historic spinning machinery during operational days.
The Formula Behind the Water Wheel (form 6)
Shaft power output is what you actually care about — that's the number that tells you whether the wheel can drive your tanning drum, your alternator, or your hammer mill. The formula combines the gravitational potential energy of the water with the wheel's hydraulic efficiency. At the low end of typical flow (40% of design flow) most form 6 wheels still deliver around 50-60% of rated power because bucket fill stays reasonable. At the high end (above design flow) the buckets overflow before they leave TDC and you lose everything above design — the curve flattens hard. The sweet spot is 80-100% of design flow with the launder sluice trimmed for clean bucket fill at zero splash.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Pshaft | Useful shaft power delivered to the load | W | hp |
| η | Overall hydraulic-to-shaft efficiency (typically 0.55-0.75 for form 6) | dimensionless | dimensionless |
| ρ | Water density | kg/m³ (≈1000) | lb/ft³ (≈62.4) |
| g | Gravitational acceleration | 9.81 m/s² | 32.2 ft/s² |
| Q | Volumetric flow into the buckets | m³/s | ft³/s |
| H | Effective head — vertical distance from launder lip to bucket discharge point | m | ft |
Worked Example: Water Wheel (form 6) in a working heritage tannery in Tuscany
A small heritage vegetable-tannery in the Chianti hills wants to drive two oak tanning drums directly off a restored form 6 water wheel. The site has 3.6 m of usable head between the spring-fed millpond and the tailrace, and the springline gives a steady 55 L/s in summer with peaks to 90 L/s in spring. Target wheel diameter is 3.5 m to fit the existing stone wheelhouse opening. The drums need roughly 1.8 kW continuous at the shaft.
Given
- H = 3.6 m
- Qnominal = 0.055 m³/s
- ρ = 1000 kg/m³
- g = 9.81 m/s²
- η = 0.65 dimensionless
Solution
Step 1 — compute available hydraulic power at nominal summer flow before efficiency losses:
Step 2 — apply form 6 hydraulic efficiency at nominal flow. A well-built wheel with tight shrouds and a properly placed launder typically hits η ≈ 0.65:
That's below the 1.8 kW the drums need at nominal flow. The tannery either accepts longer tanning cycles or adds a second wheel — direct-drive at 8 RPM still works for the drum, just slower agitation.
Step 3 — at the low end of the typical operating range, late summer drought drops flow to roughly 60% of nominal, around 0.033 m³/s:
At 0.76 kW the drums turn but agitation is sluggish — you would feel the wheel labour visibly when the leather hides hit the down-stroke of each drum revolution. The owner needs a clutch or bypass arrangement for these months.
Step 4 — at the high end of the typical range, spring melt brings flow to 90 L/s, well above the wheel's design capacity. The buckets overflow before TDC, so the wheel only captures its design flow of about 60 L/s:
The extra 30 L/s spills harmlessly down the bypass sluice. Power tops out around 1.4 kW regardless of how much water the spring delivers — that ceiling is set by bucket geometry, not by available flow.
Result
Nominal shaft power is 1. 26 kW at 55 L/s and 3.6 m head. In practice the operator would see the wheel turning steadily at 8-9 RPM with one drum running comfortably and the second drum needing to be staged rather than run in parallel. The range across the operating band — 0.76 kW in late-summer drought, 1.26 kW nominal, 1.38 kW at spring overflow — shows the sweet spot sits in late spring through early summer where flow is steady at design and bucket fill stays clean. If the measured shaft power comes in below 1.0 kW at nominal flow, the three suspects in order are: launder misalignment dropping water past the buckets (check splash pattern at TDC for water hitting the rim instead of entering buckets), shroud-to-rim gap leaking bucket volume on the descending side (look for water sheeting from the wheel faces between TDC and BDC), or tailrace silting causing 50-100 mm of backwater drag on the lower buckets (measure tailwater elevation against bucket discharge point with a string level).
