how much weight can a 6:1 block and tackle support

I ask this because I am going to be entering my first year teaching fifth grade science. Part of the curriculum is understanding how simple machines work, such as block and tackle. I plan on taking my boat to school and setting it up so my studnets can see some of the practical uses of pulleys. I was thinking of constructing a tri-pod and having the students use the 6:1 blocks to lift an amount of weight the normally wouldn't be able to. Would this be bad on the blocks? How much weight could I use before I risk damaging the blocks? What do ya'll think?

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James
86' Redline Hobie 16
Sail # 76909

A 40 mm Harken Carbo triple (2640) has a max working load of 1455 lbs and a breaking load of 3050 lbs.

The standard Harken low profile triple ratchet (194) has a working load of 500 lbs and a breaking load of 2000 lbs.

I think an 11 year old would be impressed with lifting significantly less than that.

In a very general sense, working load is about half of the breaking load. All Harken blocks have a rating one way or the other. Don't forget the working load of the line you're using too.

A very dramatic effect for the kiddoes is to lift something heavy by hand, then hook it up to the pulley system. Don't forget to illustrate that with a 6:1, you also have to pull 6x the line. These are the kinds of lessons that stick for life. Good luck!

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Sea ya,
Chris Larsen
Co-Pilot of the Hobie Getaway
"The Twins"

~~(\_~~

As others have said the Harkens can handle probably more weight than you will want to use for the demonstration. All the Harken data can be found on the the Harken web site.

You can demonstrate by having the kids try to lift something without, then with the block. You can even rig them progressively to demonstrate 2:1, 3:1, pulley ratios, etc. It might be easier to show the differences, however, using a spring scale like a fish scale, so they can actually see the numbers.

Best of luck teaching! After retiring from the Navy I taught HS math for 3 years, until a heart attack and disillusionment sent me in another direction.

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Alan
'86 H16, Sail #89057

Thanks for the replies everyone. I thought that it could handle a couple hundred pounds without any damage. I believe it is important for kids to literally get their hands on what they are learning. I think letting them see a fully rigged cat, letting them help hoist the sails, showing them how the sheets and blocks help shape the sail, and maybe showing them a cool cat video clip might help the lesson stick. Thanks again.

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James
86' Redline Hobie 16
Sail # 76909

I remember something like this when I was a kid. However, they took two 2x4's (about 2 feet long) and put handles on them and rigged up some blocks/pulleys in between them. Then they had the two biggest kids in the class hold the handles and pull it apart while the smallest kid pulled the rope and brought the 2x4's together (dragging the other two kids across the floor).

Horrible explanation, but I certainly remembered the lesson.

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1984 H16 - #80541

That sounds like great fun. I may have to steal your idea JRagg, if you don't mind.

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James
86' Redline Hobie 16
Sail # 76909

Wasn't my idea. I was one of the "strong guys' that got pulled together by a girl. I'm scarred for life...

I can't remember, but I think it was something like 8:1 using standard hardware store pulleys (so they could be screwed into the 2x4.

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1984 H16 - #80541

One other idea is to show the students that blocks can be set up to multiply as well as add. You could put a 2 : 1 drive on the end of the 6 :1 for a net 12:1 cascading system. As a kid a cascaded 5 2:1 blocks into a system I was impressed then shocked how easily I pulled my desk over. Fortunetly my da is an engineer so we understood

That is sooo cool James. The world needs more teachers like you!

J

I remember a lesson from college. (25 years ago). If I place 600 lbs force pulling down (weight)on a 6-1 purchase system, how much force will it take to lift it off the ground. You change the purchase for multiple problems. (The prof sailed a Hobie 16). I think the answer was 100 lbs.

Keep up the good work.

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1989 Hobie 18 Worlds Boat, Magnum Wings & Spinnaker
1987 Hobie Holder 20 #273

dale.vanlopik"at"att"dot"net