Rowframe uses 6061-T6 almost exclusively....some exceptions being deck plate and sheet for dry boxes. We use 6061-T6 for its endurance characteristics, i.e. the value of the area under the curve of the stress strain diagram.
6061-T6 exhibits a long strain plateau and this characteristic multiplied by the tensile strength, is the energy absorbed by the frame. 6061-T6 pipe generally exhibits tensile strengths of 42-44 ksi and yield strengths of 37ksi.
Pipe Size: RowFrame uses the 1.5" Pipe Size or NPS (Nominal Pipe Size). This pipe is 1.9" outside diameter. The pipe size designation being a holdover from the days of slide rules and the computation of the internal hydraulic radius; that numeric value precident to computing hydraulic friction. In the beginning, almost all frames were made of this size pipe since it offers the necessary resistance to support a 200 lb seat load on a 72" simple supported cross member.
In the race to the bottom, the big box raft frame retailers started offering the 1 1/4" frame size (1.66" outside diameter) raft frames as directed by their bean counters to save roughly 25% on material costs. The logic being planned failure leads to additional sales since there is few folks retailing components of that pipe size. We at RowFrame want you to know that you are not constrained by you initial pipe size selection. We offer parts at reasonable prices to fit the 1 1/4" pipe size. Shop and Compare. Please Buy USA.
Lead(Pb) is added to some of the washwater bronzes to improve machinability since that property is diluted in the alloy process. So tell your river products manufacturer to: "Get the lead out" as well.
There is absolutely no reason to add lead to anything that ultimately becomes drinking water.
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.
Some folks have end run the bond problem by radiosonic welding the fabric joints. The result is that their tubes accept more presure that tubes wuth glue bonded joints. However, there remains the sharp rock point load delimma that haunts all inflatables.
Over the course of history, there have been a few that embraced the hydraulic delemma. We bow our heads to those who made the effort. Of note is Archimedes, Euclid, Froud, Reynolds, Weber, Mach, Newton and, Bernoulli. If I left any out, please update me.
We owe the following to Wikipedia: "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object", whom it turn owes the statement either direct or indirect to Archimedes. We call it "Archimedes Principal".
Combine Archimedes Principal to Newtons First Law (or derivitive thereof) to illustrate the force balance at static equilabrium: ΣForces (Yaxis)=Zero; Such that volume displaced times the specific gravity times the unit weight of water is equal and opposite to: the weight of the frame, oars, tubes, occupants , as well as the mass of beer and its respective cooler riding shotgun in the RowFrame. The unit weight of water is around 62.39 pcf. The length of tube displaced times its displaced cross sectional area defines the displacement.
I took a few liberties with the words. However, do not confuse this with the Archimedes Screw. It in turn, is paying a boatload of money for an inflatable tube made by commies.
Find an inexpensive way to displace water, navigate whitewater, not pollute the environment, and the world will beat a path to your door.
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.
Several Notes on Material
There are several materials that lend themselves to Row -Frames. Of note is Aluminum since it is relatively light in mass when compared to steel. Excluded from the analysis is PVC since it strains plastic in the regime being considered. Also excluded from the analysis is Wood since it lacks the ability to undergo multiple stress reversals without failure at the joints. Wood is an excellent material when used in composite with other materials. This analysis uses the modulus of elasticity (Ey), yield strength (Fy), and specific gravity (SG) to compare materials.
Reasons for using Light Weight Aluminum
Aluminum exhibits a cubic density of 170 lb/cubic ft or 35% the weight of steel. Steel weighs 490 lb for the same cubic ft. The ratio of weights is 2.9 or restated: steel weighs 2.9 times heavier than aluminum for the same volume. Steel is commonly used for boat anchors since its submerged weight in water is (490-62) =428pcf. When compared to water, steel is 6.9 times heavier than water, the result of which is its free acceleration to the bottom of a water column. In contrast, aluminum has a submerged weight of (170-62) =108 pcf.
E (Young’s modulus)
E is the ratio of stress to strain inside the yield strength envelope. For aluminum that value is close to EA=10000000psi; for steel ES=29000000psi or 2.9 times that of aluminum.
I (Moment of Inertia)
I is a measure of the inertial characteristics of a geometric section. I’s value independent of a material selection. Think of it as resistance to change.
EI is a useful characteristic for describing the stiffness of a material-section selection. The inverse of stiffness is flexibility. Objects subject to bending forces will bend. There is no such thing as unbendable pipe; it’s a mathematical certainty.
FY (Yield Strength) for structural steel pipe / tube is a result of its constituent alloys and manufacturing processes. Generally FY Steel is in the 20 to 50 ksi range. Conduit not being a structural section is in the lower portion of that envelope.
Yield strength for aluminum pipe / tube is also result of its constituent alloys and manufacturing processes. Generally FY Aluminum is in the 5 to 41 ksi range. Aluminum that is cold worked such as swaging, tube bending and brake forming has a low FY by design. This low strength assists in the reduction of material grain separation at the surface of the material. The characteristic is somewhat analogous to ductility. Therein lies part of the problem.
Bendable pipe implies high elongation potential. The range of strength for almost all high strain aluminum bending is 4-12 ksi. This high strain may manifest itself as frosting or scaling on the metal surface. Heat treating may return the material to a tempered state. Heat treating may also reduce the strength. This characteristic is noted as the second digit (X) behind the 6061-T6X. The X may be designated or may be a proprietary feature. Sheet aluminum dances to a parallel tune with similar designations.
Reasons for using High-Yield Strength Aluminum Low Weight and High Strength =>Reasonable Price. RowFrame offers good frames at reasonable prices.
Boat Anchors We don’t make them …yet. A boat anchor is a rope, several pulleys, an anchor, and a cleat. REI has several decent pulleys at the sub $15 range. Unfortunately, they are not made in the USA. Bass Pro has several anchors in a wide array of shapes and masses. Many of these are not salmon friendly; all are not coral reef friendly. Please choose wisely. Any more than a hundred dollars for an anchor system is obscene.
Geometric Sizing of a Raft Frame
The following is an educated opinion. There are no absolutes.
The purpose of a double longitudinal raft frame is two-fold. First is the need of the outside rail to bare on the inflatable tube and transmit that load to the inflatable diaphram. Second is the need of the inside longitudinal rail to corral cargo, i.e. restraining said cargo from chafing in the inside of the inflatable tube. The inside rail is an excellent place to tie to.
Visualize your boat inverted, cooler intact, all your gear properly restrained; the result of a wave capsizing the boat. You dear rafter are concentrating on the safety of yourself as well as your crew. You are NOT fretting over any lost gear because gear is meaningless if someone gets injured.
The inside rail being a nominal 11 inches (x2) affords straps in direct tension. Straps tied to an outside rail get the inverse tangent multiplier and therefore more likley to deflect or break from the load. Polypropelene is a commom strap material with a nominal strength of 450 lbf. Nylon is typically 3X stronger. Properly rigged, Polypro is adequate.
The outside rail positioning is matter of convenience. First choose the width of the frame as a function of 1/3 your your oar length. Allowing for the offset of the tower, half the width of the frame and 1.5 inches of thumb to centerline clearance, choose a frame width that lands on the tube chafe (if provided).
The inner rail should afford the entrance and exit of a conventional cooler and dry box appurtenances. The inner rail should impeded the lateral movement of the aforementioned cargo into the inside of the inflatable tube. I use the word should as a non absolute statement. If you have interferences, add a fabric chafe to the wear points on the insife of your inflatable.
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