Choosing the Water Wheel (form 6): Pros and Cons
Form 6 is one of several practical choices for low-head sites. Whether it's the right call depends on flow steadiness, head height, capital budget, and whether you care about peak efficiency or long-term reliability. Here is how it stacks up against the two most common alternatives a practitioner would consider on a 2-8 m head site.
| Property | Water Wheel (Form 6) | Crossflow Turbine | Undershot Paddle Wheel |
|---|---|---|---|
| Typical efficiency | 55-75% | 70-85% | 20-35% |
| Operating shaft speed | 4-12 RPM | 300-1500 RPM | 6-15 RPM |
| Useful head range | 2-8 m | 2-200 m | 0.3-1.5 m |
| Capital cost (3 kW class) | £8k-25k restoration cost | £4k-9k turbine + civil works | £3k-12k restoration cost |
| Tolerance to debris and silt | Excellent — passes leaves, fish, sticks | Poor — needs trash rack and screen | Excellent |
| Maintenance interval (bearings) | Annual greasing, 5-10 yr bushing replacement | Quarterly inspection, 3-5 yr bearing change | Annual greasing, 5-10 yr bushing replacement |
| Service life (structure) | 80-150 years (proven on heritage sites) | 25-40 years | 60-120 years |
| Direct-drive suitability | Excellent for low-RPM mills, drums, hammers | Needs reduction gearing for most loads | Excellent but lower torque |
| Backwatering sensitivity | High — tailrace must stay clear | Low | Very high — partially submerged by design |
Frequently Asked Questions About Water Wheel (form 6)
The most common cause we see is launder geometry, not bucket leakage. If the launder lip sits too high above the bucket lip — more than about 50 mm — incoming water gains too much velocity and bounces out of the bucket on impact rather than settling into it. You can verify this by watching the fill point with a phone camera at 240 fps. You should see a clean parabola of water entering the bucket; if you see splash-back or water spraying off the lip, raise the launder lip closer or reshape the bucket leading edge.
The second cause is timing — the launder outlet might be positioned wrong relative to TDC. Form 6 wants the outlet 8-15° past TDC. We have measured restorations where the launder was set at TDC exactly, and 12% of the water simply fell down the inside of the rim instead of into a bucket.
Diameter sets shaft RPM and torque, not power. Power is fixed by H and Q. A larger wheel turns slower and delivers more torque at the shaft — useful if you are direct-driving a stone or a tanning drum. A smaller wheel turns faster, useful if you are driving an alternator with less gear-up.
Rim velocity is the controlling spec. You want roughly 1.5-2.5 m/s at the rim for stable bucket fill. For 3.6 m head, target a wheel diameter close to head height — heritage rule of thumb. So your 3.5 m wheel is correct for 3.6 m head. Going to 5 m on the same head would mean buckets entering empty for the top portion of the rotation, which kills efficiency.
This is almost always a bearing or alignment issue showing up only under torque. A bronze bushing that has glazed or run dry has very low static breakaway friction when unloaded, but under shaft torque the journal walks sideways into the bushing wall and friction climbs steeply. You'll often hear a low groan just before stall.
Check the journal for polishing wear (bright bands), pull the bushing, and look for the matching wear pattern. Shaft alignment matters too — even 2 mm of out-of-square between the two journal blocks across a 3 m span will cause the same stall behaviour. Use piano wire or a laser level across both journals to verify.
Design the bucket geometry and launder for your most common steady flow, not your minimum and not your maximum. A bypass sluice handles excess flow during high-water periods — the wheel simply won't accept more than its design intake regardless of how much water you push at it. For low-flow periods, you accept reduced power output rather than redesign the wheel.
Practical rule: pick the flow that is exceeded 60-70% of the year on your site (the Q60-Q70 point on a flow duration curve). That gives you a wheel that runs at design power most of the year, slightly under-loaded in dry months, and dumps excess in wet months.
Tailwater level rising even 100 mm above the bucket discharge point causes the lower buckets to pass through standing water on their way up the back side of the wheel. They have to push that water out of the way, which is pure parasitic load. We have measured 15-25% power loss from 150 mm of backwater on a 3 m wheel.
The fix is mechanical, not hydraulic — keep the tailrace clear. A simple grating upstream of the wheel discharge plus an annual autumn clear-out is usually enough. On sites with heavy leaf fall we install a sloped grating at 20° that self-clears as flow rises.
Heat-related vibration on a water wheel almost always points to a bushing or to felloe joint movement. Cold, the wood and bronze are tight; warm, the bushing clearance opens up to its running value and any out-of-round on the journal becomes audible. Stop the wheel after a 30 minute run, immediately feel each bushing housing — if one is significantly hotter than the other, that bushing is your problem.
The other cause is felloe joint creep. If rim segments were joined with through-bolts that have loosened, the wheel develops a 1-2 mm flat spot under thermal expansion, and you get a once-per-revolution thump that builds with speed. Tighten or re-key the felloe joints.
References & Further Reading
- Wikipedia contributors. Water wheel. Wikipedia
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