Table of Contents
A boomless rig buys access a boom cannot, and pays for it with evenness. One cluster throwing a 6 m swath replaces twelve flat fans on a 6 m boom. The whole swath leaves through two orifices. Height, angle, pressure and overlap decide whether that swath is usable or striped.
This guide is a setup and calibration procedure for a boomless spray nozzle cluster. It works the width law, the droplet penalty of wide patterns and the square root flow law. It then covers pass overlap, mounting, wind limits and calibration.
The Snapshot
- 3.46 m is the swath of a 120 degree pattern at a 1.0 m mounting height, from W = 2 x h x tan(theta / 2).
- 41 percent is the flow gain from doubling pressure, because flow follows the square root of pressure. The pattern angle barely moves.
- 10 to 30 percent is the published pass overlap band, applied to the even part of the swath.
- 15 km/h at cluster height is the usual wind limit for broadcast work. Above about 8 km/h the very coarse to extremely coarse classes are the only defensible choice.
What a Boomless Cluster Actually Is
A boomless cluster is an assembly, not a single tip. A wide-angle or off-centre nozzle throws the main swath to one side of the mount. A smaller side nozzle fills the strip behind the machine. Some rigs use one symmetric 180 degree deflector, which throws left and right from a centre mount over a narrow tractor.
The hardware sets the numbers. Published cluster swaths run 4 to 12 m, and mounting heights run 0.4 to 1.2 m. Cluster flows run 2 to 20 L/min, and the coarse droplet band runs 1.5 to 4 bar. A cluster is quoted at a stated height and pressure, usually 3 bar.
Off-centre boomless spray nozzles exist in left and right versions. A single symmetric pattern wastes part of its liquid on the machine. A pair of asymmetric tips angled away from the centreline covers more width from less flow. The penalty is a double dose where the two patterns meet under the tank.
Most ag spray equipment in this class is trailed or mounted on a utility vehicle. Tank sizes of 300 to 500 L with a 20 to 40 L/min pump are common.
A cluster therefore has four settings: height, levelling, pressure and the inner overlap between the tip pair. A boom averages errors across twenty tips, while a cluster carries two, so every error stays visible.
The Geometry Law: Swath from Height and Angle
The swath at the target follows the law that governs any hydraulic fan. Width equals two times the mounting height times the tangent of half the spray angle. The shorthand is W = 2 x h x tan(theta / 2).
A 120 degree pattern has a half angle of 60 degrees and a tangent of 1.732. Its swath is 3.46 times the mounting height. A 140 degree pattern has a tangent of 2.747, so its swath is 5.50 times the height. At 180 degrees the tangent of 90 degrees is undefined. No finite width follows from the law.
| Mounting height | Swath at 120 degrees | Swath at 140 degrees | Swath at 180 degrees |
|---|---|---|---|
| 0.4 m | 1.39 m | 2.20 m | no finite swath: the sheet leaves level |
| 0.6 m | 2.08 m | 3.30 m | width set by the deflector face, not by height |
| 0.8 m | 2.77 m | 4.40 m | same geometric limit at every height |
| 1.0 m | 3.46 m | 5.49 m | rated swath from the tip data, not the tangent law |
| 1.2 m | 4.16 m | 6.59 m | the outer sheet thins and drifts before it lands |
The 180 degree column is what the geometry does. A flat deflector throws the sheet almost parallel to the ground. The swath is fixed by the deflector width and the flow. Extra height only adds fall time.
Read the two numeric columns as a rate table. Doubling the height doubles the swath. Double the width on a band pattern halves the deposit per unit area.
At a 0.6 m mount, 8.0 L/min covers a 3.30 m swath. At 8 km/h that is 182 L/ha. At 1.2 m the same flow covers 6.59 m and applies 91 L/ha. Nothing changed but the bracket.
A 0.1 m height error on a 140 degree pattern moves the swath by 0.55 m, about a 10 percent rate error. The height belongs on the calibration sheet.
