On 22 Sep, 22:40, "The Medway Handyman"
wrote:
Three poles in a triangle, one longer that the other two, used with a block
& tackle attached to the longer one?

A "Fecking Dodgy" ? You're using one pole as a gin (i.e. an
unsupported pole) and you're doing this at an angle, not even
vertically. You've had to put the equivalent of four poles together
to make something with only the strength of one and no mobility. Gins
are inherently dodgy (a slight sway sideways changes the loads from
axial compressive to bending, then the whole thing collapses rapidly).
This seems safer as it already starts out in the worst-case
configuration, but it's not an efficient use of materials.

A tripod is three poles, with the weight carried on the junction of
the three and thus shared between them.

Sheerlegs are a rigid frame in one plane, allowed to hinge at the
base. They're usually constructed from two poles, as fixing the ends
down is sufficient to hold the frame together. The advantage of
sheerlegs is that they're a purely compressive load, not a bending
load (as they're free to move), so they don't need clever carpenty and
joints to make the frame.

There are also the two forms of timberyard crane, but I can't remember
the precise names. Both use a fixed tripod, the jib for one is a
single beam sheerleg from the ground, the other uses a rigid post &
jib, supported at the top by the tripod. These need strong joints (the
tripod carries a side-load) and the rigid post version is based on the
millwright carpentrythat evolved in the late medieval period. The
advantage is that they can slew a load sideways to position it, the
first sort can even luff it inwards, and both can be dismantled and re-
assembled to move around a yard, woodland or construction site. Names
for these are probably in Sandels, Sprague du Camp or even Vitruvius,
but those books are at home.