NavList:

Hello everyone,

From my sailing experience aboard a mid‑size sailboat of about 40 feet, achieving speeds of 7–8 knots typically requires fairly strong winds (close to 20 knots). Under such conditions, the seas are often choppy, which makes it challenging to obtain precise HS observations when lowering stars/planets toward the horizon. Inevitably, this introduces a significant margin of error.

At a speed of 7 knots, if it takes around 20 minutes to record sights of three different celestial bodies, the vessel would cover roughly 2 nautical miles (assuming no current). In practice, surfing waves tends to reduce speed slightly, so the actual distance traveled is often less. Even when taking three sights of each body, the average time difference between the first and last observation is about 15 minutes, meaning the distance covered would be under 2 miles.

When plotting lines of position (LOPs), accuracy is further constrained by the tools themselves—parallel rulers, protractors, and even the thickness of the pencil. For this reason, I do not believe it is necessary to establish a dead reckoning (DR) position for every single celestial body observed.

A practical improvement would be to estimate the DR position between the first and third celestial body shots. This is the approach I used last year when crossing the Atlantic on a large cruise ship (Norwegian Lines), which maintained an almost constant speed of 20 knots. Even at that speed, the error introduced by the vessel’s motion remained small.