Optical scopes are essentially telescopes with crosshairs. While typically used by land surveyors, they’re also in the tool crib of High Seas Yacht Service, in Fort Lauderdale, Florida, where they’re valued for their preciseness in measuring running gear alignment.
The scope works when you focus its crosshairs forward through the crosshairs of two glass targets placed in different bearing locations. When the crosshairs are perfectly aligned, this creates a reference line. Next, the scope moves forward to focus on a third target. If the third target doesn’t line up with the straight line, the engine is out of alignment.
In the above video of an optical scope alignment of a 49′ (15m) Eastbay, a High Seas technician explains each step. He starts by calibrating the crosshairs in the scope, then focuses the scope on the glass target in the center of the aft strut, aligning the crosshairs of the scope with the ones on the target. Once these are aligned, the scope moves forward to the target in the shaft log, and aligns with those crosshairs, too. This creates the straight line. Now, to find out if the shaft is aligned, the scope moves forward to a third target on the transmission.
As you can see, even though the alignment is off by only a couple thousandths of an inch, that tiny misalignment is clearly visible.
For more on shaft alignment and High Seas Yacht Service, see the article “The Necessity of Straight,” by Steve D’Antonio, in Professional BoatBuilder No. 159, page 36.