NavList:

I was trying to imagine when you might use an expensive pentaprism attachment for other than survey work.  Suppose you were navigating on an accurate chart using, for whatever reason, rapid fixing using a station pointer and horizontal marine sextant angles between your position and three recognizable points.  Again, for whatever reason, there were only three such points available.  Lines from your position to the three points, would define three sectors.  So long as only one sector angle was greater than 120 degrees, fixing would be possible with just a marine sextant.  However, if a second sector angle increased above 120 degrees, a pentaprism would be very useful.  So long as you measured the two larger angles, you wouldn’t need to keep slipping the pentaprism on and off.   So, when might you use this?  Possible during pilotage down a narrow channel or into or out of an estuary with few other aids available. As the first priority is to sail the vessel, you might find this difficult sailing single handed. Just a thought.  DaveP