Smooth bore and fog nozzles flow, react, and pump differently. Here is what every pump operator needs to know to set the right pressure and crew the line safely.
The two main fire nozzle types — smooth bore (solid stream) and fog (combination) — behave differently in ways that directly change the pump operator's job. The biggest differences for the operator are nozzle pressure and how flow is determined, which together drive pump discharge pressure and nozzle reaction.
| Factor | Smooth Bore | Fog / Combination |
|---|---|---|
| Nozzle pressure | 50 psi handline, 80 psi master stream | 100 psi (75 psi low-pressure) |
| Flow (gpm) | Set by tip size: GPM = 29.7 × d² × √NP | Rated by manufacturer at design pressure |
| Stream | Solid stream — reach & penetration | Adjustable: straight to wide fog |
| Nozzle reaction | Lower at equal flow | Higher at equal flow (100 psi) |
| Pump pressure needed | Lower (lower NP) | Higher (higher NP) |
| Best at | Reach, penetration, low-pressure ops | Flexibility, gas cooling, hydraulic ventilation |
| Clogging risk | Very low (open tip) | Higher (fine passages) |
The nozzle pressure is the starting point of every pump discharge pressure calculation. A smooth bore handline needs only 50 psi at the nozzle; a fog nozzle typically needs 100 psi. That 50 psi difference flows straight through to the pump — for the same hose lay and flow, a fog line requires roughly 50 psi more pump discharge pressure than a smooth bore line. Operators have to know which nozzle is on each line and adjust accordingly.
With a smooth bore tip, flow is fixed by the tip diameter and pressure — change the tip and you change the gpm. A fog nozzle's flow is whatever the manufacturer rated it for at its design pressure (some are selectable or automatic). Because fog nozzles run at higher pressure, they generally produce more nozzle reaction at a comparable flow, which affects how many firefighters are needed on the line. See the smooth bore nozzle chart for tip-by-tip flow and reaction figures.
It is a tactical decision, not a universal answer. Smooth bore tips excel at reach, penetration, and operating from lower pressures — useful for big-water and standpipe operations. Fog nozzles offer stream flexibility, gas cooling, and patterns for hydraulic ventilation. Many departments carry both. Whatever is on the line, the operator's job is the same: set the correct nozzle pressure and account for the flow and reaction that follow.
Compare nozzles for your own layouts with the free smooth bore flow calculator and nozzle reaction calculator, then roll the result into the pump discharge pressure calculator.
Most engine companies split the difference: fog (or combination) nozzles on preconnects for their pattern flexibility and vehicle/dumpster work, smooth bore on the 2½" attack line where flow-per-firefighter matters most, and smooth bore master stream tips on the deck gun. Whatever the mix, the pump operator has to carry both pressure standards in their head — which is precisely what the simulator's mixed-nozzle evolutions drill.
What is the difference between smooth bore and fog nozzles? A smooth bore nozzle delivers a solid stream with flow set by tip diameter and pressure, operating at 50 psi (handline) or 80 psi (master stream). A fog nozzle produces an adjustable straight-to-wide pattern at a manufacturer-rated flow, typically at 100 psi. The lower nozzle pressure of smooth bore tips means they need less pump discharge pressure for the same layout.
What nozzle pressure do smooth bore and fog nozzles use? Smooth bore handlines use 50 psi and smooth bore master streams use 80 psi. Most fog and combination nozzles are rated at 100 psi, with low-pressure fog nozzles at 75 psi.
Which nozzle produces more nozzle reaction? At a comparable flow, fog nozzles generally produce more nozzle reaction because they operate at higher nozzle pressure (100 psi vs 50 psi for a smooth bore handline). Higher reaction means more firefighters may be needed to control the line.
Which nozzle is better, smooth bore or fog? Neither is universally better; it is a tactical choice. Smooth bore tips excel at reach, penetration, and low-pressure operations; fog nozzles offer stream flexibility, gas cooling, and ventilation patterns. Many departments carry both.