George.

Of course Roger is head on.  Our system uses 3, not 4, trolley contacts 
with the boat; a large wheel at the bottom center of the trolley and a 
pair of sliders or wheels on the sides of the top end of the trolley.  
This facilitates the automatic tilting of the motor at the proper lift 
height and, so far at least, results in zero binding at any point of 
travel in the tracks.

But you can skip all of this. We think we are this winter away from 
inboard retractable electric propulsion.   Imagine never having to take 
your eyes off where you are headed by having a single handed electronic 
control that does it all.

On 9/6/20 2:25 PM, Gmorganflier wrote:
> 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“
>
>
>
> --
> Sent from: http://rhodes-22.1065344.n5.nabble.com/
>