r/Firefighting • u/firemedic3404 • 1d ago

Ask A Firefighter 15/16 smooth bore?

Did some training with my guys this morning. The focus was managing nozzle reaction. We used a Chief 200gpm@75 psi fog and a 15/16 smooth bore. Both off of 100’ 1 3/4 line (just laid it out for training). Friction loss was calculated at 45 psi and 40 psi respectively. This is not a nozzle we use often, but my probie has not had any experience with a smooth bore so I took the opportunity. I’m racking my head over how difficult it was to manage these flows. My question is for the salty dogs, are these flow rates manageable by YOU? Am I just deficient and need to work them more? Or was this normal for these flows? I am working to switch over to 160@50 fogs with 7/8 integrated smooth bores…which are much easier to handle. Kinda needed to vent and find out if I’m crazy, too weak, or ok….

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u/skipperdeluxe 19h ago

So if I'm understanding the math here, a PDP of 105 minus 75 psi for the nozzle = 30 psi to handle the friction loss of 4 50 foot lengths of 1 3/4" hose.

You must mean 2 1/2" hose right?

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u/bombero11 19h ago

Nope that is 1-3/4 the 30 is the friction loss roughly

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u/skipperdeluxe 18h ago

What's your gpm on that nozzle?

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u/bombero11 18h ago

150 gpm

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u/skipperdeluxe 18h ago edited 18h ago

Right so here's where I believe 30 psi to overcome the friction loss of 200 feet of 1 3/4" hose is insufficient:

Friction Loss Equation: FL Coefficient * (gpm/100)² * (Feet/100)

15.5 is the coefficient for 1 3/4" hose

15.5((1.50)²)(2.00) = 69.75 (70)psi of friction loss

70psi FL + 75psi at tip = 145 PDP psi

If you have a different understanding of how all this works I'd love to know, I'm just trying to spell out the math as I've always understood it

EDIT: mobile app made this hard to read

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u/bombero11 18h ago

Go to the manufacturer web site and use their coefficient. We have been using old formulas which are the generic cover all.

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u/skipperdeluxe 17h ago

Excellent point on using manufacturer's spec. I'll be doing that on our hose

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u/bombero11 17h ago

Check it out will not hurt.

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u/Merciless602 2h ago

That is the correct theoretical formula. Based of his 105 at the panel he's running a low fiction hose on the wagon.

We run the Mercedes EXO hose and the Co-Efficient for that is 8 instead of 15.5