DaySailer.org Forum

Post season's greetings all,

Is there any mechanical formula, field experience or even rationalized guessing out there as to the percentage of increased effort, or loss of efficiency incurred by adding another turning block into a line path?

Simple example, if I have a block on a tree limb with an appropriate line through it, down to a 10 pound weight. It should (I think) require 10 lbs of pull on the unattached end of the line to lift the weight. There probably is actually some additional fraction of effort needed to be added for whatever the friction of the block is, and that probably works into my question.

If I add another block at the base of the tree, converting the down, loose end of the line now to a horizontal pull, will it still require 10 lbs of pull, or by simply adding a change of direction, is there a specific fractional (or frictional) increase?

Taking it to an extreme for the purposes of the point, if 3 more turning blocks were added to the path to... let's say, guide the loose end of the line into a building where it gets attached eventually to a winch, is there a nominal increase to the pull needed (when does the friction start to add up noticeably)?
I guess the additional weight of the line between the original pull point and the new final pull point begins to count, but is that it? That's how I envision it.

Am I missing a basic mechanical law or rule in my thinking?

Thanks - Scott

A double line purchase will take half the effort for the same pull. Or you get twice the pull for the same effort. you have to move more line is the trade off.
The right size line with the right size block should keep friction to a minimum.

Two principles can be used to figure out practically all configurations.

The first is that the pull is constant everywhere along the line. Any 180 degree bend will double the pull, because now two parts of the line pull with the constant force.

The force available to move your object depend on how many parallel parts of the line are attached to it (if not parallel, then you get factors from trigonometry).

The second principle is that the mechanical advantage goes with the ratio of distance of pull on the free end to distance of movement of the object. (In other words, the product of distance times force should be the same, in the absence of friction, for both the pull you exert on the free end and the pull the whole arrangement delivers to the object you are trying to move)

Friction alters this equation a bit. For each turn, you need to account for friction, so you need to add an extra force at each turn. The force of pull along the line is no longer constant, but gets larger towards the free end. With large number of blocks that have high friction, you could end up losing all mechanical advantage.

Cascades are very efficient in getting large mechanical advantage with few blocks, each block increasing the mechanical advantage by a factor of two. Not surprisingly, they tend to be used for boom vangs.

GreenLake wrote:Two principles can be used to figure out practically all configurations.

The first is that the pull is constant everywhere along the line. Any 180 degree bend will double the pull, because now two parts of the line pull with the constant force.

The force available to move your object depend on how many parallel parts of the line are attached to it (if not parallel, then you get factors from trigonometry).

The second principle is that the mechanical advantage goes with the ratio of distance of pull on the free end to distance of movement of the object. (In other words, the product of distance times force should be the same, in the absence of friction, for both the pull you exert on the free end and the pull the whole arrangement delivers to the object you are trying to move)

Friction alters this equation a bit. For each turn, you need to account for friction, so you need to add an extra force at each turn. The force of pull along the line is no longer constant, but gets larger toward the free end. With large number of blocks that have high friction, you could end up losing all mechanical advantage.

Cascades are very efficient in getting large mechanical advantage with few blocks, each block increasing the mechanical advantage by a factor of two. Not surprisingly, they tend to be used for boom vangs.

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.

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...

Thanks - Scott

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.

GL - got it now... Thanks !

Coming late to this discussion. Remember that the cost of increased purchase is more line to run through the system on each tack. I had a 4:1 mainsheet system that I put back to 3:1 by simply replacing my main swivel block with a ratchet block. I can still sail uncleated, have smoother tacks in strong wind, and less weight drooping on the boom in light air.

If you use a ratchet block, you can hold the mainsheet easier w/o needing as much of a purchase. I find that for the DS 2.5:1 is plenty, even with a single ratchet block at a 90° turn. (I don't routinely encounter winds over 15 knots, and definitely not for longer periods, so your mileage may vary).

I guess my main is also 2.5:1 (2:1 at the end of the boom, a right-angle bend above the main swivel.) My ratchet is a Harken, mounted on a Ronstan swivel cam base. By the way, I liked it so much that I later put Harken ratchets on both jib tracks.

Talbot,

What kind of Harken ratchets do you have on the jib tracks? I've got the original O-Day fairlead-cam cleat combinations, which require a sharp forward yank to uncleat, so I'm looking for a better alternative.

Alan, are you sure your jib fairlead/camcleats aren't mounted backwards?

It should be a "rearward yank" to uncleat them ... ?

Tim,

As set up now, it's a forward yank (actually a combination of forward and athwartship) to uncleat. I tried turning one of the fittings around, but set up that way the sheet would have to enter the fairlead on the aft side, and it looks like it would hang up on the fairlead in the process. I'll try to post photos later today.

Could you just swap them port and starboard then?

Good suggestion, Tim. I looked at that today, and unfortunately If I swapped them port and starboard I'd get the same setup I have now, because the port and starboard fairlead-cam cleat units are identical (no port or starboard orientation). I did figure out, though, that if I remove the units from the cars and turn them around, I could have a rearward cam cleat release if I had a block to change the direction of the sheet 180 degrees. A ratchet cheek block on each side of the boat might be just the ticket.

Didn't get a chance to get photos today. I'll try tomorrow, because I can't think of a good way to put this in words so it makes sense.

TIM WEBB wrote:Alan, are you sure your jib fairlead/camcleats aren't mounted backwards?

It should be a "rearward yank" to uncleat them ... ?

The way I imagine these: if the "base" of the cam cleats faces the hull, then you need to pull the sheet away from the hull to uncleat. If the base is then bent, you get a forward pull. That's not necessarily a problem if you have crew, but I can see that it's a pain to do when single handed.

If you want to release by pulling to the rear, the sheet would have to cross the plane of the back of the cleat at one point. That makes design a bit more challenging (probably why it wasn't done that way), and is something that can't be retrofit.