Looking for the Perfect Inflatable Pump
The time it takes to fill an inflatable with air to the proper pressure is a function not only of the size of the boat, but of the type of pump used and the pumper’s strength and size, if a manual pump is used.
We can’t give good marks to the “Bravo 6” bicycle-type pump that comes with the QS-230 as standard issue. Its handle is only 18" off the ground, so requires deep, back-tiring bending to use, even by the shortest adults. Because the pump cylinder is large (4" diameter), one has to push downward really hard (40 to 50 pounds or more) with his upper body to move sufficient air into the boat. The Bravo 6 is supposed to operate like a “stirrup” pump, because the pumper’s feet are meant to hold the pump in place—but the 2" flanges at the bottom of the central cylinder are barely enough for a toe-hold. As a result, the pump frequently slips out of control to one side or the other.
The pump’s label claims that it will compress air up to one “mbar”, i.e. one atmosphere or 14.7 psig (pounds per square inch gauge), and that, by inserting a plug to utilize the pump’s double-stroke feature (adding pressure in both upstroke and downstroke), the pump will move 10.6 cubic feet of air in a minute, assuming 60 strokes per minute. Figuring that the QS-230 holds about 20 cubic feet of air, this seems to indicate that a normal individual ought to be able to inflate the whole boat to 3-1/2 psi in about two minutes.
We tried valiantly, but by the time the flattened boat was about half filled, we had only enough energy to push 20 strokes a minute, pumping only on the downstroke, compared with the 60 per minute “pushing and pulling” suggested on the pump label. Pushing hard, we finally got to 3-1/2 psi on the three main tubes after more than 300 strokes. On the air floor, we never even got close to 10 psi. A little more than 6 psi was the best we could do before the tapered valve hose started popping off at every stroke.
Dissatisfied, we decided to hunt for a better alternative. At first it seemed we had found one, namely the “Bravo 10” foot pump made by the same firm as the Bravo 6 (Scoprega S.p.A. in Milan, Italy) in the discount catalogs—$44.99 from West Marine, 800/262-8464 and $42.95 from Defender Industries, 800/628-8225.). We were delighted to read on the packaging that “the problem of the physical effort required to inflate large boats up to the maximum pressure, has at last been overcome… The recommended pressure of all fabric or rubberized boats can easily be achieved with very limited effort.”
Yes! But wait, the claims get better. “In fact, with a force on the foot plate of only 300 N (approximately kg 30) all boats can be inflated to their maximum pressure without effort.” A pressure of 30 kg? We thought: Isn’t that about 66 pounds? Could we have had our multipliers wrong?
Ignoring that question for the moment, this foot pump sounded like just what we needed, especially having made the natural assumption that the “10” in “Bravo 10” meant it could pressurize inflatables to 10 psi, the rule-of-thumb these days for maximum air-floor pressure. That concept seemed especially logical since we had already determined that the “Bravo 6” would pump to 6 psi.
The Bravo 10 foot pump is a “double chamber” model, with a “volume chamber” for moving a lot of air at low pressure, and a “pressure chamber” to pump in higher pressure, though at less volume per stroke.
That, too, sounded okay to us. But as we read further, we began to have doubts. There was a numerical table on the packaging, showing total volume (of the pump chamber), output volume, and the strength of the push needed to reach a certain pressure. The trouble was that the quantitative data was all in terms of “N”, “mbar”, “cc”, and “kg”. We knew that “cc” was cubic centimeters, and “kg” was kilograms (1.0 kg = 2.20 pounds), but it took some digging to figure out that “N” stands for “Newton” (a force needed to accelerate a 1-kg mass 1 meter per second per second), and that “mbar” stands for “millibar”, which is “one atmosphere (14.7 psi).” We were then able to translate the table to more familiar units. See the table on page 13 for our comparison.
Despite the label’s verbiage to the contrary, the figures in the table tell us that one can’t “easily achieve” the “recommended pressure of all fabric or rubberized boats,” namely 10 psi for most air floors and keels. The table indicates that even to get to 7.3 psi requires a force of 88 pounds, more than many people would be physically capable of applying.
Next, we noticed that, using this table and making a few calculations, we could determine how long (in theory) it would take to pump up the QS-230 to 3.3 psi, using the Bravo 10 foot pump. Estimating that the QS-230 main tank volume is 20 cu. ft., the table tells us we could get to 1.2 psi in 157 strokes (20 divided by 0.1273) if we pushed with a 66-pound force. Then we’d have to switch from the volume side to the pressure side of the pump and finish the job—up to 3.3 psi on the main tubes. If we did the whole job on the pressure side of the pump, at 66 pounds of force, we could figure the number of strokes by: Strokes = 20/0.0475, or 421 strokes. But we would already have finished about the first one quarter of the way to 4.4 psi, and could stop at the three quarter mark (3.3 divided by 4.4), so the total number of strokes on the pressure side would be only about half of 421, or about 210 strokes. That makes a total of 157 + 210 = 367 strokes, pushing with a force of 66 pounds all the way. Is that what the Bravo people call “without effort”?
In disgust, we went to the nearest Target store, and for a mere $19.99 bought a “Massair” (no other manufacturers’ information listed), a 12-volt, 250 psi air compressor with a 6 psi “raft air pump” integrated into it. It worked fine plugged into an automobile cigarette lighter, pumping the main tubes to 3-1/2 psi in a jiffy, bringing the boat up to 7 psi before the valve hose started popping off.
Peculiarly, when we checked with Target stores’ main offices in Minneapolis they could not locate the “Massair” in their computers. A research coordinator said it does not appear that Target sells the pump. The researcher said it may have been purchased as a “stray” on sale.
So, we’ll resume our search for a contender to the $100 LVM 110, rated tops in our August 15,1997 report. (LVM, 125 Mixville Rd., Cheshire, CT 06410; 203/272-7059.)