Section 06 · Deep Dives
Performance data
Rams trade flow for height, so the honest question is never "how much water?" but "how much water, to how high, per litre spared from the stream?" Here are the numbers, with their assumptions in plain sight.
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Output per 1 L/s of drive flow
The table below applies the first-order model q = η·Q·H/h with Q = 1 L/s and η = 60% (D'Aubuisson). Multiply linearly by your drive flow — 10 L/s of drive flow delivers ten times the table.
| Lift ratio h/H | Delivered q | Litres per day | m³ per day | What that serves |
|---|---|---|---|---|
| 2 : 1 | 0.30 L/s | 25 900 | 25.9 | Small hamlet or large irrigation block |
| 3 : 1 | 0.20 L/s | 17 300 | 17.3 | Several households + livestock |
| 4 : 1 | 0.15 L/s | 13 000 | 13.0 | Homestead cluster |
| 5 : 1 | 0.12 L/s | 10 400 | 10.4 | Homestead + garden |
| 6 : 1 | 0.10 L/s | 8 600 | 8.6 | Household + troughs (top of sweet spot) |
| 8 : 1 | 0.075 L/s | 6 500 | 6.5 | Household — tuning gets fiddly |
| 10 : 1 | 0.06 L/s | 5 200 | 5.2 | Household — ambitious; expect less |
Theoretical values; real installations vary with tuning, friction and wear. See the calculator for your own numbers.
Typical ram capacities
Commercial ram sizes are named for their drive-pipe diameter. The figures below are the minimum source flow each size needs in order to work at all — always check the manufacturer's measured tables, which are worth more than any formula.
| Ram size | Minimum drive flow | Character |
|---|---|---|
| 1″ (25 mm) | ≈ 23 L/min (6 gpm) | Hobby & homestead scale; the classic DIY size |
| 2″ (50 mm) | ≈ 95 L/min (25 gpm) | The workhorse; household-plus supplies |
| 3″ (75 mm) | ≈ 225 L/min (60 gpm) | Small community supplies |
| 4″ (100 mm) | ≈ 340 L/min (90 gpm) | Village scale; needs serious flow |
| 6″ (150 mm) | ≈ 570 L/min (150 gpm) | Multi-village & irrigation duty (e.g. Philippine programmes) |
Minimum source flow, from NC State Extension's comparative figures for commercial rams (drive pipes 3/4″–6″, 2–150 US gpm; the 4″ value is interpolated between 3″ and 6″). Manufacturer tables vary — a 2″ venturi-type ram runs on about 60 L/min. These minimums correspond to roughly 0.5–0.8 m/s in the drive pipe, slower than the 1–2 m/s band used to hand-size a pipe in Design & Sizing, so a hand-sized drive pipe often comes out narrower than the bore a maker's table implies for the same flow.
What cuts real-world output
- Air in the drive pipe — even a small pocket softens the column; the beat goes fluttery.
- Flexible or oversized drive pipe — velocity never reaches the valve's slam threshold.
- Friction — undersized or over-long pipes eat head before the pump ever sees it.
- Worn valve rubbers — the slam leaks; each beat delivers a little less.
- Waterlogged chamber — delivery becomes staccato and the check valve takes a beating.
- Very high lift ratios — the spike barely clears the delivery head, so beats stop pumping long before they stop beating.
How rams compare
| Hydraulic ram | Solar pump | Fuel pump | Hand pump | |
|---|---|---|---|---|
| Energy source | Falling water (free) | Sun (free) | Diesel/petrol (costly) | Muscle |
| Moving parts | 2 | Many (motor, controller, tracker?) | Many | 1–3 |
| Runs when | 24/7 unattended | Daylight (or with batteries) | When fuelled & staffed | When pumped |
| Maintenance | Hours per year | Moderate; electronics age | High: fuel, filters, repairs | Low–moderate |
| Lifespan | Decades | 10–20 yrs (panels longer) | 5–10 yrs | Decades with service |
| Head capability | Up to ~10× the available fall | Very high (with the right pump) | Very high | Modest |
| Requirements | Fall ≥ ~1 m + spare flow | Sun + capital | Fuel supply chain | A well & strong arms |
| Best when | Stream below, need above, no power | Power absent but capital available | Temporary or high-power needs | Small, intermittent demand |
A ram is a marathon runner, not a sprinter: modest, continuous, indestructible. If you need to fill a tank in an hour on demand, a ram will disappoint; if you need a tank reliably full every morning for thirty years, it's hard to beat — literally, for the price.
See these numbers play out in the field: Case Studies.