Bob wrote:I'm uncomfortable with plumbing threads, and I think clove hitches tied off with two half-hitches would secure the guy lines better than cow hitches, but if it works it works.
Re: pulleys, you can find them at tree trimmer supply outlets and hardware stores that carry such things. Jackson's in San Rafael CA should have them. Maybe REI as well.
I did some weight-testing of the mast that isn't documented in the blog post. Basically, I supported the ends and put weight on the middle to see how much weight it could take for how much deflection. With 100 lbs of weight in the middle, it flexed and then returned to true. With about 200 lbs of weight on the middle, the mast did not return fully to true after this amount of deflection, which indicates some permanent deformation of the metal. (For the record, I flipped it 180 degrees and bent it back.) I conclude that 200 lbs is too much weight, but 100 is "okay".
This test gives a perspective on the strength of the metal and the threads, but it does not directly translate to the real world, because in the real world, the ends are fixed and the load is basically at one end point. If you think about it, if the mast was perfectly vertical and the load was perfectly balanced over the center of the mast, it could support a huge amount of weight, because all force would be straight down through the pipe, and you would have to literally crush the wall of the pipe to cause a failure. So, when we're talking loads of 700 lbs or so, it's only forces perpendicular to the mast that matter, not forces parallel to the mast. Because we keep the mast close to vertical, perpendicular forces are minimized, although they can't be eliminated completely because the pulley is off to one side of the mast. The pipe in use is 2" diameter, which translates to about 0.25 degree off-axis over its 25-foot length, or about 0.3% of the dome's weight applied as perpendicular force to the mast when the mast is perfectly upright.
Also, the weight is being applied to the mast very close to the top guy point. The worst-case scenario is that you guy the end and apply weight in the middle, or that you guy the middle and put weight at the end, both of which provide maximum leverage to the weight. The best-case scenario is that you guy the pole right where the weight is being applied, which minimizes leverage and transfers the forces directly to the guy lines. That's the scenario that's in play here.
Honestly, I think that you will be more likely to bend off the pole than you would be to rip out the threads. If I had $50 to spare on the experiment, I would put weight on a mast until it bent off and see where it failed, but I don't care that much.
Bear in mind that the cow-hitch is formed from a loop made of a figure-eight knot, so both ends of the rope are captured. There is no danger of the loose end of the rope working its way out of the hitch. If the line goes slack, there is a theoretical possibility of the cow hitch working its way loose, but I have never had this happen, and of course once the lines are loaded, the hitches pull tight. You're right, though, that clove hitches tied with half-hitches would be more secure.
I wasn't able to find anything sold as a "pulley" that would take the weight of this project. I searched around forever trying to find the right search term before I found "snatch block." One common use for snatch blocks is directing falling trees, so I agree that landscaping supply stores might be a good choice. They're also used for recovery of stuck off-road vehicles, if that helps. Of course, there's always the Internet.