Congrats Daryl, you win my bet. As best I can determine, the SkySharks I break-tested were the P300 and the P3X, both of which were no better than average. The P400 you mention, I did not test, probably because it was not on Goodwind's or Kitebuilder's site at the time. I see that it is a thicker walled tube, weighing around 8.97 gsi, or .58 gm/in, compared to the 8.2 gsi, or .53 gm/in, for the P300. So, it might well be above the average strength for the tubes I tested. The P3X are no longer listed on the Goodwinds site, because they quickly sold out of them and did not restock; however, IMO they are the same tube as the Carbon Express Maxima shafts that are readily available (They appear the same, have the same O.D, and weigh the same to a thousandth of an ounce). By comparison to the P400s, the Victory V-Force V6 300s are around 9.8 gsi, due to even more thickly walled tubing.
I cannot concur with the conclusions from the post by the kitebuilder forum member you linked to. Specifically:
1. I don't think he was referring to ferrule length as being 4 times the shaft diameter, but to the length of insertion or "engagement" of the ferrules into each tube. Carbon arrow shafts generally run below .3" O.D. Four times that is 1.2". A 1.2" ferrule would be woefully short, and would not distribute the stress of a bend over a wide enough arc, an invitation to breakage. Both Fibraplex and its predecessors have used 4" tubes for internal ferrules for carbon poles. Makers of alloy poles, like Easton and DAC, typically use 3" ferrules.
There must be some amount of looseness between the main tube and the ferrule; otherwise the tube could not readily slip into or over the ferrule. Even when that looseness is minimized, there will be some slight amount of cant between the tube and the ferrule, resulting in more pressure on the main tube at its lip, and at the point where it passes over or out of the lip or end of the ferrule. The longer the ferrule, the more this pressure is distributed when the entire pole is flexed, I believe.
The kite forum tests seem to contradict this; however, as I discovered, the degree of snugness between the ferrule and the main tube markedly effects the resistance to breakage, and differences here will totally skew break test results. I did not see anything in the kitebuilder forum test methodology to assure equal snugness, the same mistake I made when I began break testing, and the reason why I have not published my earlier results.
2. What I would like to see in support of the kite forum test conclusion about internal vs external ferrules is some reason to believe that internal ferrules are more reliable than external ones. Without that, I would not accept the kite forum test results without evidence of some control for snugness. Morevoer, an external ferrule offers more of the same protection from breakage that a collar over a carbon tube does, a proposition you accept. Even with the transverse wraps, carbon poles are prone to damage or separation at their ends, and the external ferrules also prevent that when in place. Internal ferrules do not.
However, I can see how internal ferrules of equal snugness might arguably be less likely to break a stressed tube than external ferrules would. With an external ferrule, all other factors being equal, when the pole is arced and stressed, more of its surface is exposed and forced against the lip of the ferrule than is the case with an internal ferrule. Indeed, this may have something to do with the reason why Easton chose internal alloy ferrules for its carbon FX tube poles. But even if the kitebilder forum tests are correct about internal ferrules being less likely to occasion breaks in the main tube, there remain some practical considerations.
3. .240" O.D. pultruded carbon tube should be as strong as the SkyShark 400s, and is available at many of the kite sites. The ends can be reinforced by soaking with Cyanocrylic glue (Krazy glue) – best I've found for this purpose is Bondini – but since the ends of internal ferrules are protected and reinforced by the outer tube, that helps to protect against wear and tear. Nevertheless, during the period of over ten years I used internal carbon ferrules on carbon poles, I had problems with pultruded ferrules eventually splitting. This could be corrected before breakage by keeping an eye on them, but was still a problem. But the more serious problem was that the ferrules were either too loose or too strong, causing the main tube to break. Carrying an extra pole section always sufficed, and the alloy sleeves in the pole repair kit never had to be used, but still …
If you break a stick over your knee, it will break where it contacts your knee. Similarly, if a ferrule is stronger and has less flex than the main tube, the tube will break at the lip of the ferrule. So you don't want it to be stonger, or have a higher "modulus of elasticity," than the main tube.
Another more practical problem with the .240" tubing is that most of the carbon tubes used for arrows are manufactured on spindles of the same diameter, and therefore have an I.D. of around .245". Hence, the difference in diameters is .005", compared to the difference of .002" found between poles and ferrules on Easton tent pole tubing. Again, this much difference will markedly increase the likelihood of the main tube breaking at the lip or end of the ferrule.
I could not find carbon tubing of any kind with around a .243" O.D., and that is yet another reason I went to external ferrules, because the Easton .344" tubing fits over the Victory V6 300s as precisely as could be hoped without being too snug. I believe Roger also found Easton tubing that fit snugly over the carbon shafts that he uses.
If internal ferrules are inherently less likely to occasion breakage, it would make sense to use them, but only if a snug fit could be obtained between the ferrule and tube from materials available to MYOGers, and if outer collars were also used. But, so far I have not found materials that would provide such a snug fit, with close to the same elasticity. I did find a fiberglass tube that fit well as an internal ferrule, but it was so elastic, it bent at the spaces between the main tubes. I also found an alloy internal ferrule that fit well, but was only 2" long, not long enough IMO, as I'm not ready to discard the conclusions of the large pole manufacturers in favor of the kitebuilder poster's tests.
So it would be interesting to hear what Roger and others have to say about the argument for internal ferrules; but without availability of the right materials, the argument may have to remain academic.
4. When you state that your Fibraplex pole broke at the ferrule, I'm not sure if you mean the ferrule broke, or the main tube broke at the lip of the ferrule. Frankly, it could be either, because the ferrules Fibraplex was using, at least a few years ago, were much thinner walled than .240" pultruded carbon tube. I then spoke to Linus, and encouraged him to obtain stronger ferrules. With the pultruded tube as a ferrule, a break would probably occur where the tube passes over the lip or end of the ferrule, as noted above. Achieving exactly the same modulus of elasticity for both the ferrule and main tube, while possible for a large manufacturer, may be like looking for Godot for a MYOGer.
I admit that I don't know if the .344" O.D. Easton alloy tubing and the Victory V6 300s have the same modulus of elasticity; however, I did note in my break tests that they both broke at about the same pounds of force, while protruding from external ferrules of as close to equal snugness as possible. So I am optimistic, but accept that as a MYOGer, I can't expect to achieve precise scientific data on this without the benefit of laboratory facilities, and even if I could, where would I get the precise fitting materials with almost precisely equal elasticity.
That's my opinion, FWIW.
