|Topic Review (Newest First)|
|01-31-2011 01:13 PM|
Regarding nylon's stretch, I once rigged a nylon line to use as a jib downhaul. It was single braid, I think. I won't do that again; with the line run aft to the cockpit and the jib at full hoist, I could pull the line probably two feet without the jib budging. It was surreal.
|01-31-2011 12:44 PM|
Originally Posted by svHyLyte View Post
By about how much, would you say, does the amount of bend in an eye splice without a thimble reduce the strength of the line? In the soft shackle, other than the knot, that's the only bend in the line.
I think I will keep talking about this, just to spite you.
|01-31-2011 12:00 PM|
Your son was wise, since improperly handled lines are far more likely to break unexpectedly.
|01-31-2011 11:41 AM|
Originally Posted by sailingdog View Post
The block had nylon double braid and not polyester.
|01-31-2011 10:54 AM|
Unless that 5/8" line was in really rotten shape or a really weak fiber, like polypropylene, I seriously doubt you were anywhere near the breaking strength of it since 5/8" nylon three-strand anchor line has a breaking strength somewhere in the neighborhood of 10000 lbs.
Half-inch polyester double braid, which is likely what was in the block, has a breaking strength of 8000 lbs or so.
Part of the issue you may have had is friction. If the blocks were twisted, then there may have been significant friction in the tackle, making it less effective than it should have been. Also, if the line was the maximum diameter for the blocks in question, you would have seen more friction than if the line were thinner.
|01-31-2011 08:43 AM|
|lancelot9898||Speaking of tension on a block and tackle. This last summer I had occasion to use the 4 to 1 block and tackle that normally is on the boat. (used for the running back stays which I've never had occasion to use) Anyway I take this back to the house to use on a project in which mechanical advantage is needed. When I went out to the dock that prior morning I noticed that our stairs were missing that went down to the small beach. A couple of lots down on a neighbor's jetty were the stairs all still intack. The size of the stairway was about 20 feet long and about 5 feet wide and being water logged the only way to move it off the jetty was with a breaker bar. My whole estimate was that the thing maybe weighed about 500 lbs, but that was totally off considering that with that 4 to one block and tackle myself and a neighbor was barely able to raise the end of it up the 10 feet to reattach it to the dock with bolts rather than nails which the contractor had used. My neighbor and I are both pretty strong and with one of us looping the end of the rope around our hip and the other heaving with all our might we finally got the end up to the dock with the other end remaining in the sand. I had a half inch line in the block and tackle and a 5/8 inch line on the stairs as a bridle and that 5/8 bridle was stretched a lot more than I thought possible. I'm not sure how much more margin I had before breaking, but you'll see noticable stretch way before breakage espically with 3 strand. I estimate that we were putting about 400 lbs of pull on the block and tackle and that would result in lifting maybe 1600 lbs. With one end still on the sand the whole thing must have weighed around 3000 lbs.|
|01-31-2011 07:50 AM|
Originally Posted by AdamLein View Post
WRONG Boyo. Any bending in a line reduces the strength of the line due to the unequal loading of the fibers with the fibers on the outside curvature of the bend carrying most of the loading and those on the inside virtually none. That is why one can tear a towel apart by starting the tear at an edge but not by simply pulling on opposit ends of the towel's middle. Knots/sharp bends reduce a lines load capacity by anywhere from 20% to 60%. Based on your comments thus far you are in desperate need of some lessons in basic physics. I suggest you leave engineering matters to others.
|01-31-2011 02:34 AM|
The last couple of posts are reinforcing my thinking about tension.
Regarding breaking strength of soft shackles, while you are using a knot, the knot is made up of two lines loaded in one direction, and then hooked onto a loop, essentially made up of two lines loaded in the other direction. So on the side of the shackle that has the knot/loop hookup, you've got double the strength. So I would say that that side of the shackle has a breaking strength approximately equal to the strength of the underlying rope (doubled because you've got two lines equally sharing the load, half because of the knot).
The other side of the shackle is stronger, because no knot, but who cares, weakest link and all. So the breaking strength on either side of the shackle is the strength of the underlying rope. But now the shackle, when closed, is again folded in half, like a line going over a sheave. The load, hanging from the bottom of the shackle, is supported equally by both sides of the shackle, so we again get to double.
So it seems, more conservatively (taking the 50% hit from the knot into consideration), the breaking strength of a soft shackle is twice the breaking strength of the underlying rope.
|01-30-2011 09:34 PM|
Now don't pick on SWAY or SAILORTIM that way....
Originally Posted by CapTim View Post
|01-30-2011 09:14 PM|
If a rope is holding an 80 pound weight up, then the link needs to hold 80 pounds, no matter where it is in the rope.
As a fun experiment in rope tension, do this...
Put a spring scale on both ends of a rope. Fix one spring scale to a wall, or a very large, cornfed-type man (one which should not move, you see). Pull on the other scale until it reads 80 pounds. At that point, both scales will read 80 pounds. Every time. Honest.
Now, hook a third spring scale anywhere you want on the rope. Somewhere in the middle would be fine, of course. Pull on _that_ scale until it also reads 80 pounds. [edit: for clarity, you would be pulling 'away' from the rope.. not in line with it. so if somebody were watching all this from above you, they would see a triangle of sorts.] At this point, all three scales will read 80 pounds. Every time. Honest.
Tension is always evenly distributed over the span of a rope.
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