SailNet Community banner

1 - 16 of 16 Posts

·
Registered
Joined
·
5,155 Posts
Discussion Starter #1
Mechanical Advantage explained:

The following is a hypothetical series of events that has may have a surprise conclusion for you.

Lets say you are a very fit 180 lbs. You can do a dozen pull ups and bench at least 200 lbs.
Your son is sailing with you and he is an athlete that weighs 140 lbs can do 20 pull ups and bench well over 200 lbs. In other words you are both very fit, but the kid can do anything you can do physically and then some.

On a sail you find the the anchor light bulb has burnt out and you decide to climb up the mast to replace it. You don't have all the right gear but you do have a couple hundred feet of line and a block handy so you figure you will just hoist up the block with the main halyard and a blight of the line. You figure that you are in good enough shape to do 20 pull ups with your full weight and if you rest a bit at the spreaders you can do a second set and make the top.

You start out doing really well, in fact it seems easier than you figured. But about half way you start to get a cramp, probably all the beer the night before.
Your kid is on deck shouting encouragement so you figure you will let him pull you the rest of the way up.

As soon as your son gets his hand on the down line you let go and expect him to hoist you up.

What happens is a surprise to both of you.
You start to fall and your son is lifted right off the deck headed your way.

What happened.

As long as you were tailing your own line you only had to pull 90 lbs, just half of your weight plus a little for friction. As soon as your son on deck had the line and you let go the weight he has to pull jumps to 180 lbs. Regardless of his grip since he only weights 140 lbs he is going to go up and you are going to go down.

The fact that the guy in the chair only has to handle half the weight of the guy on the ground seems at first look more than a little strange, but the math works.

The following videos explain is well:

https://www.youtube.com/watch?v=76l9KZ6XcME
https://www.youtube.com/watch?v=v1p-xuZNq5g
 

·
Senior Member
Joined
·
19,489 Posts
Where's the Video??? ;)
 

·
Registered
Joined
·
4,650 Posts
Father and son together should have hauled up a barrel of bricks(180 lbs) first and used that potential energy to counteract his weight. To come down the son just grabs an armful of bricks from the barrel Mechanical advantage is easy when you use your head.
 

·
Registered
Joined
·
5,155 Posts
Discussion Starter #4
Where's the Video??? ;)
Video worked for me.

Try searching for
Rope and Pulley Systems: Segment 16- Preface to the RADS

on youtube.
 

·
Registered
Joined
·
66 Posts
Not to derail the thread, but I thought this was an appropriate place for this old "workers comp accident form."

Dear Sir:

I am writing in response to your request for additional information in
Block 3 of the accident report form. I put "poor planning" as the cause
of my accident. You asked for a fuller explanation and I trust the
following details will be sufficient.

I am a bricklayer by trade. On the day of the accident, I was working
alone on the roof of a new six-story building. When I completed my work,
I found that I had some bricks left over which, when weighed later were
found to be slightly in excess of 500 lbs.

Rather than carry the bricks down by hand, I decided to lower them i n a
barrel by using a pulley, which was attached to the side of the building
on the sixth floor.

Securing the rope at ground level, I went up to the roof, swung the
barrel out and loaded the bricks into it. Then I went down and untied
the rope, holding it tightly to ensure a slow descent of the bricks.

You will note in Block 11 of the accident report form that I weigh 175
lbs.

Due to my surprise at being jerked off the ground so suddenly, I lost my
presence of mind and forgot to let go of the rope. Needless to say, I
proceeded at a rapid rate up the side of the building.

In the vicinity of the third floor, I met the barrel, which was now
proceeding downward at an equal, impressive speed. This explained the
fractured skull, minor abrasions and the broken collar bone, as listed
in section 3 of the accident report form.

Slowed only slightly, I continued my rapid ascent, not stopping until
the fingers of my right hand were two knuckles deep into the pulley.
Fortunately by this time I had regained my presence of mind and was able
to hold tightly to the rope, in spite of beginning to experience a great
deal of pain.

At approximately the same time, however, the barrel of bricks hit the
ground and the bottom fell out of the barrel. Now devoid of the weight
of the bricks, that barrel weighed approximately 50 lbs. I refer you
again to my weight.

As you can imagine, I began a rapid descent, down the side of the
building. In the vicinity of the third floor, I met the barrel coming
up. This accounts for the two fractured ankles, broken tooth and several
lacerations of my legs and lower body.

Here my luck began to change slightly. The encounter with the barrel
seemed to slow me enough to lessen my injuries when I fell into the pile
of bricks and fortunately only three vertebrae were cracked.

