John wrote: >>>>I plan on using a smaller diameter tubing. ... What is the comparative strengths of these tubes, both of which are tempered aluminum. 1. One is 7/8" OD with a .065" wall thickness. This is round. 2. The second is 1.5" OD with a .05" wall thickness. This was round, but was somewhat flattened to make an oval. >>>>>>>> John, This is a more complex question than you realize because so much depends on the details of the actual design, the connections, how each individual member is loaded, etc. There is no website I know of where you can compare properties of these two tube configuration, or learn how to calculate them yourself because without an understanding of the principles involved you would likely get very confused what you are looking at. Below I have calculated some comparative properties that may help you understand how complicated this could be. for the 7/8 x .065 tubing calculated deflection under the same load condition: 0.011318" (in cantilever bending) max bending moment capacity; 1092 in-lb column buckling load for 60" long length; 374 lbs. relative wall crushing strength; 132 percent of 1.5 in. tube shear capacity; 2205 lbs. Weight per foot (approx.) 2.38 oz/foot of length For the 1.5 x .050 tubing calculated deflection under the same load condition: 0.003003" 4.39 times stiffer than 7/8 tube max bending moment capacity; 2796 in-lb 2.56 times stronger than 7/8 tube column buckling load for 60" long length; 1643 lbs. 4.39 times stronger " " " relative wall crushing strength; 75.7 percent of 7/8 in. tube shear capacity; 3037 lbs. 1.12 times stronger " " " Weight per foot (approx.) 4.37 oz/foot of length 1.84 times heavier " " " This buckling limit for a column assumes a round tube without dents or defects. Any imperfections or other shapes will drastically reduce this number. As you can see there is not clear answer, if your gunwales are in simple bending, or if compressive buckling was your limiting stress, you would need approximately 4 7/8" tubes for each 1.5 inch diameter tube to be equivalent stiffness. But if you were only concerned with bending strength, and did not mind that it would be a lot more flexible, you would only need about 2.5 7/8 tubes for each 1.5 tube. Both conditions would be a lot heavier. On the other hand the smaller diameter tube, with the thicker wall, would be more resistant to getting dented or crushed during handling or on beach landings. Of course if the installation was done so you have some "truss" action in the frame there are other considerations. Most installations do not get any truss action but this is where the gunnells are in compression and the keel is in tension under normal loading. Truss action would occur if you have some diagonal members bracing between the keel and gunwales to transmit the shear loads between the upper and lower members. Even if you stack the smaller tubes in the gunwale installation and attach them such that they bend together as a single unit it could be much stronger than these numbers indicate. In the case where you get truss action, or the tubes are structurally attached together, the deflection and bending moment comparisons are not as important, but the buckling and compression strength is, which would make the lateral bracing of the primary members important since the longer the laterally unbraced length of each member the less load it can carry before it buckles. For these numbers I assumed 6061 T6 aluminum, other alloys would be comparatively higher or lower for the moment, crushing, and shear capacities, but not affect the deflection, weight or buckling. BTW, I think that it is the buckling strength requirements for hard-shell kayaks that make them so heavy as compared to non-folding skin on frame kayaks. By the time you make the wall thickness strong enough to keep them from buckling the shell, which would result in total failure of the hull, they end up being much heavier. Traditional skin on frame kayaks, by using individual discrete frame members to carry the hull loads and a flexible skin, can flex without damaging the skin, allowing it to be much lighter. The flexing also allows for some shock absorption, reducing the actual transmitted loads to the frame. Typical skin on frame kayaks will weigh about half as much as typical hard-shell kayaks, in the 24 to 37 pound range for 17 foot long kayaks with no apparent loss of durability in terms of structural failures, even including botched landings on rough rocky beaches. Unfortunately the folding mechanisms, internal sp*ns*ns, and skin closure devices negates the weight savings advantage of this type of construction, so they end up weighing about the same as hard shell kayaks. Also unfortunate is that traditional skin on frame construction does not appear to lend itself to economical factory production processes, so if you want one you will have to build one yourself, or buy one from a small custom builder. That is one engineer's opinion anyway. If you have any questions feel free to email me off list if you like. Peter PS: Doug, surfing 30 foot high waves would qualify as something stupid if it was way beyond your skill and capacity to react to the conditions. IMHO, nature has a way of weeding these types out of the human gene pool, hopefully they would engage in such activities so it does not endanger anyone else, or leave behind dependants that the rest of society has to support. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Mon Mar 05 2001 - 18:34:10 PST
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