Best Material for Trampoline Support

Hey guys first I better explain first the method I have decided on for the tramps on my catamaran build. I have glassed to the hulls some thin wall PVC pipe, and plan to use 2 inch seatbelt webbing as the straps. The PVC pipes will have some kind of rod inserted for the webbing material to wrap around. The webbing is two inces wide so I figure I will cut out a two inch long hole on two inch centers all the way around, and then weave the webbing over the rods in a pattern like a big lawn chair or something. I hope that makes sense. y'all have probably seen it before.

anyway, what is best material for the rod inserts? The "tramps" are about 9x7 feet more or less. I guess they probably wouldn't be holding more than eight or ten people at the most. there wouldnt be much uV exposure, though maybe a little at the edges of the webbing .Carbon rods or stainless are pretty expensive. fiberglass costs more than aluminum. I can get 3/8 6016 aluminum for about .50 cents a foot. it is regular extruded rod The "cold finished" is much more (what does that mean?)

Any feedback appreciated
how do you figure out if it will be strong enough?

 

The only way to figure out strength is to build a model and stand on it.

8-10 people will be a hell of a load.

Personally I wouldn't use pvc, it is weak and not stiff.

There are aluminum extrusions used on awnings for RV equipment. go see what you can buy as a replacement part. On most catamarans (Hobie, Nacra, Prindle, Tornado) they just use a piece of cord or rope sewn into a folded and sewn section of the tramp.
Seat belt material will be unusually thick when folded over, you will have to find the extrusion and see if it will work.

Or just check out a trampoline for jumping. you may want to use bolted down loops and run a rope thru the edge of the tramp. Even better, just get an old trampoline matt. Most just have the thread ruined by the sun. They can be restitched and will work well.

 

Ten people will be at least 1600 lbs. That is a huge load. I don't think thin wall PVC will take it.

 

I think cold finished means cold rolled.
http://en.wikipedia.org/wiki/Rolling_(metalworking)#Hot_and_cold_rolling

Will you have to stitch the webbing where they cross each other, to stop them from spreading out and leaving a hole? I wouldn't think it would have to spread much for a foot and a leg to go through.

 

Can you confirm that Aluminium grade as 6016, are you sure it is not 6061? Also the condition which is usually a T(No) designation, for plate and extrusion.

Personally, I'm not sure you can successfully glass PVC pipe if it is the common plumbing type, which has a certain level of plasticiser in it. Better chance with the much more rigid drain piping but that is at least 75 (3") - 100mm (4") diameter. I'm rather hoping these are not the main catamaran load bearing beams... A sketch or some pics would be helpful.

 

sorry for the lack of clarity. the PVC is merely a form for the layup fabric. It has no structural purpose per se. the lay up schedule is a fairing of structural bog followed by three staggered layers of 10.5 pz uni and a layer of 12 oz biax in epoxy.

The aluminum I was looking at is 6061. 3/8 inch rod. Material parameters are strength, UV compatibility, weight and price, in that order, pretty much. there has to be room of course for the webbing to go through. I have a sample coming today.

I guess the strength of the alum rod is dependent on the span between cut outs, so the span would be two inches with two inches support on each side, etc. This is assuming that the rod is stronger than the layup holding it in place. I will try and dig out a photo later. thanks for any input.

 

Fiberglass rods would be nice but as you said, price is an issue. I have bought fiberglass rods in shorter lengths and scarfed them together which may work out cheaper. I would worry about aluminum corroding when enclosed inside the pvc with wet webbing wrapped around it, not a good environment for aluminum imh. On the webbing net, I once built a mid net for my Mac 36 cat with black 2" webbing and once i had it all strung i glued the junctions with hot melt glue, then removed it and took it in to a canvas shop and had them sew it.

Steve.

 

that is exactly what my wife has been saying. god I hope she is wrong I will never hear the end of it.
Steve,s suggestion sounds good but I am wondering how to tension the thing once it is sewn together like that? I ordered a ss ratchet to try out with the 2 inch webbing.

