Paddle shaft constrution and flex

[peterwSUPr]

Hi, I'm not sure how many folks out there have been making many paddles, but I thought I'd bounce this off the crew to see if anyone has any insights. 

The first paddle I made had a carbon shaft made around a copper pipe, 28mm (1.1") diameter.  I started with a wrap of 4oz E-glass to make sure I could get it off the pipe, and then once cracked loose, I added carbon from there.  I was using the 24K carbon tow, and all layers were done with strips right next to each other in a single layer.  I did 2 lengthwise layers, plus a spiral wrap at almost 90 degrees to the axis of the shaft, to help with the structural strength of the cross-section (using 110g of carbon tow off the roll - I think).  I then added a single layer of 5.9oz twill carbon cloth on the outside.  I used 3M super 77 adhesive to stick the final carbon cloth to the shaft before wetting it out and then wrapping with VHS tape, and was very happy with the final look and finish.  I ended up with a nice light paddle around 520g (I think the shaft was in the 300g range), and the shaft is pretty stiff, stiffer than the few commercial paddles I have tried.

So, for paddle #2, I'm not looking to build a noodle, but would like to add a bit more flex.  My final outside shaft diameter is a touch bigger than most (32mm), but I'm sticking with it at least for now, since #2 is started, and I don't know what other mandrel options are easy to get and nice and round and smooth like the copper pipe I have.  Maybe making a nice tapered oval shaft mandrel is a subject for another thread!  I don't think I want any less total material in the shaft, since the weight is right and it has to be strong.  I also was thinking that going to fibreglass or S-glass is too big a step with too much different material properties, and that I'd ideally like to just tweak the fiber angles of the carbon.

I think the logical next step is a shallow angle spiral instead of running the tow straight along the shaft.  I'd also like to not do the 90 degree spiral wrap if I can avoid it, a long wrap like that was actually a pain, since it was hard to keep the tow from sometimes not having a kink or fold when coming off the roll, but when cutting 6 foot lengths they could be smoothed out nice and straight before applying them.  The question is, how much of an angle do I do the spiral wrap at instead of running it parallel?  I know there is no magical number, and everyone's results may vary, depending on all kinds of other factors, but all other things being equal, will a 20 degree angle of the spiral wrap make a small difference or big difference compare to along a straight line?  Anyone have any experience along these lines?   

I'm hoping for lots of answers from lots of folks who have been here before, but based on the posts I found in the archives, I'm not holding my breath....

Thanks,
Peter

[PonoBill]

Glass reinforced by tow is a fine option. You could randomize the tow and spread it out to get a lot of the benefit of CF without adding weight or cost. I'd certainly consider a tapered shaft for this kind of homebuild, you could turn the mandrel out of wood since you're going to get it out early and use the fiberglass as a hollow mandrel. Tapered shafts have the advantage that once they are released, they come out easily. I'd probably do an egg-shape for cross section too, since all you need to do is shape the wood. Horrible for production, not so bad for homebuild.

[peterwSUPr]

Thanks Bill, I don't have the resources right now to turn down a tapered mandrel, so like I said, for now I'm sticking with my copper pipe, especially since it has a wrap of glass on it ready for carbon.  My concern about going to some fibreglass is that  I was under the impression that with it being much less stiff, the load would be carried mostly by the carbon at first until it fails, rather than loading both materials similarly.  (A good analogy I once heard  is imagine making something out of aluminum foil and bubble gum.  Even if both are strong the foil will take all the load since the bubble gum is so stretchy that it is not loaded when not stretched out).

So, I'm not sure if I could take out carbon and put in an equal weight of glass and get something  equally strong and light.  I would expect that a mix of glass and carbon makes for a heavier shaft.  As for cost, the carbon tow from Soller Composites is dirt cheap.  The $26 roll will do several paddles.

Playing with numbers and sines and cosines, I think that the key here is the cosine of the wrap angle squared.  By these calcs, a wrap at 90 degrees and one at 0 degrees works out the same as two wraps at 45 degrees. I also figured out that the carbon tow, the way I was laying it up edge to edge works out to being very close to 6oz unidirectional cloth, in terms of weight per area.  My original shaft would have 2.5 layers lengthwise and 1.5 around it, once I consider my outer carbon cloth as 0.5 in each direction.   

I'm starting to think that changing my two lengthwise tow layers to a 10 to 15 degree angle may be a starting point, so I have about 2.3 to 2.37 layers or units of carbon in the lengthwise direction instead of 2.5.

BUT, here's a question, what is the typical failure mechanism, or is there one?  Tube buckling on the compression side or failure on the compression or tension side while still staying circular.  Anyone know??

Peter

[PonoBill]

Most of the failures I've seen look to be compressive, originating from a scratch or flaw. There's a lot of ways to build a tapered mandrell--including wrapping a smaller diameter tube with tape. Yes, the different materials present complex stress, but unless you are working of the edge of material performance it doesn't much matter. Most of the commercial shafts are fiberglass under carbon. The addition of fiberglass plus tweaking the carbon fiber angles makes flex tunable and damps out vibration. full carbon shafts are harsh.

