Smooth Bore vs. Combination…and the Beat Goes On

By Paul Shapiro

One of the most “passionate” topics in the fire service is which nozzle works best, a smooth bore tip or a combination nozzle (combination). Some really deep feelings are expressed. Sometimes, it gets so personal that it’s like talking about the kids. I recently participated in a really intense computer bulletin board debate on the Internet about this subject. In fact, the Web page owner had to cancel the discussion to stop the name-calling.



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It was interesting to read the comments on this topic. Some were comical, some were technical, and some made no sense at all. For the most part, this debate involved the use of the nozzle of choice in the interior attack mode. People were talking about steam production (mainly unwanted), stream reach, stream penetration, and nozzle reaction. It was also interesting to see the background of the people involved in the debate. They came from large and small departments from all over the country; they all had their firm beliefs. There were also some who were more inquisitive than opinionated about the topic.  


My forte in fire education is the mechanics of moving water. When I don’t understand a certain application, I like to go out and do it to see for myself how it does or does not work. So, I decided to go squirt some water. With the help of the Brea Fire Department in Southern California, a series of flow tests was set up to evaluate straight streams produced by smooth bore and combination nozzles.

Note: The results reported here are just the facts as I see them. They do not represent the position of the Brea Fire Department. This information is presented to help you evaluate the points made in this debate and determine which nozzle works best for you.


Three flow tests were conducted in Brea. All were geared to verifying straight stream quality results.

The following flow tests were designed to illustrate the reach and penetration portion of the debate, the portion involved with the mechanics of moving water. Steam-production tests involve live fire testing, which I was not able to do. I think everyone will agree that as far as nozzle reaction goes, the lowest nozzle pressure at a given flow will have the lowest nozzle reaction. This is a rule that Mother Nature gave us and cannot be changed.

Before reviewing the test results, let’s look at a few definitions as they relate to fire streams.

  • Reach. The dictionary definition is “to carry.”
  • Penetrate. “to spread through, to make or force away.”
  • The point of break-over, as discussed in the IFSTA Fire Streams book, refers to the point in a solid or straight stream from which it loses its continuity—or, for lack of better words, it’s the point at which the solid body of water in the stream begins to lose its forward velocity. The stream then begins to turn into more of a spray.

The book also tells you that a good quality stream should be able to land nine-tenths of its body of water into a circle 15 inches in diameter and three-quarters of the stream inside a 10-inch circle. However, it does not tell you from what distance this is to be accomplished.

Another good stream characteristic, according to the books, is a stream that is stiff enough to reach the required height of the target through a moderate breeze, again from an unspecified distance. It stands to reason that the distance figure should not have an actual number. Instead, it should represent each individual application–in other words, the streams hitting the target.

Flow Test #1


The first round of tests involved comparing various straight streams delivered at an approximately a 15° angle from portable master stream appliances. Even though they were handline flows, it was decided to use master stream appliances to provide a stationary personnel-free method of delivering the water. Three flows were produced to get a well-rounded flow range representing interior attack, standard flow, and exterior high flow streams. The flows were 120, 180, and 260 gallons per minute (gpm).

A 260 gpm side-by-side with a 1⅛-inch tip @ 50 pounds per square inch (psi) nozzle pressure (NP) on the left and an automatic @ 100 psi NP on the right. Both streams look almost identical to at least 50 feet.

The smooth bore tip will appear to have a cleaner looking stream than the combination nozzle, but is appearance everything?

At a flow rate of 260 gpm, the nozzle flowing at 100 psi NP had about a 30foot advantage in reach over the 50psi nozzle, 120 feet vs. 90 feet. Both were good streams for an interior attack.

Let’s first analyze the 260-gpm test. The nozzles and their operating nozzle pressures were as follows:

  • 1-inch smooth bore tip, 50 psi NP
  • automatic combination, 100 psi NP
  • 15/16-inch smooth bore tip, 100 psi NP
  • selectable gallonage combination nozzle, 100 psi NP

The 50-psi stream had a reach of approximately 90 feet–in other words, that is where the body of water hit the target. Its break-over point was at approximately 70 feet.

All three of the 100-psi streams had a reach of approximately 120 feet. The break-over point was right at 100 feet.


The topic of penetration constantly comes up when discussing fire streams. I’m not exactly sure what some people consider its definition. Earlier, the dictionary definition was given. Following are some of the varied opinions I have heard: “the actual reach of the stream,” “the velocity or punch the stream has,” and “the actual digging power needed for tearing through debris.” As I see it, all of the above relate to one common denominator–the force from which the body of water in the stream is moving. With this being the case, in my opinion, the

stream with the highest nozzle pressure will be the best penetrating stream, whether it is from a smooth bore or a combination nozzle.

Now let me ask a question, how important is this so-called penetrating stream? In an exterior attack mode on a large volume of fire, in windy conditions, or where the stream needs to travel over approximately 50 feet to reach the target, penetration could be a significant factor for a successful stream.

What about an interior line–let’s say, a 1¾-inch handline flowing anywhere from 120 to 180 gpm? Most (notice I said most, not all) interior streams are used in close proximity to the fire. In fact, if I had to put a number to an average in my community, I would say that I could hit a target interior from no more than 25 or 30 feet away, but usually less. With the flow tests performed, both the low-pressure and standard 100-psi pressure streams were very successful and powerful looking up to at least 50 feet.