Why a Wide Pattern Costs Droplet Size
A wide pattern spreads the same litres across a thinner sheet, and sheet thickness sets how the liquid breaks into droplets. At a 1.0 m height a 65 degree fan covers about 1.27 m. A 140 degree pattern covers about 5.49 m from the same flow, a little over four times the width. The film is thinner, so the ligaments leaving the edge are smaller and the spectrum shifts finer there.
The drift-relevant part is that edge fraction. The centre of a wide pattern carries the larger droplets. The outer band carries the finest ones, and it hangs in the air longest. A 140 degree cluster therefore drifts more at its edges than a 110 degree tip at the same flow.
Droplet class is quoted by volume median diameter (VMD), the drop size that splits the sprayed volume in half. The published ASABE S572.1 bands below are the reference when a label names a class.
| Class | VMD (microns) | Behaviour |
|---|---|---|
| Very fine | below 145 | hangs in the air; travels far in the lightest breeze |
| Fine | 145 to 225 | good leaf coverage, high drift risk |
| Medium | 226 to 325 | moderate drift, general coverage |
| Coarse | 326 to 460 | lower drift, usable for most broadcast work |
| Very coarse | 461 to 650 | low drift, weaker retention on waxy leaves |
| Extremely coarse | above 650 | minimal drift, suited to soil and stubble targets |
For a drift-prone product from a boomless rig, work at very coarse or coarser. Step to extremely coarse above 650 microns for soil, stubble and burn-down targets. Pressure is the other half of the decision. Doubling it on a plain fan moves the class about one step finer. A tip that is very coarse at 1.5 bar is coarse at 3 bar.
Pressure, Orifice and the Square Root Law
Flow through a sprayer nozzle follows Q = K times the square root of P. K folds the orifice area and the discharge coefficient into one constant. Doubling pressure raises flow by 41 percent, from a factor of 1.414. Halving pressure cuts flow by 29 percent.
| Pressure | Square root of P | Flow at K = 4.62 | Change against 3 bar |
|---|---|---|---|
| 1.5 bar | 1.225 | 5.66 L/min | 29 percent below |
| 2.0 bar | 1.414 | 6.53 L/min | 18 percent below |
| 3.0 bar | 1.732 | 8.00 L/min | rated point |
| 4.0 bar | 2.000 | 9.24 L/min | 15 percent above |
| 6.0 bar | 2.449 | 11.31 L/min | 41 percent above |
A cluster rated at 8.0 L/min at 3 bar has K equal to 8.0 divided by 1.732. That is 4.62. Every pressure then maps onto one flow.
Pressure buys flow and fines the droplet without widening the pattern. A 120 degree tip stays within about 1 to 2 degrees of 120 degrees across its useful band. Raising pressure to chase a coverage gap adds liquid where it is not needed. It also adds drift where it is not wanted.
Doubling pressure also raises pumping power about 2.8 times, because power follows pressure times flow. The energy cost per litre delivered doubles, so pressure is the expensive lever and the orifice is the cheap one. Flow scales with the square of the orifice diameter. A bore 10 percent oversize passes 21 percent more flow, so lifting flow by 41 percent costs one orifice size, not one bar.
Hold pressure at the low end of the band on a boomless rig. A cluster at 2 bar in the very coarse class keeps its droplets. The same cluster at 5 bar is coarse or medium. Filtration follows the orifice, with a screen opening near one quarter to one third of the orifice diameter. A 2.0 mm orifice wants a 0.5 to 0.7 mm screen.
Application Rate Arithmetic in Litres per Hectare
Rate (L/ha) = (flow per cluster in L/min x 600) / (swath width in m x speed in km/h)
The constant 600 carries the unit conversions, sixty minutes per hour and ten thousand square metres per hectare. A cluster passing 8.0 L/min across a 6.0 m swath at 8 km/h applies 4,800 divided by 48. That is 100 L/ha.
