[Rhodes22-list] Converting Electric Winch Motor Lift to a Manual System

[Gmorganflier]

I emailed Roger for an engineering answer to the question Peter and I had
about why it was taking more pull than we expected. Below is his response. I
obviously had a misconception about how a block and tackle system works and
I didn’t consider the horizontal component Roger described. 

George

_____________________

“Hi George,
 
I see two issues with your design.  The force is only evenly distributed
among the various falls between the blocks until the motor starts to move. 
Once the motor starts to move, the tension in each fall drops off by the
force ratio in the block and tackle up to that point.  i.e. Fall 1 has ~106
lbs of tension, fall 2 has ~53 lbs, and so forth.  If you look at the way
the line is rigged thru the various blocks and sum up the tension on the
block mounting points, you will see the block closer to the port side has
considerably more force on it than the other block.  This unequal loading
might be tending to cause the sliding motor mount to be nonsquare in the
tracks, thus causing it to bind up slightly.  The other issue is the
changing geometry as the motor gets near the top of the track.  Your design
pulls on the motor mount at an angle instead of straight up and down the
track.  So, there is a horizontal component of the block and tackle force
that is trying to pull the motor into the transom and only the vertical
component is raising the motor.  This vertical component is proportional to
the tangent of the angle made by the block and tackle relative to vertical. 
As the motor approaches the top of the track, this angle increases.  Thus,
more and more of the force is wasted trying to pull the motor into the
transom.  So, not only is there less vertical force available to lift the
motor as it nears the top of its travel; but, this horizontal component
might also be causing the sliding motor mount to bind in the track, with the
binding getting worse as the motor nears the top of its travel.
 
The GBI manual motor mount design clusters all the blocks on the centerline,
the tension from the block and tackle is directly in line with the tracks,
and the force available to lift the motor doesn’t change with motor
position; thus eliminating or minimizing all the effects described above. 
Using standard off the shelf hardware, I don’t see anyway to fix your
design.  You may just have to live with it.  If there really is the binding
I am describing; then, you will also experience increased wear on the
sliding components.
 
Roger Pihlaja
S/V Dynamic Equilibrium“



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