Many times, evaluation of a fire stream is based on its actual appearance, how clean the stream looks. Based on this evaluation, the smooth-bore nozzle will usually win every time. The smooth-bore stream gives off a rope- or glass-looking stream, and the combination nozzle will show itself more as a hazy look with a mist of water coming from the entire length of the stream. Is this mist of water hurting the operation? Are we losing gpm from it? If there were a method of measuring gpm loss from a combination nozzle straight stream, I think you would find the actual loss would be extremely insignificant. Both of the streams look almost identical. Do you think either one of them would perform satisfactorily in an interior attack mode? For that matter, how about the exterior high-flow mode? Remember these streams are 260 gpm!

The 120- and 180-gpm flow tests conducted in Flow Test #1 produced similar results with slightly less reach. It seems as though the larger flows, even at the same nozzle pressures, produce further reaches because of the volume and weight of the water involved.

Let’s look at the nozzles with their corresponding nozzle pressures used for the 120- and 180-gpm flow tests.

  • ¾-inch smooth bore nozzle 50 psi NP, 120 gpm.
  • Lowpressure automatic combination nozzle 75 psi NP, 120 gpm.
  • Automatic combination nozzle 100 psi NP, 120 gpm.
  • Selectablegallonage combination nozzle 100 psi NP, 120 gpm.
  • 15/16-inch smooth bore nozzle 50 psi NP, 180 gpm.
  • Lowpressure automatic combination nozzle 75 psi NP, 180 gpm.
  • Automatic combination nozzle 100 psi NP, 180 gpm.


Flow Test #2

This test involved comparing all of the above-mentioned nozzles with their corresponding nozzle pressures and flows delivered through portable monitors again; however, this time the angle of the stream was a little steeper. The monitors

were set at a 90° angle, straight up in the air. The purpose of this test was to compare the velocities of streams using Mother Nature and her gravity for the resistance. As predicted, the highest pressure streams had the highest reach into the air.

▪ The vertical test delivering the streams against gravity to determine the hardest-hitting stream at 260 gpm.


▪ The 50-psi tip on the left and the 100psi nozzle on the right both look good at about 40 feet.

▪ The final results in the gravity challenge: the higher nozzle pressure has a harder impact.

Notice in the photos of the full-length shoot of the vertical stream comparisons that the one on the left is a 260-gpm stream with a 1⅛-inch tip at 50 psi NP; the one on the right is a combination nozzle with a 100-psi NP. The center portion of this photo is about 40 feet high. Although the smooth bore stream is tighter in appearance than the other, both seem to be high in velocity at about this point. As you can see, the stream on the right is the higher-reaching stream because of its higher nozzle pressure.

Flow Test #3

How about this one? How much water is reaching the fire? To evaluate the streams to answer this question, we flowed the streams into the second-story window of the burn tower from 50 feet away and at about a 25° angle. The window in question was 3 feet × 4 feet. All streams were able to hit the target dead center with an insignificant amount of water misting off. The combination nozzles had slightly more mist than the straight stream. In my opinion, any stream that can make it into an area the size of this window from 50 feet away is moving or putting the wet stuff on the red stuff. Besides that, if we’re pulling one of these handlines into a fire, I think you will all agree that the area burning is bigger than 3 feet × 4 feet.


▪ The stream is directed into the secondstory window from 50 feet away at a 25° angle.

▪ 260 gpm with a 1⅛-inch tip @ 50 psi NP.

▪ 260 gpm with a 100-psi automatic.

There is one other test I feel is worth conducting that is indirectly related to this nozzle issue: What effect does a low-pressure stream vs. a higher- or

Standard-pressure stream have on the kinking effect of the line? This is a very simple test to conduct. Establish discharge pressures for both types of streams, low pressure and high. Next, charge the lines, keeping the nozzles shut. Try putting bends and loops in the line. Drag the line through your burn tower to see what effect the walls, corners, and obstacles will have on the line. The quality of the hose will also have an effect on the kinking potential. My tests revealed a slighter kinking problem with the lower-pressure lines than the higher-pressure lines.

Check this out: I charged a 1¾-inch line to its maximum operating pressure of 360 psi (the service test pressure of this hose is 400, which is very common for today). Well, guess what! Not only were there no kinks under the previously mentioned testing scenarios, but the hose was very flexible in terms of advancing the line. The only point at which the line was noticeably stiffer was right at the nozzle.

The main conclusion I came up with for these flow tests was that the stream with the higher nozzle pressure, whether a smooth bore nozzle or a combination nozzle, produced the hardest hitting stream while moving enough water to hit the target. It should be noted that all nozzles tested were found to be adequate.

I hope this information improves your understanding of fire streams. Don’t base your decisions on this subject solely on these flow tests. Go out and try similar tests as well as any others that you can think of. Base them on reality rather than on appearance. When doing these tests, ask yourself, is the stream getting the job done to my satisfaction? That is what really counts.

Paul Shapiro is director of Fire Flow Technology. He is a nationally recognized instructor on large-flow water delivery. He is also a retired engineer from the City of Las Vegas (NV) Fire Department. He has authored numerous articles for fire trade magazines. He has been in the fire service since 1981 and is author of Layin the Big Lines and produced the first in a series of videos on large-flow water delivery. He is available to answer questions; he can be reached at (702) 293-5150 or Layinline

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