Turn the formula around when the label sets the rate. A pasture herbicide calls for 120 L/ha from a 6.0 m swath at 10 km/h. Flow per cluster is (120 x 6.0 x 10) / 600, which is 12.0 L/min. That is an 8.0 L/min main tip plus a 4.0 L/min side tip.
Speed error is rate error in the opposite direction. The same 12.0 L/min cluster at 12 km/h applies 100 L/ha, a 17 percent under-application. On a 60 hectare block that is a fifth of the chemical spread too thin.
Do not fix a flow shortfall with pressure. Closing a gap from 8.0 to 12.0 L/min wants 2.25 times the pressure by the square root law. That means 3 bar rising to 6.75 bar, outside the coarse band. Drift control ends there, so change the tip instead.
Swath Spacing and Overlap Between Passes
A boomless pattern tapers at both edges, so the nominal swath is wider than the even part. Published practice puts the usable band at about 75 to 85 percent of the quoted swath. The outer 15 to 25 percent carries less than the average.
Set the pass spacing from the usable band. A 6.0 m nominal swath takes a spacing of 4.2 to 5.4 m. That is a 10 to 30 percent overlap between successive passes. Spacing wider than 5.4 m leaves a light strip that shows only once the crop responds.
| Nominal swath | Pass spacing at 10 percent overlap | Pass spacing at 30 percent overlap | Passes to cover 100 m |
|---|---|---|---|
| 4.0 m | 3.6 m | 2.8 m | 28 to 36 |
| 6.0 m | 5.4 m | 4.2 m | 19 to 24 |
| 9.0 m | 8.1 m | 6.3 m | 12 to 16 |
| 12.0 m | 10.8 m | 8.4 m | 9 to 12 |
The overlap band gets more liquid than the rest of the pass, deliberately. The tapered edges of two passes sum to about one full pass of deposit. A gap between them gets nothing. A 1.0 m overlap on a 6.0 m swath is a 17 percent overlap, inside the working band.
On a two-cluster rig there is a second overlap to check, in the centre of the machine. A 20 percent inner overlap doubles the dose in a band about 0.5 to 1.0 m wide. Pitch the tips further apart and accept a light centreline.
Mark the passes with foam, tramlines or a first guideline. A 1 m spacing error on a 6.0 m swath is a 17 percent rate error across the field.
Mounting Height, Levelling and Machine Attitude
Height is a rate variable, so keep it within about 0.05 m of the rated figure. Measure from the canopy once the crop passes about 0.2 m.
Level the cluster frame side to side within about 2 degrees, using a digital angle gauge. A 5 degree roll at a 1.0 m mount moves a tip about 0.09 m sideways. It also tilts the sheet, so one edge of the swath thickens while the other thins.
The swath follows the roll more than the tip does. A 2 degree roll tilts a 6.0 m swath so each edge moves about 0.10 m. A 5 degree roll moves the edges about 0.26 m. That is a 4 to 9 percent shift of deposit, and it reappears as a stripe on the next pass.
Pitch is the other attitude error. A 5 degree nose-down attitude on a tow behind sprayer moves the footprint about 0.09 m forward. Over a rise the front tip of a pair runs closer to the ground than the rear tip. The two patterns then stop matching.
A tow behind sprayer adds a load case a mounted rig does not have. Ride height drops 0.05 to 0.10 m between a full tank and an empty one. On a 140 degree pattern that changes the swath by 0.27 to 0.55 m and the rate by 5 to 10 percent. Set the height with the tank half full, using an adjustable levelling link and a locking clamp.