I am sorry to report, however, as I lay there on the pile of bricks, in
pain, unable to move, I again lost my composure and presence of mind and
let go of the rope and I lay there watching the empty barrel begin its
journey back down onto me. This explains the two broken legs.

I hope this answers your inquiry
 

·
Registered
Joined
·
5,155 Posts
Discussion Starter #6
Father and son together should have hauled up a barrel of bricks(180 lbs) first and used that potential energy to counteract his weight. To come down the son just grabs an armful of bricks from the barrel Mechanical advantage is easy when you use your head.
Well I tried that once and it didn't work out too well.

We got the bucket of bricks hoisted up pretty well and the line connected to my harness. Apparently we used too many bricks because I shot up at warp speed and banged my knees on the bucket about halfway down.

Then my fingers got jammed on the pulley at the top of the mast but I held on for dear life.

Sadly the bucket we used was a little old and the bottom fell out.
So now I'm cumming down faster than I went up and of course I got bashed by the durn bucket on the way down.

I landed on the pile of bricks which was very painful and apparently I was disoriented and let go of the line.

The bucket came down of course and bashed my head.

I don't remember much of this indecent myself but the above is what I was told is how it all happened.

So in short I don't recommend hoisting bricks in a bucket as a counter weight.:)

I see someone else had the same experience!!!
 

·
Senior Member
Joined
·
19,489 Posts
  • Like
Reactions: davidpm

·
Registered
Joined
·
76 Posts
Not to derail the thread, but I thought this was an appropriate place for this old "workers comp accident form."

Dear Sir:

I am writing in response to your request for additional information in
Block 3 of the accident report form. I put "poor planning" as the cause
of my accident. You asked for a fuller explanation and I trust the
following details will be sufficient.

I am a bricklayer by trade. On the day of the accident, I was working
alone on the roof of a new six-story building. When I completed my work,
I found that I had some bricks left over which, when weighed later were
found to be slightly in excess of 500 lbs.

Rather than carry the bricks down by hand, I decided to lower them i n a
barrel by using a pulley, which was attached to the side of the building
on the sixth floor.

Securing the rope at ground level, I went up to the roof, swung the
barrel out and loaded the bricks into it. Then I went down and untied
the rope, holding it tightly to ensure a slow descent of the bricks.

You will note in Block 11 of the accident report form that I weigh 175
lbs.

Due to my surprise at being jerked off the ground so suddenly, I lost my
presence of mind and forgot to let go of the rope. Needless to say, I
proceeded at a rapid rate up the side of the building.

In the vicinity of the third floor, I met the barrel, which was now
proceeding downward at an equal, impressive speed. This explained the
fractured skull, minor abrasions and the broken collar bone, as listed
in section 3 of the accident report form.

Slowed only slightly, I continued my rapid ascent, not stopping until
the fingers of my right hand were two knuckles deep into the pulley.
Fortunately by this time I had regained my presence of mind and was able
to hold tightly to the rope, in spite of beginning to experience a great
deal of pain.

At approximately the same time, however, the barrel of bricks hit the
ground and the bottom fell out of the barrel. Now devoid of the weight
of the bricks, that barrel weighed approximately 50 lbs. I refer you
again to my weight.

As you can imagine, I began a rapid descent, down the side of the
building. In the vicinity of the third floor, I met the barrel coming
up. This accounts for the two fractured ankles, broken tooth and several
lacerations of my legs and lower body.

Here my luck began to change slightly. The encounter with the barrel
seemed to slow me enough to lessen my injuries when I fell into the pile
of bricks and fortunately only three vertebrae were cracked.

I am sorry to report, however, as I lay there on the pile of bricks, in
pain, unable to move, I again lost my composure and presence of mind and
let go of the rope and I lay there watching the empty barrel begin its
journey back down onto me. This explains the two broken legs.

I hope this answers your inquiry
Enjoyed that! :laugher
Thanks :D
 

·
Old as Dirt!
Joined
·
3,487 Posts
Back in the mid-‘80’s, my (much) better half and I were visiting the Isthmus on Catalina Island for a long weekend. While seated in the bar at Two Harbors, we overheard a heated discussion between some of the Island's denizens as to the best way a single hander could make his way to the masthead while underway, if necessary. One fellow came up with the idea of hoisting a three part tackle to the masthead, attaching the lower block to his bosuns chair, passing the fall through a snatch block on the toe rail and attaching it to a 5 gallon collapsible bucket. In theory, by pitching the bucket into the sea, it would expand and fill and the drag on the bucket would hoist our seaman to the masthead. To descend, he attached a retrieving line to the bottom of the bucket that could be tightened thereby spilling the bucket and allowing him to return to the deck. This idea met with rather a lot of derision and joshing which entertained the crowd but made the fellow rather unhappy.

