A smooth bore nozzle produces a single, solid stream of water with no option to switch patterns. Its defining characteristics are a tapered internal waterway with no moving parts, a solid cylindrical stream, greater reach and penetration than fog nozzles, and lower air entrainment. If you’re studying for a firefighting exam, the most commonly tested characteristic is that smooth bore nozzles produce a solid, unbroken stream at a standard nozzle pressure of 50 psi for handlines.
Simple Design With No Moving Parts
The interior of a smooth bore nozzle is a smooth, tapered cone that narrows from the hose coupling to the tip. There are no baffles, deflectors, or adjustable components inside. Water flows through this narrowing passage, which accelerates it and shapes it into a tight, cylindrical jet. Because there are no moving parts, smooth bore nozzles are extremely durable and easy to maintain. The tradeoff is a complete lack of versatility: you cannot switch to a fog pattern or a broken stream. What exits the nozzle is always a solid column of water.
Solid Stream With Greater Reach
The solid stream is the signature output of a smooth bore nozzle, and it behaves differently from the cone-shaped spray of a fog nozzle. At 30 feet from the nozzle, a solid stream covers roughly two square feet. A fog stream at the same distance covers about 100 square feet. That tight concentration is what gives the smooth bore its reach advantage.
A smooth bore nozzle throws water farther than a fog nozzle because it converts pressure to velocity more efficiently. At 50 psi nozzle pressure, horizontal reach is roughly 65 feet. Reach increases approximately with the square root of pressure, so dropping to 30 psi brings effective reach down to about 52 feet. The stream holds together as a coherent column before eventually breaking into visible “slugs” of water, then finally into spray. The point where those slugs are still visible marks the effective reach.
This concentrated stream also punches through obstacles that would deflect or absorb a wider spray pattern. A solid stream can penetrate hot fire gases above 1,100°F, resist wind deflection, and strike deep into a burning structure. Fog streams, with their larger surface area exposed to crosswinds and thermal currents, lose effectiveness at the same distances.
Larger Droplets and Less Steam
Smooth bore nozzles with tip diameters between 7/8-inch and 1-1/8-inch produce water droplets as large as 2.0 mm in diameter. Fog nozzles, by comparison, produce droplets smaller than 1.0 mm. This difference in droplet size has significant practical consequences.
Larger droplets travel farther before evaporating, which means more water reaches the burning fuel surface rather than turning to steam in midair. When a solid stream strikes a ceiling in a fire room, roughly 50 percent of the water absorbs heat and evaporates at that level. At droplet sizes of 2.0 mm and above, only about 20 percent evaporates at the ceiling, with 80 percent falling to the floor. The result is less steam blown back toward firefighters, better visibility inside the compartment, and a lower risk of steam burns. Smooth bore streams also entrain far less air. One measurement found a 180 gpm stream from a 15/16-inch tip moved only 500 to 710 cubic feet per minute of air, causing less disturbance to the fire’s thermal layers.
Standard Operating Pressure
Smooth bore handline nozzles operate in a pressure range of 40 to 60 psi, with 50 psi considered optimal. Master stream devices (deck guns, ladder pipes) typically operate at 80 psi. This is notably lower than the 75 to 100 psi that most combination fog nozzles require, which affects how hard the stream pushes back against the firefighter holding it.
At 50 psi, a 7/8-inch tip flows 161 gallons per minute with a nozzle reaction force of about 60 pounds. A 15/16-inch tip flows 185 gpm with roughly 69 pounds of reaction. The lower operating pressure makes smooth bore nozzles easier to handle for extended periods, and the straightforward physics means the flow rate is predictable. The formula is simple: multiply 29.7 by the tip diameter (in inches) squared, then multiply by the square root of nozzle pressure. For a 7/8-inch tip at 50 psi, that’s 29.7 × 0.875² × √50, which gives you about 161 gpm.
Lower Heat Absorption Efficiency
The same tight, cylindrical stream that gives smooth bore nozzles their reach also limits their heat absorption per unit of water. Because the stream has a small surface area relative to its volume, less water is exposed to hot gases at any given moment. Fog nozzles, with their wide cone of fine droplets, create far more surface area and absorb heat faster per gallon delivered. This is why fog patterns are sometimes preferred for gas cooling in confined spaces.
In practice, smooth bore nozzles compensate for this lower efficiency by delivering water directly to burning surfaces rather than trying to cool the entire gas layer. The solid stream reaches the seat of the fire, cools the fuel, and produces less disruptive steam in the process. Many departments, after testing both types, have settled on smooth bore nozzles for interior attack precisely because the stream is more controllable and the thermal environment stays more predictable for the crew inside.
Key Characteristics at a Glance
- Stream type: Solid, cylindrical, unbroken column of water
- Internal design: Smooth tapered waterway with no moving parts
- Operating pressure: 50 psi for handlines, 80 psi for master streams
- Reach: Greater than fog nozzles at equivalent flow rates (roughly 65 feet at 50 psi)
- Droplet size: Up to 2.0 mm, significantly larger than fog nozzle output
- Air entrainment: Minimal, causing less disruption to fire conditions
- Pattern options: None. Solid stream only