Tackle mechanical advantage

[sanibel sailor]

Am I correct in thinking the mechanical advantage of these two arrangements are the same?

[Breaking Waves]

Originally Posted by sanibel sailor

Am I correct in thinking the mechanical advantage of these two arrangements are the same?

By counting the number of ropes pulling the moving part (the boom) . . . . I would have said the left system was 5:1 while the right was 4:1.

[Tin Tin]

Agree with the above and also the 4: 1 has a higher point loading on the boom if that is what the top rail is.

[iansan5653]

Yep, left is a 5:1 purchase. Will 20% lower load than the 4:1. For example, if the boom load is 1000 lb, the sheet load will be 250 lb on the right tackle and 200 lb on the left tackle.

[glenn.225]

You also have to take in to account the resistance of each sheave. Way back when, I was taught 10% added resistance for each sheave. Mind you that was with heavy armoured vehicle recovery equipment, I'm sure modern bearing blocks have lower resistance but I would still tack on at least 5% per sheave to be safe.

[MicHughV]

The diameter of the sheaves as also the diameter of the line can also make a difference.

Ideally the sheaves will have roller bearings, but not always the case, at some point adding in a extra sheave, etc, becomes rather pointless, as any mechanical advantage get surpassed by friction, etc, etc.
A case in point, my Beneteau requires the tail of the line to end at a winch, which is true for many other boats as well, despite going thru' a series of sheaves located elsewhere.

[barnakiel]

Wrong answers. Sorry to kill the party.


Only the blocks attached to the moving part (the boom) matter.


Both solutions provide THE SAME ratio (4:1)


Now you can improve the purchase by rewiring the blocks into a compound system.



https://www.translatorscafe.com/stat...dvantage-3.png


Take care and avoid learning physics from Youtube.


;-)
b.

[barnakiel]

https://www.translatorscafe.com/unit...cal-advantage/


for those who want to see where the compound method steps in - so that the same numbers of pulleys gives us a better advantage - note this can be done on the boom downhaul, but NOT on the mainsheet, etc



heyhey


I get a beer or I get a stick ?


b.

[PippaB]

Take care to read the FPN on 'authoritative' sources:

A simple pulley system, where the end of the line is attached to the anchor, has the mechanical advantage, which is equal to 2n where n is the number of moving pulleys. [emphasis added]

In the OP's left drawing the end of the line is not attached to the anchor, but is instead attached to the moving object/load. This, in turn, does create additional advantage.

From another source:

Rule #1: If the pulley closest to the haulers is on the anchor, the pulley is only considered a change of direction (cd). The same rule applies to ANY pulley system.
Rule #2: If the rope used in the pulley system is tied to the anchor, the ideal mechanical advantage (IMA) will be EVEN (i.e., 2:1, 4:1. 6:1, etc.)
Rule #3: If the rope used in the pulley system is tied to the load, the ideal mechanical advantage (IMA) will be ODD (i.e., 1:1, 3:1. 5:1, etc.)
Rule #4: To determine the IMA of a simple pulley system, count the ropes between the anchor and the load. Do not count the ropes between two anchors.

[Bowdrie]

Originally Posted by PippaB

In the OP's left drawing the end of the line is not attached to the anchor, but is instead attached to the moving object/load. This, in turn, does create additional advantage.

This is so.

[BonefishGuru]

Originally Posted by MicHughV

The diameter of the sheaves as also the diameter of the line can also make a difference.

The torque required to move a line a certain distance is halved if the diameter of the pulley is doubled. It doesn't matter if it's the first, middle or last pulley.

Certain lines also slide more readily through sheaves given the same diameter, and when you're able to size down using PDPE(acronym?) like dyneema, you can significantly increase the force generated by the line. Then you get into line stretch too which can be critical.

Originally Posted by MicHughV

Ideally the sheaves will have roller bearings, but not always the case, at some point adding in a extra sheave, etc, becomes rather pointless, as any mechanical advantage get surpassed by friction, etc, etc.

In an ideal world, many simple mechanical devices would contain roller bearings or similar efficiency-enhancing parts, but not a single one on my 34' boat has rollers, nor would I expect them to. I simply keep them lubricated every month or two, which is as frequently as I lubricate pretty much everything on the boat.

[BonefishGuru]

MODERATORS CAN SOMEBODY PLEASE FIX THE ISSUE OF THE SITE ADDING EXTRA CARRIAGE RETURNS (BLANK LINES) EVERY TIME YOU POST?



Very annoying to have to edit every post to get rid of all of the extra white space they add. I worked as a programmer for years so I know it's your site and it's an easy fix. Thank you!
Edit: There it goes again! My reply had one return I keyed so one blank line but there's 3 for some reason. You can replicate it super easy. Just hit return and keep typing. I use Firefox btw.

[Lee Jerry]

Originally Posted by barnakiel

https://www.translatorscafe.com/unit...cal-advantage/


for those who want to see where the compound method steps in - so that the same numbers of pulleys gives us a better advantage - note this can be done on the boom downhaul, but NOT on the mainsheet, etc

Not exactly the same thing, but it's common to have a gross and fine trim (or "tune") on the mainsheet. For example, see the Harken website (scroll down) for this and other examples.

Harken example mainsheet.jpg Views: 49 Size: 71.3 KB ID: 294355" style="margin: 2px" />



Edit: and the left figure is 5:1 IMHO.

[cal40john]

Originally Posted by barnakiel

https://www.translatorscafe.com/unit...cal-advantage/


for those who want to see where the compound method steps in - so that the same numbers of pulleys gives us a better advantage - note this can be done on the boom downhaul, but NOT on the mainsheet, etc



heyhey


I get a beer or I get a stick ?


b.

Jumping on the bandwagon of where the anchor point is, your site only allows for even number advantages, due to the end on the anchor. How are odd number advantages achieved?

Another way to look at it, there is equal tension throughout the line, ignoring friction. If you pull on the end with 10 lbs of force, a block on the boom has 10 lbs on each side of the block, so 20 lbs force on the boom, the other block another 20 lbs, and 10 lbs on the bitter end at the anchor point on the boom. Equals 50 lbs, 5:1 advantage.

[donradcliffe]

Originally Posted by barnakiel

https://www.translatorscafe.com/unit...cal-advantage/


for those who want to see where the compound method steps in - so that the same numbers of pulleys gives us a better advantage - note this can be done on the boom downhaul, but NOT on the mainsheet, etc



heyhey


I get a beer or I get a stick ?


b.

You are wrong and get a stick.