 

some pics

from the build two years ago, and one of a sister ship. this is the look I am after.

 

I don't know if you've seen this site, but it's kind of humorous and informative too.

http://catamaranconcepts.com/2010/06/02/tramps/

.

 

Tensile strength?

I have about decided on fiberglass rods to go into the PVC tubes.

Grainger has it in ten foot Lengths.

For their 3/8 stock their specs say "tensile strength" 100,000 PSI. What does this really mean? the span will be about 2 1/2 inches unsupported.

I prefer 3/8 over 1/2 inch because there will be more room inside the pipe, in case I want to double up on something. 1/2 inch will work too though

thanks

 

It means that it will break if you pull more than 100,000 pounds per square inch of section. That is if the rod had one square inch of section surface.

 

Your 3/8" rod has a cross sectional area of 0.1104 in2 x 100000=11040lbs tensile but in your application you are dealing with shear, with metal it is usually taken as 0.60 x tensile, not sure if the same applies to other materials. However you cut it it is more than adequate.

Steve.

 

Hi Charlyipad,
I used to have a multi myself, and, knock on wood, will again one day soon. And yeah, it took a little while to get used to the trampolines (nets), so I applaud your looking into yours being safe. Especially if you're going to be carrying passengers.

Just an FYI, 100,000psi is a higher strength than a good percentage of the steels out there, so you might want to look into those numbers. Like this 304 Stainless comes in at about 82k psi.
http://matweb.com/search/DataSheet.aspx?MatGUID=e2147b8f727343b0b0d51efe02a6127e And most plain Jane 316s come in at less than 306s due to having a higher %age of alloys to improve corrosion resistance, at the expense of strength.

Getting 100k psi is tough with autoclaved, carbon fiber G10, let alone with fiberglass. And yeah I know, you don't need rods anywhere near that strong, as such is many, many times burlier than the hull bits holding the rods in place. But you might ask Grainger for a tech specs sheet on the stuff, prior to handing them your Visa. As there are some pretty weak, factory made, fiberglass laminates out there.

Also, you might want to inquire as to how these rods holds up to UV exposure... unless you have a real good plan for keeping el Sol off of them.

If the specific numbers on those materials don't come through in this post, and you'd like them, either do some digging on www.matweb.com or hit me up via PM. Out of curiosity, I gather that you wont have rods on all 4 sides of the nets. How does the design specify tightening them up on the non-rod sides?

 

Thanks guys. From what I gather so far, the rod in this application would not be the weakest link. The webbing is (I think) rated at 6000 lbs, and that may not be comparing apples to apples. I guess the "chain" here will have three links- the glass holding the pipes to the hull/ the rod insert/ and the webbing material. My best "wild a** guess is that the weakest link would be the fiberglass laminate, since it is so dependant on a consistent quality layup job.

UNCIVILIZED, yes the rod would be largely shielded from the sun, and actually this arrangement will encompass all four sides of the tramp. I figure that I can secure a pvc pipe by wrapping it in glass, curing it and then after painting the aluminum compression tubes, I can strap it along the tramp side of each one by encircling the tube with some smaller dia webbing and buckling the pvc on in place. If I put some caulk in between these "dog collars" and the aluminum it should hold the pvc assembly tight to the aluminum comp tubes, and prevent any chafe issues. And the bonus is there would be no perforations in the aluminum. The other three sides are glassed directly to the hulls or the bow beam.

The photos below show the seatbelt webbing roll stock, and the way the pvc tube and rod will accept the webbing. And check out that stainless ratchet! It only cost about 25 bucks. There has to be something wrong with it . All I have to do now is figure out how to work it. The goal is to have only two uncut lengths of webbing for the whole tramp, and rig up some way to ratchet the tension, then remove the ratchet. There must be a way.

Kurt Hughes stock plans for this boat do not get into the specifics of tramp assembly.