here is a real newbie question but i am gonna throw it out there. What is a good rate of decent when decending using a blakes hitch. I do not want to burn my ropes or have the wear prematurely. are we talking 3-4 feet per second or what? please advise on this issue.
rate of decent using blakes hitch
- Thread starter cntrybo2
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Fireaxman
ArboristSite Guru
Cntry, there may be too many variables for a streight forward answer to your question. Your body weight, how tight you keep the knot, how much friction your tie in point generates, and what rope you are using for instance. But I climbed on a Blakes on Arborplex for a while, and my answer to your question would be "Fast enough for work within the canopy, but not fast enough for descent to the ground". Particularly not when bleeding or pursued by a nest of angry wasps. Of course, in those cases, nothing is fast enough, but the Blakes is way too slow.
I do a good bit of pruning in the tops of mature loblollies. When I get ready to come down I dont like to mess around. I keep a figure 8 on my saddle and let it take the heat of the rappell. Its faster and more fun.
You can usually feel and smell the heat building up in the blakes quickly enough to stay out of trouble, but I recently had a freind tell me he loves the smell of burning poly. Not me. When I was climbing on the blakes I learned pretty quickly to (1) check it frequently for glazing and broken strands and (2) use a split tail system, so I wasn't constantly shortening my climbing rope with cutoffs.
One other caution from personal experience. If you use a splitail of a different kind of rope than your climbing rope be sure it is suitable for the pourpose. I burned through a piece of kernmantle once that luckily, but barely, got me to the ground before it came apart in my hands.
I do a good bit of pruning in the tops of mature loblollies. When I get ready to come down I dont like to mess around. I keep a figure 8 on my saddle and let it take the heat of the rappell. Its faster and more fun.
You can usually feel and smell the heat building up in the blakes quickly enough to stay out of trouble, but I recently had a freind tell me he loves the smell of burning poly. Not me. When I was climbing on the blakes I learned pretty quickly to (1) check it frequently for glazing and broken strands and (2) use a split tail system, so I wasn't constantly shortening my climbing rope with cutoffs.
One other caution from personal experience. If you use a splitail of a different kind of rope than your climbing rope be sure it is suitable for the pourpose. I burned through a piece of kernmantle once that luckily, but barely, got me to the ground before it came apart in my hands.
The nature of your knot could be important. Since all the energy of your descent turns into heat, how that heat is distributed will matter. The Blakes tends to have one turn that supplies most of the friction. That one spot will develop the most heat and become the hottest. Some of the closed hitches (VT, Distel, Schwabisch, etc.) seem to distribute the friction better and thus may run a little cooler.
As Fireaxman pointed out; the total friction in the system; accumulates to give the braking force. So this will be mainly at your friction hitch and the redirect/support point. So, increasing the friction at redirect and/or lines rubbing other things; will give cumulative brake force, with less of the friction on your hitch. Another "cheat" to achieve the needed brake force, while minimizing the brake force in the hitch(thereby it's frictions) is to add friction/ drag of your legs around the tail of your line as you descend. This increases frictional brake force to your speed; while not increasing the brake force on your hitch; or even lessening the brake force on your hitch.
As Moray points out; curiously you've chosen the most notorious of the hitches for building friction / heat concentrated at one point in your hitch, rather than spread out. This would give glazing of 1 section repeatedly over time, if bridge to hitch was always the same; and perhaps in a single descent.
As Moray points out; curiously you've chosen the most notorious of the hitches for building friction / heat concentrated at one point in your hitch, rather than spread out. This would give glazing of 1 section repeatedly over time, if bridge to hitch was always the same; and perhaps in a single descent.
Bermie
Addicted to ArboristSite
I use a blakes hitch on a split tail, never descend too quickly, certainly not as fast as if you were abseiling, take it nice and slow. If you decend with a bare hand on the hitch you can feel it heating up, that will give you an indication how slow you should descend, also the longer the descent the slower you should descend!!!!
The other posters are right, speed is a variable thing, and the blakes will wear more on one point.I have also heard it said that it is a good idea to back up a blakes with a figure 8 when descending, perhaps a good idea for a long descent.
I untie and check my Blakes quite often, that spreads the wear pattern, it gets chucked in the washer from time to time as well. I had one split tail last about two years, average of two to three days per week use, but I am very light and the maintenance helps.
Once the strands show 25% wear, its time for the bin.
The other posters are right, speed is a variable thing, and the blakes will wear more on one point.I have also heard it said that it is a good idea to back up a blakes with a figure 8 when descending, perhaps a good idea for a long descent.
I untie and check my Blakes quite often, that spreads the wear pattern, it gets chucked in the washer from time to time as well. I had one split tail last about two years, average of two to three days per week use, but I am very light and the maintenance helps.
