Design a Kaplan turbine, the axial-flow turbine shaped like a ship's propeller. Adjust the head, flow, efficiency, runner diameter and rpm to see the plant output, specific speed, runner tip speed and speed ratio update in real time, and learn how low-head, high-flow hydropower works.
Parameters
Net head H
m
Net head available to the turbine
Flow rate Q
m³/s
Volume flow of water through the runner
Turbine efficiency η
Efficiency converting hydraulic power to shaft power
Runner diameter D
m
Outer diameter of the propeller runner
Rotational speed N
rpm
Rotational speed of the runner shaft
Results
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Output P (MW)
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Available hydraulic (kW)
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Specific speed Ns
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Runner tip speed u (m/s)
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Speed ratio φ
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Shaft torque T (kN·m)
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Kaplan runner — axial-flow rotation animation
A view along the axis of the broad blades reaching out from the hub. Water flows through the blades from front to back while the runner spins. Colour shows the speed ratio φ.
Output vs net head
Output vs flow rate
Theory & Key Formulas
$$P=\eta\,\rho g Q H,\qquad N_s=\frac{N\sqrt{P}}{H^{5/4}}$$
Shaft output power P (η: efficiency, ρ: water density 1000 kg/m³, g: gravity, Q: flow, H: net head) and specific speed Ns (output P in kW, H in m, N in rpm).
$$\phi=\frac{u}{\sqrt{2gH}},\qquad u=\frac{\pi D N}{60}$$
Speed ratio (peripheral-velocity coefficient) φ and runner tip speed u. D: runner diameter, N: rotational speed. √(2gH) is the theoretical velocity from the head.
The Kaplan turbine suits low head and high flow and runs at a high specific speed (typically 300-1000). Its adjustable blades keep efficiency high even under large flow variation.
What is the Kaplan Turbine Simulator?
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A "Kaplan turbine" is one type of water turbine used in hydropower, right? What makes it different from an ordinary one?
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Put simply, a Kaplan turbine is "a turbine that looks just like a ship's screw propeller". It is an axial-flow turbine — water passes straight through the runner along the axis. The one most people picture is the Francis turbine, where water spirals in, but the Kaplan throws a large volume of water straight onto a big propeller. That lets it handle huge flows even at low head. It shines in plants on river weirs and in tidal power that uses the rise and fall of the sea.
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So it can generate even at low head. When I drop the "net head H" slider all the way to 2 m, it still produces real output.
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Exactly — that is the Kaplan's true strength. Output is set by P = η·ρgQH. Even with a small head H, you can win back power by raising the flow Q. With a head of 2 m, push 800 m³/s through and the shaft output works out above 10 MW. You do not need a big mountain-dam head; a large lowland river is plenty. In Japan, run-of-river plants on the Shinano and Tone rivers use Kaplan turbines.
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I see. But river flow swings a lot with the seasons. If the flow drops, won't the efficiency collapse?
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Good point. An ordinary propeller turbine has fixed blade angles, so the moment the flow strays from the design value, efficiency drops fast. The breakthrough of the Kaplan turbine is that "the blade angle can be changed while running". The inventor Viktor Kaplan gave it his name. When the flow drops, the blades flatten, working in concert with the inlet guide vanes, so they always hold the best angle to the flow. So even when the flow halves, efficiency stays near 90%. That is the decisive difference from a fixed-blade propeller turbine.
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The results card shows a "specific speed Ns". What does that represent?
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The specific speed is an index showing "whether a turbine type suits high head or low head". You compute it as Ns = N√P / H^1.25. The specific speed of a Kaplan turbine is very high, 300-1000. A high-head Pelton wheel, on the other hand, is low at 10-60. So the specific speed is a ruler for a turbine's character. With the default values, Ns comes out to about 577 — a Kaplan-like value for a "low-head, high-flow axial-flow turbine". In design, the standard move is to compute the specific speed from head and flow first, then pick the turbine type from it.
