How long an inflatable actually takes to inflate

Of the eleven competitor buying guides measured during our research, two published an inflation time and none published a stroke count. It is the question every buyer asks and the number nobody supplies.

It is not hard to work out. Manufacturers publish board volume in liters and a target pressure in psi; hand pumps publish a cylinder volume and electric pumps publish flow rates. Everything else is division, and the whole model is below so you can check it.

The model, in full

To take an empty chamber of volume V liters to a gauge pressure of P psi, you need to push in enough atmospheric-pressure air to fill the shape and then enough again to compress it to pressure:

free air = V x (14.7 + P) / 14.7

For a 300 L board at 15 psi: 300 x 29.7 / 14.7 = 606 liters.

A dual-action hand pump moves air on both the push and the pull at low pressure, then has to be switched to single action when back pressure makes that impractical. So the stroke count splits:

• Phase one, 0 to 7 psi: the pump delivers two cylinders per full cycle. For a 2 L cylinder that is 4 L a stroke.
• Phase two, 7 psi to target: one cylinder per stroke, so 2 L.

For the 300 L board: phase one needs 443 liters, which is 111 strokes at 4 L. Phase two needs 163 liters, which is 82 strokes at 2 L. Total 193 strokes, and a third of them are the hard ones.

For a two-stage electric pump it is simply each phase's air divided by that phase's published flow rate.

Every board we can calculate

What this model deliberately ignores

Every figure above is a planning estimate, and it is worth being precise about where it is soft:

• Air is treated as an ideal gas at constant temperature. Pumping warms it, so a board reading 15 psi while you work will read a little under once it cools.
• The chamber is treated as rigid at its final volume. Drop-stitch boards are very close to this. Round kayak tubes stretch as they come up to pressure, so their real figures would run higher.
• Hose volume, valve losses and the air lost each time you disconnect are all ignored.
• The 7 psi switchover is an assumption. Bravo's R.E.D. mechanism switches at 400 mbar, which is 5.8 psi, so the real split is a little more weighted toward the hard phase than the table shows.
• We have not timed any of this with a stopwatch, because we do not own any of this gear. These are calculations from published specifications, not measurements.

Why the kayak column is empty

Not one of the eight inflatable kayak manufacturers in our registry publishes a chamber volume. Without it, the calculation has no input, and there is no honest way to produce the number.

We could estimate it. Sea Eagle publishes a 10-inch tube diameter and a 12 ft 6 in hull, and a cylinder volume is easy arithmetic. But that estimate would need assumptions about how much the tubes taper at bow and stern, how much of the hull is floor chamber, and how the three chambers divide -- none of which is published, and any of which could move the answer by a third.

So the honest answer for kayaks is: much less than a board, because they run 1.1 to 6 psi rather than 15, and Sea Eagle's own claim for the SE370 is eight minutes. That is the only inflation figure any kayak manufacturer in our registry publishes.