[peterwSUPr]

Hmmm... interesting, I was under the impression that most high-end paddles were pretty much all carbon.  In the world of windsurfing, you can buy a mast that's 30% carbon, or 100% carbon, and they will both be built to have the same amount of flex under the same load.  But, the 30% carbon mast will feel dead and sluggish, kind of like it absorbs energy when it is flexing, rather than returning it to you.  Based on this I was thinking that high carbon in a paddle was the ticket, as long as you can prevent it from being overly stiff.  By "harsh" do you mean stiff?  Or something else?

Thanks,
Peter

[PonoBill]

It's basically like the masts you're describing. the carbon mast is "lively" and the part-glass one is deader. Which is a tuning factor for what you want your mast to do. With my shoulders and old muscles "deader" is good.

I don't know if this is still true, but originally the KeNalu 100Flex shaft was 100 percent 3K carbon twill. Very stiff, very "lively". The 90Flex and 60Flex were 90 and 60 percent 3K as you might expect. The sixty was a bit too flexy, but the 90 was almost as lively as the 100, but easier on the shoulders. Then we did the xTuf and xTuf(S), which were 60 percent and 70 percent CF with unidirectional CF. Silky smooth, but they lock up after initial flex. The shaft vendor sent us some cross-bias wound uni shafts with the glass at 80 degrees. Loved the tunability, but we would have wound up confusing everyone with too many shaft choices that feel similar to most people. So we stuck with a reasonable set of with clear differences.

[peterwSUPr]

Wow, thanks Bill, that's great info.  I didn't know you had that kind of inside knowledge.  My last shaft had essentially 4 "layers" in it (not including the 4oz E I start with).  I might try swapping out one of those layers.  I have some long narrow off-cuts of 8.9 oz S-glass kicking around, wide enough for a single wrap.  That might be an option for one layer. 

Peter

[peterwSUPr]

Here's another question.  If I wanted to spiral wrap some tow, should I be doing 2 layers would in opposite directions to balance things out?  In a perfect world I think the answer is yes, but I wonder if there would be any weaknesses introduced by a single spiral wrap in one direction only (assuming this is not stressed to close to the breaking point)

Or, maybe spreading the tow and not doing edge to edge strips and winding both ways is the better option.

Peter

[PonoBill]

I'm playing with spreading the tow out and randomizing it. There's a guy on Oahu that builds very pretty custom paddles who does that. That reminds me, he still owes me two paddles I paid for. Not only does it look great, it also can be strong in the directions you want.

My inside knowledge of Ke Nalu is pretty straightforward. I started the company.

[peterwSUPr]

Sounds like I'm talking to the right person then, very cool, thanks.  I did notice the use of the term "we" and "us" but wasn't aware of your level of involvement.  It's great that you're willing to share all this.    I've learned more than I expected to today!

Time to go mix some epoxy I think, although I'm not completely sure what I'm about to put on the shaft for this round. Back to the spreadsheet now for some final number crunching..... 

Peter
 

[peterwSUPr]

I ended up having fun getting the shaft off the pipe this morning.  After the usual methods failed, I clamped one end of  the pipe and wrapped the other end of the paddle shaft with inner tube and webbing and used a lever to twist the other end and cracked it free.  Pulling and twisting would only get me so far after that though.  I tied the pipe to the rafters and climbed up the shaft like climbing a rope and slid it maybe 2 feet before things stopped (no, not because the end hit the ground).  Adding more lube didn't help.  In the end I used the car to pull and tied the other end to a tree.  It came off cleanly, fun fun!

I think the difference this time was that last time I did not discover wrapping the shaft with vcr tape until later in the process.  This time I probably squeezed more resin inward through pinholes and re-bonded it, but there was less flex with the S-glass and a layer of carbon, so harder to un-stick this time.  That tapered mandrel is sounding pretty nice right now!  Wait, no, no problem, I now know how to get this apart!

Peter

[Dwight (DW)]

Are you using tubular fabric? or wrapping?

Wrapping is hard to do. I failed.

[Bean]

Here is an earlier paddle shaft tread that you might find interesting.

http://www.standupzone.com/forum/index.php/topic,18897.msg220180.html#msg220180

[blackeye]

...fun fun!

I think the difference this time was that ...

I'm imagining the scene, sped up and with a zany soundtrack.

Do you think the copper pipe deformed since the first shaft? And deformed further with the fun fun? Copper is very soft.

[PonoBill]

That's the construction advantage of tapered--shift it a tiny bit and it's free. I've seen people try to make shafts and have to resort to slitting tube to get it free. Not that big a deal if you're using the mandrel just to make the base tube, but a bitch for multiple layers. I've thought of making prototype bent shafts that way, using a pipe bent the way I want, put on a single layer and bag it, then slit it off and use it as the base for the rest of the layers.

Did you make some kind of shaft spinner to put the mylar down? You need to get it really tight and uniform to have it work well. there's value in those VHS cartridges after all.