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It also bothers me that the "speed ratio φ" is above 1. Does that mean the blades spin faster than the water?
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That is right. The speed ratio φ = u / √(2gH) is the runner tip speed u divided by the theoretical velocity from the head. For a Kaplan, φ is around 1.4-2.0, so it does exceed 1. That is because the Kaplan is a "reaction turbine". Unlike a Pelton, it does not push only with the water's kinetic energy; it also extracts work from the pressure difference across the blades. So the blades can run faster than the theoretical velocity. By contrast a Pelton wheel has φ around 0.45, with buckets running at less than half the jet speed. Just looking at φ tells you whether it is an impulse or a reaction turbine.
Frequently Asked Questions
A Kaplan turbine is an axial-flow reaction turbine shaped like a ship's screw propeller. Water passes through the runner along the axis, and the runner has 4-6 broad blades. Its defining feature is adjustable runner blades that can be tilted while running; combined with adjustable guide vanes, they keep efficiency high even when the flow changes greatly. The Kaplan turbine is the turbine of choice for low heads (roughly 2-70 m) and very large flows, and it is widely used in run-of-river plants and tidal power stations.
The specific speed Ns is a near-dimensionless index showing whether a turbine type suits relatively high head with small flow or low head with large flow. In metric units (output in kW, head in m, speed in rpm) it is Ns = N√P / H^1.25. Kaplan turbines have a high specific speed, typically 300-1000, which reflects a design that handles large volumes of water at low head. A high-head, low-flow Pelton wheel sits at the opposite end with Ns of only 10-60.
The speed ratio φ is the runner peripheral speed u divided by the theoretical velocity √(2gH) obtained from the head H: φ = u / √(2gH). It tells you how fast the blades move relative to the velocity due to the water's potential energy. For Kaplan runners φ is typically 1.4-2.0, so it exceeds 1. The tip moves faster than the theoretical velocity because an axial-flow turbine is a reaction turbine that also extracts work from the pressure difference across the blades.
The choice depends on head and flow. High head with small flow suits a Pelton wheel (impulse turbine, Ns ≈ 10-60); medium head and flow suit a Francis turbine (reaction, mixed-flow, Ns ≈ 60-350); low head with large flow suits a Kaplan turbine (reaction, axial-flow, Ns ≈ 300-1000). For example, a mountain dam with 300 m of head calls for a Pelton wheel, while a river weir with 15 m of head and a large flow calls for a Kaplan turbine. Because its blades are adjustable, the Kaplan turbine is especially strong for run-of-river sites with large flow variation.
Real-World Applications
Run-of-river hydropower plants: Plants that use a river's flow directly without a large dam are the Kaplan turbine's home ground. The head ranges from a few metres to a few tens of metres, and the flow varies greatly with the seasons. Thanks to the adjustable blades, efficiency stays high from the high flow of the snowmelt season to the low flow of dry periods. In Japan, many run-of-river plants line large rivers such as the Shinano, Tone and Mogami, and most of them use Kaplan or similar propeller turbines.
Tidal power: In tidal power stations that use the mere few metres of head produced by the rise and fall of the tide, the Kaplan turbine — and the bulb turbine that can pass flow in both directions — takes the lead role because it handles low head and large flow. The Rance tidal power station in France is the classic example, making up for the small tidal range with a large flow cross-section. With tides reversing the flow direction, being able to adjust the blade angle is especially important.
Evolution into bulb and tubular turbines: The bulb turbine places a Kaplan runner on a nearly horizontal axis and encloses the generator in a capsule (bulb) within the water flow. Because the flow path is not bent, hydraulic losses are small, making it suited to very low heads (about 2-15 m) for small hydropower and generation on agricultural canals. The behaviour of this simulator near the minimum head approaches the regime of these very-low-head machines.