The following morning, in an effort to prove his theory (and still likely suffering the effects of a few too many Dark’n Stormy’s), the fellow set out to prove his method. He rigged his “elevator”, cast off his mooring and confidently motored out of the anchorage, headed for Avalon and a more receptive audience. Once having cleared Bird Rock and turned southeast, the fellow reportedly set his autopilot, attached his bosuns chair to the tackle, and kicked his bucket into the sea. It caught, and the moment the slack was pulled out of the line, he was launched to the masthead like a rocket. Unfortunately, or fortunately, depending upon ones perspective, his trigonometry left something to be desired such that the retrieving line tied to the bottom of his bosuns chair was too short to permit him to reach the masthead. Reaching its full extent, the line promptly snatched the bottom of the bucket up and out of the sea. With this, our aspiring inventor stopped his upward progress and dropped toward the deck like a rock. Of course, with his descent, the retrieving line slackened, the bucket caught a wave and filled and he reversed his direction of travail. Evidently, this up and down business repeated for several cycles before someone in a dinghy took pity one him, caught up with his bucket, and, with that allowed him to return to the deck with only minor scrapes and a severely bruised ego, so much so that he skipped Avalon and was last seen headed toward Oceanside.
 
  • Like
Reactions: davidpm

·
Registered
Joined
·
583 Posts
Thanks for the video, it was interesting but I think I'll just stick to using my electric windlass. And if my anchor light burns out I'll just hoist a lantern of some sort half way up the mast. I only weighs a few ounces.
 

·
Registered
Joined
·
5,155 Posts
Discussion Starter #11 (Edited)
The reason I posted this is because the dynamics, at least to me, are not obvious even though they are certainly correct.

If for example I have a two bock system (single block at top with becket and single block attached to harness) and weigh 150 lbs and am holding the line that holds my weight just off the deck I am holding just 50 lbs.

If I pass the line to someone on standing on the deck they have to hold 75 lbs to keep my weight off the deck.

Pretty amazing, at least to me.
 
  • Like
Reactions: jimgo

·
Senior Member
Joined
·
19,489 Posts
The number of advantage 'parts' of a tackle is up to the last moving part.. sitting in the harness (which moves) makes that a 3 part tackle.. the guy on deck is a fixed point, so that last run to the deck doesn't figure in the tackle.

But doesn't that make it a 2 part tackle, with the guy on deck having to support/hold half the load (ie 75#, not 100?)
 

·
Registered
Joined
·
5,155 Posts
Discussion Starter #14
The number of advantage 'parts' of a tackle is up to the last moving part.. sitting in the harness (which moves) makes that a 3 part tackle.. the guy on deck is a fixed point, so that last run to the deck doesn't figure in the tackle.

But doesn't that make it a 2 part tackle, with the guy on deck having to support/hold half the load (ie 75#, not 100?)
Yes of course, edited my post for accuracy.
 

·
Registered
Joined
·
5,979 Posts
On my previous boat I purposely over-sized my boom vang so I'd have a nice heavy duty 5:1 block and tackle set with quick disconnect shackles capable of hauling 'things'.

I used it once to pull a 4500 pound boat that had broken loose at high tide and was sitting solidly aground on it's side. It took two full sized adults heaving away - and to get it moving I added a 3:1 to the tail. I think what I had there was a 15:1 - with only 5 feet of actual pull - meaning we could pull all the rope in and move the boat six inches or so, then reset and do it again.

The problem we ran into was that the average adult human can only pull about 150 pounds on a horizontal pull (think tug of war style). My buddy and I combined if we were lucky could only put 300 pounds of pull on the tail - and at x 5 that's still only a 1/3 of the weight of the boat.
Subtract for friction and the fact that team work pulling is not pull x2, and we had no chance but fortunately we were pulling the boat not lifting it.
 

·
Registered
Joined
·
5,155 Posts
Discussion Starter #16
Subtract for friction and the fact that team work pulling is not pull x2, and we had no chance but fortunately we were pulling the boat not lifting it.
Did it work, did you pull the boat free?

What did you use for an anchor point.

The stretch must have been enormous.

At some point you would have so much stretch is a system that you would two blocked before getting all the stretch out.
 
1 - 16 of 16 Posts
Top