I found that confusing as well, why waste the cost of a double block where a sturdy padeye would have done the job?
Mechanical Advantage - MA
- Thread starter Hunter216
- Start date
I have a few ideas on that:
1) When you lift the board, especially when it's fully down, you're not only pulling the board up, but also pivoting it back. Since the line is vertical (or nearly so), you lose efficiency due to the force vectors involved between the line, gravity, and the CB pin opposing some of your pull.
2) There could be excessive friction somewhere in the system. Was Jim measuring the force needed to lift the board, or to hold it static at some height? In a frictionless system those two should match. As friction increases the force needed to hold the board static will decrease, and the force needed to lift it will increase.
3) Are we sure Jim's rigging matches the diagram? In particular, if someone rerigged the lower block 8 and actually used it as a block, with line 1 running through it, down to block 5, and back up to block 7, that would be quite bad. Block 5 would put half of the weight on each of the two legs of line 1 that go through it. Block 7 would be pulled all the way to the left, binding against blocks 8 and providing not mechanical advantage. So all you'd have is 2:1 from block 5, and lots of friction from blocks 7 and 8 binding.
All valid points although I would be surprised if its 3) as my interaction with @JimInPB have left me with the impression he’s a smart dude.
This all started with trying to help a fellow 216 owner in the UK
https://forums.sailboatowners.com/index.php?threads/216-hydraulic-keel.197551/#post-1548838
This got me thinking I should have some plan in place to deal with a failed Sailtec unit. I got Googling and trying to remember my HS physics as a mental exercise (gotta work out the brain to keep it in shape
) and I got stuck trying to get an accurate answer on MA. Turns out that the “correct” answer seems to have a lot to do with how one visualizes the system.
All depends on the relative locations of the pivot point, lifting point, and center of load of the board. A moment arm could multiply the force required to lift the board - depending on where the cable is connected. You really need a diagram of the rest of the system to analyze it properly. So far, the discussion has been limited to an analysis of the block and tackle.
Also, @Jackdaw 's point regarding friction/misalignment in the system will always prevent us from achieving the T=L/MA nirvana which we seek...
As @Davidasailor26 points out, the force will likely vary based on the angle of the board.
Excessive corrosion at any of the blocks (basically negating the MA) could increase required lifting force significantly.
Also, @Jackdaw 's point regarding friction/misalignment in the system will always prevent us from achieving the T=L/MA nirvana which we seek...
As @Davidasailor26 points out, the force will likely vary based on the angle of the board.
Excessive corrosion at any of the blocks (basically negating the MA) could increase required lifting force significantly.
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Sorry, this is not a matter of opinion, it's simple physics. Ignoring the (valid) points that @Jackdaw made regarding friction. There is no way to come up with an odd number of mechanical advantages from a moving block system. As @Will Gilmore stated, the non-moving blocks do not give mechanical advantage, only change line direction. The only thing that counts is the blocks that move with the load. If you have a single moving block, it gives a 2:1 advantage, regardless of the direction that the eventual line goes. If you double the moving blocks, it gives 4:1, and multiplying the moving blocks by the 2:1 gives a 8:1. There's just no way to give an odd number. This pull goes to the last moving block which doubles the mechanical advantage of the entire first set.
So, for the block and tackle, your answer appears to be MA=16... right?
The other part of his question is, if the board weighs 150lb, why does it take 60+ lbs of tension to raise it with a MA ~16? So, the question has evolved from what is the MA - to what is it, and why does it act like ~2?
Well maybe "simple" for you and likely lots of others - me not so much!
Your explanation of only the moving blocks driving the MA number really strikes a chord with me as it vastly reduces the complexity of the diagram. Find the parts that move and how they relate to each other - relatively simple.@Jackdaw image of inches of effort - work also works for me as it makes calculating the length of line required in a system something I can get my head around.
So, if this is the correct image, you can see that the MA is largely undone by the connection point of the lifting cable.
Exactly. You're not just lifting the weight of the CB, you're pivoting it with a relatively short lever arm.
So, if this is the correct image, you can see that the MA is largely undone by the connection point of the lifting cable.
This is not true. You get odd number of MA when the moving block has a becket. In this case the moving block is a quad block with no becket so 8:1. If it had a becket so that the terminal end of the line was on the becket you would have 9:1
Not so. You can reeve to advantage, have the fixed line attached to the moving block, and get an odd number for mechanical advantage.
That said, I'm thinking my original assertion of 18 might be incorrect; but I have to draw a diagram.
Just hard to wind up with an odd number on this arrangement, since the last thing you do is multiply it by 2.
I'm not sure what this hand drawn diagram is referencing - all the other diagrams are professionally drafted with a legend, this one not (or at least that I can see). It may not even have anything to do with a Hunter 212 but accidently ended up in the 212 manual. However your point about the connection point makes sense to me.
So, if this is the correct image, you can see that the MA is largely undone by the connection point of the lifting cable.
I've never seen a H212 and likely never will so can't comment further. The "system" must have been OK or the original purchasers would have caused a ruckus!
My original ask was to help me understand how to determine the MA of a "complex" system. The system used on the H212 is definitely on the upper end of complex proven by the ongoing debate.
Personally I'm learning a lot. Lot of smart people on this forum and I'm better just by participating.
It was the first 212 owners manual I found. Thought the sketch was weird, but it looked similar to other info, so it's probably in the neighborhood. I don't own one nor have I seen one either.
But the above discussion has been fun! Thanks for bringing it up.
Was gonna suggest the "Riggers Handbook", but this leads to a bunch of great sites including wikipedia that have better demonstrations.Not so. You can reeve to advantage, have the fixed line attached to the moving block, and get an odd number for mechanical advantage.
That said, I'm thinking my original assertion of 18 might be incorrect; but I have to draw a diagram.
Google search
Even if Wil Gilmore , Hunter216 and Jim26m liked the above it is wrong.
The right answer is 17.
Read and understand post number 12 above.
That's a compelling argument, "I'm right because I said before what the answer is."
I don't buy your calculus. Do you have a reference for that method of analysis?
Some interesting links I came across in my quest for MA knowledge. Disclaimer I have absolutely no affiliation with any of these sites or products.
https://l-36.com/block_sys.php
https://www.westmarine.com/WestAdvisor/The-Vang
https://www.ronstan.com/marine/vang_systems.asp
https://rigginglabacademy.com/go-0-to-60-in-your-understanding-of-mechanical-advantage/
https://l-36.com/block_sys.php
https://www.westmarine.com/WestAdvisor/The-Vang
https://www.ronstan.com/marine/vang_systems.asp
https://rigginglabacademy.com/go-0-to-60-in-your-understanding-of-mechanical-advantage/