Breakin Wind wrote:Thanks GL - nearly all of that makes sense to me and is consistent with what I was thinking, except one comment I don't understand.
" Any 180 degree bend will double the pull, because now two parts of the line pull with the constant force." I want to be careful not to get my terms mixed up. In that sentence, is "pull" different than effort or force needed? In my tree, single block, single line and 10 lb weight scenario I have to pull down with 10 lbs of force (is that the right word?) to lift the 10 lb. weight off the ground. If I put a single block at the bottom of the tree so now I'm pulling up on the loose end of the line rather than pulling down from the limb, I still only need 10 lbs. of force to lift the weight right? (maybe 10.1 lbs due to friction). Just want to verify that.
Take your single block and let the two parts of the line run parallel (180 degree bend). The force in the line is 10lbs everywhere (ignoring friction). The block needs to hold two times that force, because there are two parts of the line exiting it, both exerting a force of 10lbs.
Now, if you hang your block on a tree, the tree holds 20lbs, but you can raise a weight of 10lbs. (That's a single block purchase "rove to disadvantage"). In this example, there's no mechanical advantage, because the line moves as far as the weight you are lifting (and you are using the same force).
If, instead, you tie one end of the line to the tree, place the block at your weight and pull the free end up, you only need to use 5lbs of force at the end of the rope. The block will see 2 times 5, and lift a weight of 10 lbs (still ignoring friction). The weight will move half as fast as the end of the line, there's your factor 2 mechanical advantage - tallies with you needing only a pull of 5lbs. (That's a single block purchase "rove to advantage").
Breakin Wind wrote:My question has ultimately to do with line routing. One of my goals in a different post was to move the jib sheet cams back and to the sides of the cockpit to get the lines away from crisscrossing the cockpit terminating on the CB housing. I realize now from that discussion and my subsequent "simulations" in my boat, that moving the cams to the sides is not a good solution to solve that issue, but there are other alternatives for rerouting sheet lines around to get them to the same relative locations on the CB for cleating and easy reach, but they will have to make a number of turns to get there without crossing the middle of the cockpit, and I'm wondering if that creates too much drag to work.
I've seen pictures and other posts where halyards, and boom vang lines are routed through the cuddy top down and back along the centerboards and wonder if jib sheets could do something similar. Unfortunately I don't seem to be able to just ask that question, because some part of me wants to understand the mechanical pluses and minus behind the scenes...
For jib sheets, you really need to watch friction. Otherwise, the sail won't be able to fill in light airs, not having the force needed to pull the jibs sheet out which is held
back by friction. If that was not an issue, you could run the jib sheet through any number of blocks to get it to where you think it works best (but the traditional arrangement works well, I think, so there's not that much worth improving).
For halyards and many other lines, it's possible to manually encourage the line to run through the blocks in the "other" direction (i.e. when you want to loosen the halyards you can pull on the sail).
Fairleads and deck organizers only contribute friction, not mechanical advantage.