Double Click Thumbnail To View Diagram An analysis of thin shell pressure vessel, aka cat tube.
Recommendations on pressure and performance. For every 1 psig air pressure that is applied to a tube, the tube pushes back with 144 lbs (12in)^2. The pressure required to make a cataraft/raft navagable is that pressure required to maintain is form under the forces by its occupants, frame, and the shearing forces of a hydraulic jump. The inflatable boat will tell the astute occupant what it needs with respect to pressure. A little more pressure than needed is insurance. A lot more pressure than needed is reducing the life span of the seams and fabric. People who tell you otherwise may be motivated by additional boat sales. Or,perhaps they heard it from a friend who heard it from a friend.
A rock hard boat accepts little energy. A soft boat accepts a lot of energy and can possibly fold under a wave. Hard boats can slide off rocks easier, however, hard boats can puncture easier on sharp rocks. Rowframe recommends only that pressure to maintain form. Normally 1 psi is adequate.
Re Diagram on Hoop Stress. 1 psig pplied to a 22 in dia. tube produces 11 lb/lineal in stress. For bonding with a 2 in lap seam only, the factor of safety on stress(FS) FS=48/(11/2) = 8.7 on Bond. Since temperature most affects the bond, the bond controls the design. The 48 lbs comes from a statistical curve for 99 percentile certanty. The fabric has a lower FS, however it has a higher certanty of performing at the required temperatures.
Tubes illustrated on this website are designed by engineers and professionals in the inflatable industry. Seams are lap spliced and taped inside and out to exclude debris from the joint.
A note on cone baffles. Baffles will only fail on an over-inflated boat. We recommend uniform inflation beyond atmospheric pressure. Inflate each chamber until soft (3" deflection under point finger load). Next inflate the cells to 1 psi. If making the boat hard, do it uniformly.
A Note On Temperature and Solar Gain. The Pressure Inside The Tubes is Variable and Over-Inflated Tubes Can Be Damaged If They Are Removed From The Water. Darker Tone Tubes Can Be As Much As 50 degF Warmer Than The Ambient Temperature.
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There exists two general rules affecting the oar length. 1/3 Rule applies to that portion of the overall oar length that is inside the pivot point. The pivot point is the fulcrum. This rule exists for ease of navigating and correct depth of oar blades. One may deviate from the rule 10% or so by counterbalancing the oar shaft such that a second rule applies. The second rule is that the forces exerted on the handle should be less that 1 lb or nominally weightless. This rule exists to prevent repetative motion injuries. Keep in mind that a 100 mile raft trip @ 5 ft/stroke is better than 100K oar cycles. (50K for bicycle action). We recommend oars in the 9 to 10 ft range for boats in the 12-18 ft range; 10 oars being the latter. Frames should be wide enough to accomodate the rules stated above or be fitted with outriggers, rainbows or wings to accomplish the task. Since oars have many of utilitarian functions, we recommend a strong inexpensive oar which will function as a tent pole and can be easily replaced without breaking the bank. We provide such an oar.
With respect the diagram left: In this particular case the frame had an elevated rowing chair on a 12 ft raft and the offset cross oar tower combination could be rotated outward to use a 9'-06" oar. To view a frame with wings, see: http://www.rowframe.com/rowframeraftframecom/raftframe.html