Wind Limits and Droplet Class Choice
Droplet fall speed decides how much time the wind has. A 100 micron droplet falls at about 0.25 m/s. A 250 micron droplet falls at about 0.9 m/s, and a 500 micron droplet at about 2.0 m/s. From a 1.0 m mount in a 4 m/s crosswind those droplets land about 16 m, 4.5 m and 2 m downwind. Real fields add turbulence, so treat those distances as the floor.
| Wind at cluster height | Working class | Operating move |
|---|---|---|
| Below 8 km/h | Coarse minimum, label permitting | Normal height, hold the pressure band |
| 8 to 15 km/h | Very coarse | Mount low, widen the buffer, check the class at the tip |
| Above 15 km/h | Extremely coarse only, or stop | Only with the buffer in place; broadcast work waits |
| Under 3 km/h at dawn or dusk | Treat as an inversion risk | Fine droplets hang and travel; wait for the inversion to break |
The inversion row matters more than wind speed itself. A temperature inversion holds fine droplets in a shallow layer, and they travel on the gentlest air movement. A calm morning can be the worst drift window of the day.
For a drift-prone product, hold very coarse at minimum and step to extremely coarse near water, dwellings, orchards and pasture boundaries. The label’s minimum class is a legal floor, not a preference. A record of the tip, the pressure and the class is what defends the application later.
A Catch-Can Calibration Routine
A boomless cluster needs six calibration checks rather than a boom’s five.
Measure the usable swath first. Run clean water over a dry strip at working pressure with catch cans at 0.5 m centres. Mark where the catch falls below about 85 percent of the mean. That span is the swath the rate formula should use.
Measure speed over a marked 50 m course with a half-full tank, in the gear and throttle you will actually run. At 8 km/h the run takes 22.5 seconds. Divide 180 by the seconds to get the speed in km/h.
Solve the rate formula for the flow the cluster must deliver, and write it on the sheet. For 100 L/ha from a 6.0 m swath at 8 km/h, the cluster needs 8.0 L/min.
Catch the cluster output at working pressure for 60 seconds in a bucket. A cluster within 10 percent of its rated flow passes. Anything above 10 percent is worn or is the wrong tip.
Run the pattern check across the swath. A mid-band dip more than about 20 percent below the mean means the tip pair overlaps badly. One partly blocked or worn tip gives the same reading. A peak in the centre means the inner overlap is too wide.
Re-check height, roll and pressure after the first tank. Wear, ride height and gauge drift all move across a tank. The 10 percent flow rule catches wear before the field shows it.
The 1/128 acre shortcut also works here with the swath replacing the nozzle spacing. The course is 340.3 ft divided by the swath in feet, which on a 6.0 m swath is 17.3 ft. That is too short to time well, so use the metric catch test once the swath passes about 4 m.
Boomless Versus Boom: The Honest Comparison
| Factor | Boomless cluster | Boom |
|---|---|---|
| Swath per pass | 4 to 12 m from one or two tips | 10 to 36 m from 20 to 72 tips |
| Drift behaviour | Higher: wide sheet, long fall, finest droplets at the edges | Lower: height controlled tip by tip, class held per section |
| Passes to cover 100 m | 9 to 36 depending on swath | 3 to 10 |
| Capital cost | One or two clusters, no boom plumbing | 20 to 72 tips, sections, breakaways and valves |
| Terrain fit | Rough pasture, timber, fence lines, steep ground, narrow gates | Level open ground with room to turn and set down |
| Edge uniformity | 15 to 25 percent taper at each edge | Even within about 10 percent when the overlap is set right |
| Carrier volume | 100 to 200 L/ha in the coarse classes | 80 to 120 L/ha in the medium and coarse classes |
| Pump demand | High flow at low pressure through a single fitting | 2.4 L/min per metre of width at 0.5 m spacing |
The pass count is the cost owners notice last and pay most. Covering a 100 m wide paddock at a 6.0 m swath takes about 19 passes. A 24 m boom covers the same width in 5 passes. Every extra pass adds a turn, a marker check and a chance to lose the spacing.
The capital line runs the other way: a boomless rig is cheaper on a small machine.