Once the strands show 25% wear, its time for the bin.
beowulf343
Addicted to ArboristSite
Used a blakes for several years. Imo it's easier to burn on a blakes than any other friction hitch i've tried. Fortunately, my hands are huge and don't fit on the hitch well so my index finger drags down the rope-when my finger gets too warm, i know my hitch is getting warm too. What rope you are using makes a difference too-no problems with xtc splitails but made one out of arbormaster and burnt it to the core first time i used it with a blakes.
when you say no problems with the xtc splittails, can you be more specific? area we talking just lean back and let it rip descent or descending at 3' per second? not being a punk just want to know more.
beowulf343
Addicted to ArboristSite
No problem. I meant that in over two years of daily climbing with a blakes and an xtc split tail, i only burnt one tail to the point of it becoming unusable. But then again, i'm not a lean back and let it rip kind of descender any more-those days are way behind me.
Now days i mosey down the tree taking my own sweet time-seems to be easier on me and my gear. (Actually broke a gaff burning out of a stem once.) I rarely burned out of trees anyway-i usually keep my feet on the stem or tree when descending (not saying i climbed down, but i like some point of contact on the tree so i can stay in control and keep from spinning.) Bermie said it well though-if you climb barehanded, you can feel the hitch heating up and it's a good indicator to slow down.Cntrybo2, you seem to want real numbers. Let's assume you are suspended from a pulley so your knot is the only friction element. If you weigh even 150 lbs, like me, and drop at 3 feet per second, you are generating about 500 watts of power during the whole descent, all of which is going into your knot and rope. This is far faster than the knot can radiate all that heat to the outside air, especially since your tight little hand is interfering with the knot's ability to dissipate heat. The knot is going to keep getting hotter until you can no longer hold it.
The heat test described by Bermie is the best guide. When it gets uncomfortable to hold, you know it is a lot hotter still right at the friction surface and it is time to stop descending or slow way down. This is one reason I like Tenex for my split tails--my hand is closer to the action so I am not likely to overheat the knot without knowing it. I suspect Tenex dissipates heat a little better as well, since there is less distance from the friction surface to the outside air.
You have a lot of leeway, since what is uncomfortable to you or me is totally harmless to the rope. My knot got so hot one time it nearly burned me through my glove. But the Tenex suffered only a couple glazed spots the size of apple seeds.
The heat test described by Bermie is the best guide. When it gets uncomfortable to hold, you know it is a lot hotter still right at the friction surface and it is time to stop descending or slow way down. This is one reason I like Tenex for my split tails--my hand is closer to the action so I am not likely to overheat the knot without knowing it. I suspect Tenex dissipates heat a little better as well, since there is less distance from the friction surface to the outside air.
You have a lot of leeway, since what is uncomfortable to you or me is totally harmless to the rope. My knot got so hot one time it nearly burned me through my glove. But the Tenex suffered only a couple glazed spots the size of apple seeds.
Actually, i like Tenex too. It starts out at pretty fair strength and has less loss of strength as it bends around the tight bight of the host/mount/lifeline becasue of it's A) flexability and B) flat/Zero height(therefore less leveraging) on the bent axis.
Also, i think more of the chord is gripping the host lifeline than with round on round. i've thought the heat and glazing tend to be less too.
Metal devices, especially aluminum would tend to conduct the heat away, but wood and rope would be insulators; trying to hold the heat in. But, in each case; total friction for same work of stopping/braking same forces; will be same. It just depends on where you allow friction in to do the this job, how concentrated a point, how much of that is dependent on your hitch, and now as we see, how that heat can be dissipated.
In rigging i like several friction points, theorizing that built up friction can dissipate in between.
Also, i think more of the chord is gripping the host lifeline than with round on round. i've thought the heat and glazing tend to be less too.
Metal devices, especially aluminum would tend to conduct the heat away, but wood and rope would be insulators; trying to hold the heat in. But, in each case; total friction for same work of stopping/braking same forces; will be same. It just depends on where you allow friction in to do the this job, how concentrated a point, how much of that is dependent on your hitch, and now as we see, how that heat can be dissipated.
In rigging i like several friction points, theorizing that built up friction can dissipate in between.
beowulf343
Addicted to ArboristSite
We're looking at the rate energy is being consumed, which is where the heat is coming from. This is power. If 150 lbs is dropping 3 ft per second, that is 450 foot pounds per second. This is the energy, per second, going into the friction zone. It is exactly the same power that would be required to RAISE 150 lbs 3 feet per second (or to raise 450 lbs 1 foot per second).
I remembered that one horsepower is about 550 ft pounds per second. I also remembered that one horsepower is about 750 watts. I then made a conservative estimate that 450 foot pounds per second is about 500 watts. It is actually over 600.
So imagine a 500 watt light bulb buried inside your knot. It won't take long for that knot to get hot.
I have oversimplified. Half the heat generated from descending actually starts in the rope, not the knot. Since the knot is much hotter than the rope, heat is flowing from the knot to the rope. The rope is carrying away not only its own half of the initial heat energy, but another unknown amount conducted from the knot.
Even so, the knot gets very hot. The lesson I take from all of this is that the knot/rope friction setup we use is such a poor dissipator of heat it cannot keep up with a steady input of 500 watts or so.
I have a rappel rack I use for descending SRT. It is the only friction element present. It also gets hotter than the rope. But the metal is a much better conductor and radiator of heat than a knot made of rope, so I find in practice it can easily keep up with much faster descents than I can manage with a prusik knot.
beowulf343
Addicted to ArboristSite
Thanks. Scary with my weight inserted.
