Uh- they blow up, like balloons. I have a 22 gallon one in a box under the cocxkpit, another in a old water tank in the bow, Figure the volume your "box" has, and order a tank with that much capacityBut the bladders listed are only 2 dimensional?
I have a space under the setee in the center of the cabin where i want to put it ( a box)
How do i get a three dimensional one