Learning turbine-type selection: In planning a hydropower plant, you first survey the site's head and flow, compute the specific speed, and pick the turbine type. Raising the head and lowering the flow in this tool shows the specific speed falling and moving out of the Kaplan range (Ns 300-1000). In practice, this is where you would consider switching to a Francis or Pelton turbine. It lets you feel how a single index — specific speed — is the starting point of turbine selection.
Common Misconceptions and Pitfalls
The most common misconception is assuming the turbine efficiency η is a fixed constant. This tool lets you set η with a slider, but the real efficiency changes moment by moment with the combination of flow, head and blade angle. What makes the Kaplan turbine excellent is that its adjustable blades keep efficiency almost flat over a wide flow range — not that efficiency is constant under any condition. Far from the design flow, at extremely low flow, or under conditions where cavitation (bubble formation in the water) occurs, efficiency certainly drops. The efficiency curve is really shaped like a "hill", and the adjustable blades are a trick to flatten and widen that hill.
Next, the belief that "a higher specific speed means a better turbine". The specific speed is not an index of merit; it is simply a ruler for "the turbine type that fits the site". Force a high-specific-speed Kaplan onto a high-head site and the runner tip speed becomes excessive, with strength, cavitation and vibration problems erupting. Conversely, use a low-specific-speed Pelton on a low-head site and you get a huge, inefficient installation. The essence is to pick the type that "matches" the specific speed derived from head and flow; high or low carries no merit ranking.
Finally, "the design is done once you have P = η·ρgQH" is not true. That formula gives only an idealised shaft output. A real design must account for many factors: cavitation margin (setting the suction head), leakage through the gap between runner and casing, pressure recovery in the draft tube, and water hammer during transients. The Kaplan in particular, being low-head, sees efficiency hurt by even small losses, so the draft-tube design governs the output. Remember that this tool's result is a "theoretical ceiling", and a real machine ends up with various losses subtracted from it.
How to Use
Enter gross head (m) in headNum field and select range via headRange dropdown to explore 5–25 m typical for Kaplan installations
Input volumetric flow rate (m³/s) using flowNum and flowRange to simulate discharges from 10–500 m³/s across variable-pitch blade operations
Set runner efficiency (%) via efficNum and efficRange, typically 85–94% depending on blade angle and cavitation margin
Adjust runner diameter (m) in runnerNum to modify specific speed Ns and optimize for your site's head-flow characteristics
Click Calculate to generate output power (MW), available hydraulic power (kW), Ns, runner tip speed u (m/s), speed ratio φ, and shaft torque T (kN·m)
Worked Example
Design a Kaplan turbine for a 6.5 m head river installation with 85 m³/s flow. Set runner diameter to 1.8 m, efficiency to 90%. Hydraulic power available = ρgQH = 1000 × 9.81 × 85 × 6.5 = 5,444 kW. Actual output power = 5,444 × 0.90 = 4,899 kW ≈ 4.9 MW. Runner tip speed u = πDN/60; at 150 rpm and 1.8 m diameter, u ≈ 14.1 m/s. Specific speed Ns = N√Q/(gH)^0.75 ≈ 380 min⁻¹, confirming optimal Kaplan range (300–500). Shaft torque T = P/ω ≈ 312 kN·m at synchronous generator coupling.
Practical Notes
Kaplan turbines excel at 2–25 m head with variable discharge; increase runner diameter when Ns drops below 250 to avoid part-load instability and cavitation risk in shallow-gradient rivers
Blade pitch angle adjustment in real installations mimics your efficNum slider—maintain 88–92% efficiency to balance cavitation margin (σ ≥ 0.5–0.7) against speed and torque
Shaft torque directly scales with flow; at 500 m³/s and 6 m head, expect T > 400 kN·m, requiring robust bearing and coupling design per DNV or IEC 60193 standards
Runner tip speed u should stay below 20–22 m/s to prevent material erosion and noise; verify blade stress using FEA if u exceeds design envelope