Worked for me...
Hi, We used to have an Albin 43 Sundeck. I always watched whenever a yard pulled the boat. I found that while they were trying to be conscientious about the prop shafts, sometimes they would set the straps so far aft that they put side pressure on the shafts, and sometimes erroring in the opposite direction, would place the straps too far forward, to make sure they didn't bend the shafts. Obviously, both can cause problems - bending the prop shafts, or getting too close to the center of gravity. So, one day, while standing close to the boat I held a square up so that the horizontal arm was in line with the actual waterline and moved it to the point where the square's vertical arm was in line with a point where the aft strap could not touch the prop shafts, but was about 4-5" forward of the point that it could. I painted a small (~1") "T" laying on its side to indicate where the aft-most edge of the strap could be and not touch the prop shafts. I hope the following illustration comes out as it does on this computer. The "T" looked like this on the port side: ---| . The vertical line represents the aft-most edge for the sling, while the horizontal (dashed) line represents the sling's placement. The markings were located just below the cap rail, so that they were easily seen. The yard folks really appreciated the markings once they were explained to them.
I had done essentially the same thing on our old sailboat to indicate where the aft end of the keel was so that the aft strap would not be under the rudder.