One thing that constantly surfaced during the discussion on
using a block and tackle with a 3:1 mechanical advantage to
reduce the load on the gin pole was the objection to having
to use a LOT of expensive rope. Here is a technique to
minimize that problem. Yes, its more work, but uses a LOT
less rope. There is always a trade-off . . .
Say you are raising a 200 pound mast up to the top of a 100
foot tower. If you are a believer and plan to use a 3:1
block and tackle at the top of your gin pole (which is
sitting at about 110 feet above ground), you will need a
continuous length of rope that will run 4 times up and down
the tower from the top of the gin pole to the ground, or 440
feet of rope. This is a LOT of expensive rope . . .
But wait! It only has to be one continuous piece if you
expect all of it to fit through the pullys. That last piece
that runs down the tower to the workers pulling the mast up
never has to pass through the block and tackle pullys so it
can have a knot in it at the top of the tower. This still
means you need 440 feet of rope but it can be made of two
pieces . . . one is 330 feet long and the other is 110 feet
long tied to the first piece. Yes, 330 feet is still a very
long and expensive piece of rope, but wait . . . there is a
way to save even more rope!
In this next scenerio, you lift the mast in two stages.
Stage one gets the mast to the top of the tower. You can do
this by rigging a single pulley on and near the top of the
tower itself (not the gin pole). The tower is generally
PLENTY strong enough to support the entire load with no
benefit of the mechanical advantage of a block and tackle.
This step requires a length of rope 200 feet long, and
anyone contemplating a 100 foot tower should already own a
strong rope of this length.
Next, you temporailty tie off the mast near the top of the
tower and prepare to hoist it the last 10 feet over the top
and down inside the tower. This is the only time you need
to use the gin pole to help lift this mast in place and
therefore, the only time you really need the mechanical
advantage of the block and tackle. Since you won't be using
the gin pole and block and tackle to do a lift from the
ground, you only need about 36 feet of rope to run 3 times
through the pullys to do a 10 foot lift at the top of your
tower. You still need to get the end of the rope to the
ground 110 feet below so the rope needs to be about 146 feet
long. We already decided you have to own a 200 foot piece
anyway so you have all the rope you need to do this. But
wait . . . there is more!
This last 146 foot piece can really be two pieces tied
together since the 110 foot piece going from the top of the
gin pole to the pulling crew will never have to pass through
the block and tackle pulleys. It can be made of a 110 foot
piece with a 36 foot piece tied to it.
So that's it . . . the only thing you need now are a couple
of good blocks . . . one single and one double. I have
found them at estate sales for as little as $5 or $10. New
ones cost about 5 or 10 times that much but think of all the
money you are saving on the rope you don't have to buy now!
The above suggestions would have been pretty useless a
couple of days ago when we were still arguing about whether
it makes any sense at all the use a block and tackle for
lifting heavy masts.
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