Choose boomless where access, obstacle density or ground roughness bars a boom, and accept the wider rate tolerance as the price. Choose a boom wherever the ground allows, because an even swath and tip-by-tip class control are worth more than the hardware saving.
The Known Limits of Boomless Coverage
Edge tapering is the first limit. The outer 15 to 25 percent of a boomless swath carries less than the mean. The nominal width is not a working width, and the label rate holds only inside the usable band.
Uneven distribution between adjacent clusters is the second. On a two-cluster rig the inner patterns overlap under the machine. A 0.5 to 1.0 m double-dose band results unless the tips are pitched apart on purpose.
The droplet penalty is the third. A wide sheet breaks into finer droplets at its edges. A 140 degree cluster cannot hold an extremely coarse class as reliably as a 110 degree tip at the same flow.
Pump demand is the fourth. One cluster carries the whole swath flow through a single fitting and strainer. A 12.0 L/min cluster needs a feed line one size larger than the tip thread suggests. Coarse droplets also need more carrier volume. Boomless rigs commonly run 100 to 200 L/ha, against 80 to 120 L/ha for a boom.
Height sensitivity is the fifth. Doubling the mounting height halves the rate at a fixed flow and speed. A bracket that slips 0.10 m on a 140 degree pattern moves the rate about 10 percent.
Section control is the last. One cluster covers the whole swath, so point rows, waterways and damp patches cannot be shut off tip by tip. The usual answer is a second, smaller cluster.
Frequently Asked Questions
How high should a boomless cluster be mounted? Start at 0.4 to 0.6 m for a 120 degree tip and hold the rated height within about 0.05 m. Height sets the swath, so doubling it doubles the width. Measure from the canopy, not the soil, once the crop passes about 0.2 m.
Does more pressure widen a boomless pattern? No. The angle is fixed by the orifice and the deflector. A 120 degree tip stays within about 1 to 2 degrees across its band. Doubling pressure raises flow 41 percent and moves the class about one step finer. Change the tip for width and keep pressure as a trim.
How much should successive boomless passes overlap? Plan 10 to 30 percent, which on a 6.0 m nominal swath means a pass spacing of 4.2 to 5.4 m. Both edges of the pattern taper. The usable width is about 75 to 85 percent of the quoted figure, and below 10 percent overlap the light strips show.
Which droplet class should a boomless rig hold for a drift-prone herbicide? Very coarse at minimum, in the 461 to 650 micron band. Step to extremely coarse above 650 microns for soil or stubble targets. Keep the tip inside its pressure band, because a tip run 1 bar high can lose a class.
Can a tow behind sprayer run a boomless cluster in a burn-down program? Yes, and the setup carries three numbers. Hold 100 to 200 L/ha, keep the class at very coarse or coarser, and stop broadcasting above about 15 km/h at cluster height.
Getting the Swath Even
A boomless cluster is a two-tip system doing a boom’s work, so the setup carries every error without an average to hide it. Set the height from the width law and hold the pressure in the coarse band. Space passes at 70 to 90 percent of the usable swath, and level the frame within 2 degrees.
The arithmetic does most of the work. A 6.0 m swath at 8 km/h needs 8.0 L/min for 100 L/ha. The same cluster mounted at 1.2 m instead of 0.6 m applies half of it.
Where a boom fits, a boom is the easier system. Where access bars a boom, the cluster is the answer, and the numbers above keep it honest. Browse the agricultural nozzle range for wide-angle and off-centre tips. Send your swath, speed and label rate to the application team for a cluster and pressure match.
For the tip families behind the choice, the agricultural nozzle types guide sets out flat fan, cone and air-induction behaviour. The pull-behind sprayer nozzle guide covers boom arithmetic on the same machines. The herbicide drift nozzle selection guide handles the class decision near a sensitive boundary.
Next Step
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Send your flow, pressure, fluid and target coverage. We come back with nozzle options and figures, not a catalogue number.
Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.